8 Late Cenozoic Glaciations in Patagonia and Tierra del...

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Late Cenozoic Glaciations in Patagonia and Tierra del Fuego

Jorge Rabassa

CADIC-CONICET, C.C.92, 9410 Ushuaia, Argentina and Universidad Nacional de la Patagonia at Ushuaia

1. Introduction

Patagonia and Tierra del Fuego show one of the longestand most complete sequences of glacigenic deposits andlandforms in the Southern Hemisphere outside ofAntarctica and, perhaps, of the entire world. Starting inthe latest Miocene, these units have been preserved,though sometimes rather in a fragmentary manner, thanksto their interbedding with volcanic flows that have pro-tected the sediments from erosion, besides allowing theirabsolute dating. Similarly, the relative tectonic stabilityof the area, after the final emplacement of the southernAndes, and the dry climate that has dominated the regionsince the Late Miocene have contributed to keep theglacigenic deposits from denudation.

The climate of Patagonia and Tierra del Fuego, fol-lowing the general conditions on the Earth, has sufferedsignificant variations during the Cenozoic, particularlysince the Miocene. These climatic changes are relatedto various causes such as continental displacement due toplate tectonics, modification on greenhouse gases contentin the lower atmosphere and changes in astronomicalparameters, namely eccentricity of the Earth orbit, obli-quity of the planetary axis and equinoccial precession.

Though this process of climate deterioration wasinitiated possibly toward the end of the Mesozoic, butmost likely, at the beginning of the Paleogene, it culmi-nated with the recurrence of multiple cold-warm climaticcycles starting in the Miocene, which led to the develop-ment of global ice ages.

The knowledge of the Late Cenozoic glaciations inPatagonia and Tierra del Fuego (Fig. 1a) has made signifi-cant progress in the last decade, thanks mainly to theapplication of absolute dating techniques, following thepioneer work of John Mercer (Mercer, 1976, among manyother benchmark contributions; Meglioli, 1992; Clapperton,1993; Ton-That et al., 1999; Singer et al., 2004a). The citeddating techniques have allowed to link the Patagonianrecords with other glaciated regions and with the globalmarine isotopic sequence (Shackleton, 1995).

This chapter presents the status of our knowledge onthe Patagonian and Fuegian glaciations, starting in theLate Miocene, when the junction of global, cooler cli-matic conditions and the final rise of the southern Andesenabled the formation of mountain glaciers in the area.

The objective of this chapter is to present the absolutechronology of the Patagonian terrestrial glacialsequences, basically dated by means of 40Ar/39Ar datingtechniques on volcanic rocks associated with glaciallandforms and deposits, and more recently, cosmogenicisotope dating techniques on erratic boulders and glacial

erosional surfaces (Kaplan et al., 2004; Singer et al.,2004a). In some cases, the magnetostratigraphy of glacialdeposits is available, thus allowing the correlation withthe Pampean (central eastern Argentina; Fig. 1a) conti-nental sequences (mostly loess units) and with the globalocean record (Rabassa et al., 2005).

Likewise, the stratigraphic and biostratigraphic unitsof the Pampean Region of Central Argentina have beenchronologically linked by means of paleomagnetic datingtechniques, thus providing a basis for regional and pla-netary correlation between the glacial events and thePampean loess deposition (Cione and Tonni, 1999).

The regions discussed in this chapter are shown inFig. 1. Argentinian Patagonia extends southward of theRıo Colorado (Fig. 1b, Site 1), with a total length of almost2500 km, between 36� and 55� S, on the eastern side of theAndean Cordillera, including Isla Grande de Tierra delFuego (Fig. 1c). If a map of Patagonia is superimposedin an upside-down position on top of a map of Europe atthe same scale, its extremes would be coincident with thelatitudes of the island of Malta and Copenhagen, respec-tively, a very large distance that explains the great varietyof climates and ecosystems of this region.

This chapter includes also the most significant infor-mation available on the glaciations of the Chilean (wes-tern) side of the Andes, along the same latitudinal belt. Itdeals particularly with the Chilean Lake District (Fig. 1b,Sites 13, 14), where very important work has been doneby several authors during more than three decades(Mercer, 1976; Porter, 1981; Denton et al., 1999a, b).

Patagonia is formed by two main physiographicalunits: the Patagonian Andes (Fig. 1a), which extend in aN–S direction, except in Tierra del Fuego where they turneastward to achieve a W–E arrangement, and extra-Andean Patagonia, mostly low-lying, semiarid flatterrains, volcanic tablelands and low ridges of variedgeological composition.

The localities cited in the text are found along thePatagonian and Fuegian Andes between 38� and 55� S,and the corresponding Patagonian plains, the Fuegian–Magellanic Basin and the adjacent Chilean areas (Fig. 1a).

2. Glaciers in Patagonia and Tierra del Fuego

Patagonia and Tierra del Fuego are some of the regions of theworld still largely covered by ice and snow. Three majormountain ice sheets can be observed along the Patagonianand Fuegian Andes, several smaller ones and countlesscirque and niche glaciers and permanent snowfields of variedsize. These three ice sheets are the Northern Patagonian

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Ice Field (46�300–47�300 S; 73�–74� W), the SouthernPatagonian Ice Field (48�300–51� S; 73�–74� W) and theDarwin Cordillera Ice Field (54�300–55� S; 69�–71� W). SeeFig. 1a. These large ice bodies are, by far, the most importantof the Southern Hemisphere outside Antarctica. They arethe remnants of the Late Pleistocene mountain ice sheetthat covered the southern Andes. This Pleistocene ice sheethad a total length of almost three times the size ofthe coeval European Alpine ice sheet, but elongated in a

N–S direction, allowing for significant changes in glaciertype, size, volume, elevation, regime and climate.

Local ice caps of much reduced dimensions are foundusually at the summit of Tertiary and Quaternary volca-noes, that is, endogenetic, constructional features thathave grown above the regional summit accordancesurface. Examples of these local ice caps are those onVolcan Lanın (39�300 S; 71�300W, 3778 m a.s.l.; Fig. 1b,Site 17; Fig. 2), Monte Tronador (41�300 S; 71�500W;

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Fig. 1. Location maps. (a) Patagonia, main geographical regions; (b) Patagonia, location of localities cited in the text(see attached list); (c) Tierra del Fuego, localities cited in the text.



152 Jorge Rabassa

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1. Río Colorado 2. Río Negro 3. Río Limay 4. Río Neuquén 5. Río Chubut 6. Río Deseado 7. Río Collón Curá 8. Río Aluminé 9. Lago Nahuel Huapi and

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Fig. 1. Continued.



Late Cenozoic Glaciations in Patagonia and Tierra del Fuego 153

3556 m a.s.l.; Fig. 1b, Site 12; Fig. 3; see Rabassa et al.,1978, 1981), Monte San Lorenzo (47�450 S; 72�150W;3706 m a.s.l.; Fig. 1b, Site 32) and Isla Riesco (53�140 S;3�000W; 1183 m a.s.l.; Casassa et al., 2002a, 2002b;Fig. 1a), among many others, particularly on the Chileanside of the Andes.

Hundreds of smaller cirque and short valley glacierscan be found elsewhere in the Patagonian and FuegianAndes. Due to the impact of global warming (Rosenbluthet al., 1997), most of these mountain glaciers have beenreceding very intensively in the last two decades, and it isvery likely that most of them will be totally gone by themiddle of the present century (Casassa, 1995; Naruse et al.,1995; Aniya et al., 1997; Aniya, 1999; Rivera and Casassa,2004; Rabassa, 2007AU1 ). The loss of ice will have drastic effectson many environmental issues, such as water resources(Coudrian et al., 2005; Rabassa, 2007) and sea level rise(Rignot et al., 2003).

3. Snowline Position and Distribution of Pastand Present Glaciers

The permanent snowline or firnline is the line that con-nects the lowest topographical positions of snow fallen

during the previous winter on the surface of a glacier thathas not melted away at the end of the Southern Hemi-sphere summer, that is, March and early April. Theequilibrium line is an imaginary line that separatesthe accumulation area, with a net gain of mass, fromthe ablation area (net loss of mass) on the surface of aglacier. Permanent snowline and equilibrium line arecoincident in most maritime and temperate glaciers(Clapperton, 1993). These lines differ only in polar orsubpolar regions, where regelation takes place below thepermanent snowline. Regional snowline, or equilibriumline altitude (ELA), is a very important geographical andclimatic parameter in Patagonia, which tends to be verystable through time for a certain area as it is related to theposition of the summer 0�C isotherm. However, recentclimatic change due to global warming has determined asignificant rise in ELA in most of the studied area, with arise of up to 200 m in only the last 20 yrs (Casassa et al.,2003; Rabassa, 2007).

The snowline and ELA and the distribution of modernice bodies have been discussed extensively by Clapperton(1993). The altitudinal position of snowline is highlydependent on local topographic and climatic conditions.The snowline decreases gradually from North to South,between around 2200 m a.s.l. in northern Patagonia and

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Fig. 1. Continued.



154 Jorge Rabassa

less than 1000 m a.s.l. in the western Fuegian Andes. Itsaltitude increases sharply from West to East, as a conse-quence of the strong precipitation gradient in this direc-tion, generated by the interference of the Andeanmountain chains with the weather coming from theSouth Pacific anticyclonic center (Chapter 3AU2 ).

In northern Patagonia, between 36� and 44� S, theposition of present and past snowline has been studiedby Flint and Fidalgo (1964, 1969) and Rabassa et al.(1980). Present ELA has been estimated by means of adetailed glacier inventory, based on aerial and terrestrialphotographs, completed in 1978 (Rabassa et al., 1980).Pleistocene ELA has been calculated using the cirquefloor elevation, assuming that these cirques (presentlywith or without ice) have been reoccupied several timesduring the Quaternary (Flint and Fidalgo, 1964, 1969). Inthe region of temperate regime and dominant winterprecipitation, the position of the ELA is roughly coinci-dent with the atmospheric summer 0�C isotherm,but further south, increased year-round precipitationbrings ELA to much lower positions, reaching as lowas 800 m a.s.l. in western Tierra del Fuego (Clapperton,1993), around 1000 m a.s.l. in Ushuaia (Fig. 1b, Site 40;Coronato, 1995a, b) and possibly in between 500 and900 m a.s.l. at Isla Riesco (Fig. 1a; Casassa et al., 2002b).

4. Glaciations in Patagonia and Tierra del Fuego

Pliocene and Pleistocene glaciations were frequent in thisregion. Moreover, glacial tills of a latest Miocene glaciationhave been found in southern Patagonia, and indirect evi-dence points at, at least, isolated mountain glaciers alreadyin Late Miocene times, both in northern and southern Pata-gonia. Pliocene glaciations have been recorded in northernNorth America (northwest Canada, Alaska) as of LateGauss paleomagnetic age (Barendregt and Duk-Rodkin,2004; Duk-Rodkin et al., 2004; Harris, 2005), and as oldas 2.5 Ma in central Missouri, USA (Balco et al., 2005).

It is considered that valley and piedmont glacierscoming from the ice sheet or from local ice caps extendedup to several hundred kilometers eastward during themost extensive glaciation, as well as to the deep PacificOcean waters in the west. The present Atlantic submarineplatform was reached by the ice several times during thisperiod, but only south of the present Rıo Gallegos valley(Fig. 1b, Site 27). On most of the Argentinian side of theAndes, the glaciers only extended to the piedmont areas,not far beyond the mountain front. On the western sidesouth of Isla Chiloe (Fig. 1a), the ice probably calved intothe Pacific Ocean during glacial events.

It should also be noted that the total length of thePleistocene Patagonian Mountain Ice Sheet was almostthree times the extent of the European Alps ice cap andmore than five times that of the New Zealand Alps duringthe same period.

4.1. The History of Glacial Investigationsin Patagonia and Tierra del Fuego

The first scientific observations about Patagonian glacia-tions were presented by Charles Darwin who, during thefamous ‘‘H.M.S. Beagle’’ voyage and together withRobert Fitz Roy, explored the Rıo Santa Cruz valley(Fig. 1b, Site 23) in 1833. There, Darwin describederratic boulders at Condor Cliff and several other sitesalong this valley (50� S; 71� W; Fig. 1b, Site 41), very far

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Fig. 2. Volcan Lanın (39�300 S; 71�300W, 3778 m a.s.l.;Fig. 1b, Site 17), southern slope, facing LagoHuechulauquen (Fig. 1b, Site 18), province of Neuquen,Argentina. (Photo by J. Rabassa, 1983).

Fig. 3. Monte Tronador (41�300 S; 71�500W; 3556 m a.s.l.;Fig. 1b, Site 12), western slope. Seen from Casa PangueGlacier valley, western slope, Chile. (Photo by J. Rabassa,1979).




from the Andean ranges, to which he assigned a glacialorigin though interpreting them as the product of icebergdeposition, as it was the paradigma of those times(Darwin, 1842; Imbrie and Imbrie, 1979). This was thefirst paper ever published on the Patagonian glaciationsand probably one of the very first after the publication ofLouis Agassiz’s glacial theory in 1840 (Imbrie andImbrie, 1979), though Darwin’s actual observations pre-ceded it by several years.

Several decades later, the famous Swedish geologistand explorer Otto Nordenskjold (1899) made the firstscientific study of the Patagonian and Fuegian glacia-tions, at both ends of the Pleistocene ice sheet, aroundSan Carlos de Bariloche (41� S; Fig. 1b, Site 9) and inTierra del Fuego. Nordenskjold followed the originalwork of Francisco P. Moreno (1897) who, during hisexploratory work in the Patagonian Andes, made veryearly and significant observations about the nature andextent of the glaciations. Nordenskjold (1899) providedthe first detailed map of the extension of the Quaternaryglaciations in southernmost Patagonia and Tierra delFuego (Fig. 4), and recognized different moraines,which he correctly interpreted as representing severalglacial stages. He was the first to suggest that the icehad partly extended over the present submarine platform.

Other important contributions of this pioneer epoch arethose by WehrliAU3 (1899), Rovereto (1912) and Willis (1914).

In the first decades of the twentieth century, Patagoniawas visited by many European scientists, who providedthe bases of our knowledge of the region. Vaıno Auer, aFinnish geographer, over several decades explored exten-sive areas of Patagonia. His contributions (Auer, 1956,

1958, 1959, 1970, among many other papers) are an out-standing catalogue of field localities and sections. Unfor-tunately, his interpretations about the extent of theglaciations were biased by the influence of Czajka(1955) who proposed a total glacierization of Patagonia,a model that we now know to be incorrect. Auer (1970)partially revised these ideas of a totally ice-covered Pata-gonia, but he still insisted in local glaciations in the centralPatagonian massifs, for which proof has never been found.

The maximum extent of the different glacial advanceswas first presented full scale by Carl C:zon Caldenius, aSwedish geologist who, between 1928 and 1931, mappedthe glacial deposits and landforms of Patagonia (Fig. 5).Caldenius’ academic advisor at Stockholm University wasthe famous glacial geologist Gerard De Geer. The latter hadasked Dr Jose M. Sobral, an Argentinian member of OttoNordenskjold’s 1901–1903 Antarctic Expedition and as hisgeology student graduated at Uppsala University, and whowas by that time the Head of the Argentinian GeologicalSurvey, to support the study of Patagonian glaciations, inthe same way as it had been done on the ScandinavianPeninsula. Mostly, De Geer’s interest was to compare gla-cial varve sequences in both hemispheres, the early chron-ological tool that he had developed for the ScandinavianPeninsula. De Geer (1927) described in detail this bina-tional arrangement and presented the first preliminary dataof Caldenius’ expedition. A warm biography of Caldeniushas been presented by Lundqvist (1983, 1991, 2001).

In his paramount contribution, Caldenius (1932) pre-sented a map that covered more than 1 M km2 (Fig. 6)extending from Lago Nahuel Huapi (41� S; Fig. 1a, Site 9)to Cape Horn (56� S; Fig. 1a). Caldenius (1932) identified

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156 Jorge Rabassa

moraines corresponding to four glacial events which henamed ‘‘Initioglacial,’’ ‘‘Daniglacial,’’ ‘‘Gotiglacial’’ and‘‘Finiglacial,’’ assuming a direct correlation with the Scan-dinavian glacial model. He considered these units as suc-cesive recessional phases of the Last Glaciation (LG) andobserved, additionally, the existence of inner morainicbelts, younger than the Last Glacial Maximum (LGM),which he named as ‘‘post-Finiglacial’’ advances. Althoughhis stratigraphic scheme is quite sound and his glacial mapis an outstanding work for its detail and precision, in spiteof the lack of appropiate maps and reliable roads at thetime, the chronostratigraphic scheme is unfortunatelywrong. Caldenius (1932) underestimated the age of someof the morainic belts, most likely impressed by the excel-lent state of preservation of the landforms, even thoseoccurring in some of the outermost (and older) arcs. Thisis due to the extremely dry climate of the Patagoniansteppes. Such a high degree of preservation would neverexist in the Scandinavian or Baltic regions, where no well-preserved pre-LG moraines are known.

Later authors have provided new data and evidence insupport of Caldenius’ basic model, modifying only hisoriginal chronology. Although the currently identifiedboundaries of the different glacial advances are highly coin-cident with those mapped by Caldenius, the total number ofglaciations and their chronological correlation has changed,based on absolute dating, new paradigms and interpreta-tions. Nevertheless, his original terminology is still pre-served, because it has a high value as unifying criteria forthe different glacial events throughout the region.

Caldenius (1932), following the methodology thenimposed by De Geer, studied varves and other glaciola-custrine deposits, and used them to telecorrelate glacialevents in Patagonia with those of Scandinavia. We knowtoday that these attempts were unsound, and therefore,this methodology has been abandoned.

Groeber (1936) recognized correctly that the glaciersin northern Patagonia never extended much beyond theAndean foothills. In later works, Groeber (1952) AU4pro-posed a fourfold glacial model, and extended the glacia-tion not only over all of Patagonia, but even to thewestern Pampas, reconstructions that are today clearlyunacceptable. Most likely, Groeber was strongly influ-enced by the works of Czajka and Auer, thus changinghis original, correct points of view. Unfortunately,Groeber’s last works and his immense prestige amongArgentinian geologists for a long time delayed a properunderstanding of the real extent of glaciation. However,his ideas were soon firmly opposed by Polanski (1965),in his studies in the Andean piedmont of Mendoza, cen-tral Argentinian Andes (33�–34� S, 300 km north of thePatagonian northern boundary), who had a great influ-ence on the later work of his student, Francisco Fidalgo.

Egidio Feruglio, an Italian geologist working for theArgentinian government, had a deep knowledge of thePatagonian regional geology and was, after Caldenius,the great innovator in the study of the Patagonian glacia-tions. Feruglio (1944) described with great precision asequence of basaltic lava flows with interbedded tillsat Cerro del Fraile, Santa Cruz Province (51� S, Fig. 1b, Site28), just north of the Magellan Straits, recognizing thegreat antiquity of the glacial deposits and assigning them

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62 Fig. 6. Caldenius’ (1932) original glacial map of Patagonia.

Fig. 5. Carl C:zon Caldenius. (Photo by Jan Lundqvist;Lundqvist, 1983, 1991, 2001).




a Pliocene age, older than the maximum glacial extent(which was later known as the ‘‘Great Patagonian Glacia-tion’’ or GPG; Mercer, 1976). This was certainly anextraordinary, pioneer contribution to the knowledge ofthe Pre-Quaternary glaciations of Patagonia since abso-lute dating was then still unavailable, and at that time,speaking about ‘‘Pliocene glaciations’’ was certainly arevolutionary concept.

Years later, and working at the full regional scale,Feruglio (1950) also recognized the existence of fourmajor Pleistocene glacial events, which he named as‘‘Pichileufuense inferior,’’ ‘‘Pichileufuense superior,’’‘‘Barilochense’’ and ‘‘Nahuelhuapense’’ (local names ofnorthern Patagonia, see Fig. 1b for type localities), retain-ing Caldenius’ (1932) fourfold scheme, but linking eachevent to geomorphological positions that were indicatorsof clearly different (and older) ages. Thus, he recognizedthat the ‘‘Pichileufuense’’ landforms and sediments arefound on the topographical divides, whereas the depositsof later glacial events are located within the valleysexcavated in them. Therefore, Feruglio (1950) estab-lished the basic criteria that much later allowed to iden-tify a Quaternary ‘‘Canyon Cutting Event’’ (Rabassa andClapperton, 1990) in Patagonia. Likewise, he firstlyestablished the possible correlation of the glacial depositswith (a) the ‘‘Rodados Patagonicos’’ or ‘‘RodadosTehuelches’’ (‘‘Patagonian Gravel Formation,’’ ‘‘Patago-nian Shingle Formation’’; Darwin, 1842; Caldenius,1940), which he considered to be of glaciofluvial origin,and (b) the loess acumulation events in those regionswhich he called ‘‘infraglacial’’, that is, the nonglaciatedPampas of eastern central Argentina (Feruglio, 1950).

Richard F. Flint, the distinguished American Quatern-ary scientist at Yale University, was invited in the early1960s by the Argentinian Geological Survey to work onthe Patagonian glaciations. Flint, together with FranciscoFidalgo (Flint and Fidalgo, 1964, 1969), studied the gla-cial deposits in the northern Patagonian Andes (39�–43� S;Fig. 1a), proposing a threefold glaciation model, based onwhat they named as the ‘‘Pichileufu,’’ ‘‘El Condor’’ and‘‘Nahuel Huapi’’ drifts, which they considered to be phasesof the LG. However, they already suggested in their 1969paper that the ‘‘Pichileufu Glaciation’’ might be olderthan the Late Pleistocene.

Fidalgo and Riggi (1965) identified four main glacialdrifts at Lago Buenos Aires (47� S; Fig. 1b, Site 24), aswell as the glaciofluvial origin of at least a portion of the‘‘Patagonian gravels,’’ but without assigning absoluteages to the studied units.

John H. Mercer (Fig. 7), an English geographer work-ing at Ohio State University, was a tireless explorer of thePatagonian mountains, and he combined his work inSouth America with simultaneous studies in Antarcticaand New Zealand. His knowledge of the South Americanglaciations was unique for his times and his work wasprobably not appreciated as it deserved. Mercer broughtnew concepts and ideas to the problem of Patagonianglaciations since 1969 (Mercer, 1969, 1972) and he wasthe first to use modern techniques such as radiometrictechniques (K/Ar and 14C dating) and paleomagneticstudies in glacial sequences. He put forward many origi-nal ideas, most of them confirmed by later work, and his

papers are a source of new research lines even today(e.g. Mercer, 1972, 1976, 1983). Fleck et al. (1972) andMercer and Sutter (1981) studied many outcrops of gla-cial deposits interbedded with volcanic rocks, in whichradiometric and paleomagnetic dating techniques wereapplicable, also restudying Feruglio’s (1944) Cerro delFraile Locality (Fig. 1b, Site 28). It was Mercer (1976)who first chronologically established the existence ofPatagonian glaciations throughout the entire Quaternaryperiod, of frequent Pliocene glaciations and even of LateMiocene tills, also recognizing the correlation of theseglacial episodes with global cold periods. He proposed afour-glaciation model for the Chilean Lake District(Fig.1b, Site 13) and demonstrated the ancient age ofthe older glaciations (Mercer, 1976). In this work, hegave the name of ‘‘Llanquihue Glaciation’’ to the lastPleistocene glaciation [18O marine isotope stages (MIS)4–2], a term later extended by Clapperton (1993) for theentire South American continent, and coined the name‘‘Great Patagonian Glaciation’’ (GPG) for the oldest andoutermost morainic complex.

Steve Porter (1981) identified also four major glacia-tions in the Chilean Lake District (39�–41� S; Fig. 1b,Site 13) and defined their chronology throughout the Pleis-tocene, using radiometric dating and relative age techni-ques. Paul Ciesielski and colleagues (1982) were the first topresent a correlation model for the Patagonian glaciationswith the erosional and depositional history of the MauriceEwing Bank (55� S), Southwestern Atlantic Ocean east ofTierra del Fuego (Fig. 1a), based on Mercer’s (1976) chron-ostratigraphic scheme. In this model, the great antiquity ofthe Patagonian glacial events and their relations with globalpaleoclimatic episodes are confirmed. The pioneer work ofEdward Evenson and his colleagues from Lehigh Univer-sity and other American universities since the mid-1980sbrought for the first time a modern approach to the study ofnorthern Patagonian glaciations on the Argentinian side ofthe Andes, combining detailed field mapping, radiometricdating and paleomagnetic studies (Kodama et al., 1985,1986; Rabassa et al., 1986, 1990a; Schlieder, 1989; Rabassaand Evenson, 1996).

Rabassa and Clapperton (1990) presented the firstreview of the Patagonian glaciations and a general

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Fig. 7. John H. Mercer at an outcrop of the Llanquihuemoraine with wood fragments, near Puerto Varas,southern Chile. (Photo by J. Rabassa, 1973).



158 Jorge Rabassa

chronological correlation of all units known at that time.Recently, Morner and Sylwan (1989), Sylwan (1989),Meglioli (1992), Wenzens (1999a, 1999b, 2000), Wenzenset al. (1996), Schellmann (1998, 1999, 2003), Rabassa andCoronato (2002), Strelin et al. (1999), Malagnino (1995),Singer et al. (2004a, 2004b), and Sugden et al. (2005),among many others, have stressed the great antiquity andcomplexity of the Patagonian glacial sequence.

Chalmers Clapperton (1993) presented the first con-tinental summary of our knowledge of South Americanglaciations, including a complete description of those inPatagonia, for the first time showing great detail on eitherside of the Andes. His book is an outstanding compilationof all the available information at that time, with a globaloverview and correlation with other sectors of the SouthernHemisphere as well as with the Northern Hemisphere.Clapperton et al. (1995) expanded the investigations alsoin southern Patagonia, in the Magellan Straits area (Fig. 1a).

Working in Patagonia since 1995, Bradley Singer(University of Wisconsin) has introduced powerful toolsfor the study of Patagonian glaciations. Detailed mappingof extensive areas, profound volcanological research, thewide use of 40Ar/39Ar dating on lava flows stratigraphi-cally related with glacial deposits, careful paleomagneticstudies with Laurie Brown and, more recently, togetherwith Robert Ackert and Michael Kaplan, the use of cos-mogenic dating techniques on morainic boulders (Singeret al., 1998, 1999, 2004a, b; Kaplan et al., 2004)are significant contributions to the knowledge of LateCenozoic glaciations in southern South America.Ton-That et al. (1999) proposed for the first time tocorrelate the glacial sequences of Lago Buenos Aires andCerro del Fraile (Fig. 1b, Sites 24, 28) with the globalmarine isotopic sequence, as presented by Shackletonet al. (1990, 1995). A recent revision of the Patagonianglaciations has been presented by Coronato et al. (2004a,b), in which they indicated the development of the GPGaround 1 Ma, and evidence of (a) several pre-GPGcold periods, between 7 and 2 Ma, (b) three post-GPGsduring the Early and Middle Pleistocene, (c) the LastPleistocene glaciation and (d) two main episodes of glacialstabilization during the Late Glacial (15–10 14C ka BP). Atentative correlation of glacial events with loess depositionin the Pampas has been recently presented by Rabassa et al.(2005).

In the last years, the activity of large, multidisciplinaryresearch groups focusing on certain geographical regionshas provided excellent studies on the chronology of Pleis-tocene glaciations of the Chilean Lake District (Dentonet al., 1999a, b; Fig. 1b, Site 13) and the LGM andyounger, Late Glacial recessional events and ice read-vances in the Magellan Straits region (Sugden et al.,2005, and other papers in the same volume; Fig. 1a).These studies are clearly the model to be followed infuture studies of Late Cenozoic Patagonian glaciations.

4.2. The Late Tertiary Glaciations in Patagonia

A significant amount of evidence suggests that the Patago-nian Andes were already glaciated during Late Tertiarytimes. Based on the concentration of d18O and other isotopes

found in Late Miocene Santa Cruz Formation carbonateconcretions, Blisniuk et al. (2006) have suggested that thesouthern Patagonian Andes were uplifted >1 km, betweenca. 17 and 14 Ma, significantly enhancing aridity. Suchimportant uplift of the mountain belt would have brought alarge landmass above the regional snowline, adding to aglobal cooling trend, forcing the development of at leastlocal ice caps at the mountain summits or upon huge com-posite volcanic cones, which were rapidly growing at thattime, as part of the same tectonic event.

From a neotectonic point of view, the final pushalong the Liquine–Ofqui fault zone in southern Chile(41�–42�150 S; Fig. 1a), associated with the arrivaland subduction of the Chilean Rise beneath the TaitaoPeninsula (Fig. 1a), generated a major denudation eventimmediately before 5 Ma (Adriasola et al., 2005) andprobably the glacierization of the rising Andean summits.

Similarly, Thomson (2002) has applied fission trackthermochronology in the investigation of low-temperaturecooling and denudation history of the Patagonian Andesalong the southern part of the cited fault zone between 42�and 46� S. Enhanced cooling and denudation initiated inthe earlier part of the Late Miocene, between ca. 16 and10 Ma, but much faster rates of cooling and denudationtook place after ca. 7 Ma and up to 2 Ma, being coevalwith the collision of the Chile Rise with the Peru–Chiletrench between 47� and 48� S and also with the initiationof significant Patagonian glaciations. Thus, Thomson(2002) stated that glacial and periglacial erosion processeswould have been the main contributors to denudationalready since ca. 7 Ma.

A latest Miocene age for the first Patagonian glacia-tions is also supported by carbon isotopic data on toothenamel (Cerling et al., 1997). These authors suggest thata global decrease in atmospheric CO2 took place between8 and 6 Ma, enabling an expansion of C4 photosynthesisplants. This lowering of CO2 is compatible with globalglaciation, as it has been demonstrated for the LGM.Supporting data from Tierra del Fuego have been pub-lished by Cerling and Harris (1999).

Glaciations of the Latest Miocene–Early Pliocene

In the northern margin of the Meseta del Lago BuenosAires (47� S, Fig. 1a, Site 24), which is entirely coveredby volcanic rocks, till deposits over 30 m in thickness arefound interbedded with basalt flows (Mercer, 1976;Clapperton, 1993; Fig. 8). Mercer (1976) and Mercerand Sutter (1981) obtained whole-rock K/Ar ages on theunder- and overlying lavas of 7.34+ 0.11 to 6.75+ 0.08and 5.05+ 0.07 to 4.43+ 0.09 Ma, respectively, whichmost likely assigns a Latest Miocene age for these glacialdeposits (Busteros and Lapido, 1983; Ardolino et al.,1999). This allows these deposits as belonging to someof the oldest Late Cenozoic glacial events in Patagonia,and indicates that the Patagonian Andes in those timeswere bearing at least isolated ice caps with outlet glaciersthat were clearly extending more than 30 km eastof the mountain front. In the same locality, Ton-Thatet al. (1999) obtained 40Ar/39Ar (incremental-heatingtechnique) ages of 7.38+ 0.05 Ma for the underlying

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flow and of 5.04+ 0.04 Ma for the overlying flow,confirming in general terms the probable Latest Miocene(or, at most, Miocene–Pliocene boundary) age of this firstpreserved Patagonian glaciation.

Four basalt flows dated by 40Ar/39Ar (incremental-heating) techniques between 10 and 6.7 Ma have overlyingtills and three other ones with underlying tills have beendated between 4.9 and 4.3 Ma at the Lago Buenos Airesregion. Additionally, no till was found below the MesetaGuanabara Basalt, Lago Buenos Aires (Fig. 1b, Site 24),dated at 9.87 Ma (Ton-That et al., 1999AU5 ). Though theabsence of evidence should never be considered as the

evidence of absence, the Meseta Guanabara is located inan area that should have been glaciated if the glaciers hadextended away from the Patagonian Andes before that age.No precise, absolute ages may be yet assigned to those tillsoverlying the basalts, but their comparison with the globalpaleomagnetic sequence indicates that they could corre-spond to the C3 (a and b) Chron. During this period, theoceanic sequences (Opdyke, 1995; Shackleton, 1995)locate the strongest thermal lowering between 5.7 and5.9 Ma, a period which is comprised between the limitingages of these tills. This correlation allows to suggest that atleast a major extra-Andean glaciation could have takenplace in southern Patagonia between isotopic stages TG 20and TG 22, during the Gilbert Chron.

Schlieder (1989) had already recognized very coarsediamictons along the Rıo Alumine valley, northern Patago-nia (Fig. 1b, Site 8), and he assigned them to Late Mioceneglacial events, based on whole-rock K/Ar ages of the limit-ing basalts. He additionally proposed that the Alicura For-mation, originally assigned to the Lower Quaternary byDessanti (1972) and later, as the Alicura Member of theCaleufu Formation, to the Miocene–Pliocene (GonzalezDıaz et al., 1986), actually corresponds to the Late Miocene,its upper age limited by overlying basalts dated at6.41+ 0.13 and 5.26+ 0.14 Ma, respectively. In this inter-pretation, the Alicura Formation would be the distal glacio-fluvial unit of the Latest Miocene Patagonian Andeanglaciations, whose water and sedimentary discharge wouldhave been concentrated by the Rıo Alumine and the RıoCollon Cura (Fig. 1a, Site 7), both tributaries of the paleo-Rıo Limay, a main regional stream of the Atlantic slopealready in those times (Rabassa, 1975; Fig. 1b, Site 3). Theinterpretation of a glaciofluvial origin for the Alicura For-mation related to ancient glaciations was already proposedby Gracia (1958), though no absolute ages were then defined(in Gonzalez Dıaz and Nullo, 1980).

Recently, Wenzens (2006a) has indicated the existenceof Late Miocene glacial deposits around Lago Cardiel, anarea that had been considered unglaciated by all previousresearchers (Fig. 1b, Site 44), with ages as old as 10.5 Ma,and nine glacial advances between 10.5 and 5.4 Ma. Theseunits would correspond to ice advances of the Lago SanMartın lobe of the Patagonian Ice Cap (Fig. 1b, Site 45) oreven by local glaciers at Mount San Lorenzo (Fig. 1b, Site32). As Wenzens (2006a) has stated, these glacier expan-sions would have been up to three times larger than theirPleistocene counterparts. This is quite difficult to explain,since the larger extent would require very cold and wetter(at the ice divide) environmental conditions as well aslonger glaciation periods. This assumption does not agreewith the oceanic record, which shows that the Late Miocenecold events are shorter and much less intense than theQuaternary glacial periods (Kenneth, 1995; Rabassa et al.,2005). The existence of these very early, extensive LateMiocene glaciations is extremely interesting, but intriguingand further studies are needed to confirm these interpreta-tions and explain this apparently anomalous behavior.

Some of these late Tertiary glacigenic deposits and land-forms have been identified well beyond the outermostboundary of the most extended Pleistocene glaciation. Con-sidering that the Patagonian Ice Sheet is assumed to haveformed only in the Early Pleistocene, when the astronomical

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Fig. 8. Latest Miocene–earliest Pliocene till at MesetaLago Buenos Aires (Fig. 1b, Site 24), Santa CruzProvince, Argentina (Mercer, 1976; Ton-That et al., 1999).(a) Location of till in between lava flows; (b, c) till andstriated glacial boulder (Photos by Bradley Singer, 1996).



160 Jorge Rabassa

forcing cycles were dominated by the eccentricity period of100 ka (Rabassa et al., 2005), it is difficult to understandwhy the ice margin reached such eastern position. It hasbeen suggested that the ice expanded over a very flat originalsurface, with almost no incised drainage, which corre-sponded to the Late Miocene sedimentary accumulationplains. Thus, the glaciers would have extended as verylow-gradient, wide ice fans over an almost reliefless surface,probably eastward-sloping, latest Miocene pediments.

Glaciations of the Middle Pliocene

Evidence of Middle–Late Pliocene glaciations can befound in southern Patagonia as well. In the Lago Viedmaregion (Fig. 1b, Site 25), glacigenic deposits interbeddedwith basalt flows have been identified at Meseta Chica andMeseta Desocupada (49� S; Mercer et al., 1975; Mercer,1976). At Rıo Cangrejo valley, Meseta Chica, a till bed isfound between two flows K/Ar dated at 3.55+ 0.19 and3.68+ 0.03 Ma, respectively, and another till unit is over-lying a lava flow dated at 3.46+ 0.22 Ma. At MesetaDesocupada, a till layer occurs in between lava flowsdated at 3.48+ 0.09 and 3.55+ 0.07 Ma.

Wenzens (2000) obtained limiting ages of 3.0 and2.25 Ma for glacigenic deposits north and east of LagoViedma. Sylwan (1989) indicated the presence of till atLago Buenos Aires corresponding to MIS 88, during theGilbert Geomagnetic Epoch, which is coincident with thelimiting ages proposed by Wenzens (2000) and those ofthe basalt flows which underlie till at Cerro Fortaleza,Lago Argentino (Schellmann, 1998, 1999; Fig. 1b, Site 26).

Mercer (1976) obtained an age of 2.79+ 0.15 Ma for alava flow that buries till at Condor Cliff, Rıo Santa Cruzvalley (50� S; Fig. 1b, Site 41). Younger glacigenic depositsappear over these flows, whereas the materials correspond-ing to the GPG are located at the base of these ‘‘mesetas’’ ortablelands. This clearly shows that even as early as theMiddle Pliocene, in some regions the Patagonian glaciersexpanded from the ice caps as far as or close to the extent ofthe outlet glaciers of the maximum Pleistocene expansion(GPG). However, these conditions are probably exclusivefor southernmost Patagonia, since there is yet no conclusiveevidence for a similar extension of the ice cap in NorthernPatagonia, with the exception of Schlieder’s (1989) obser-vations in the Alumine valley (Fig. 1b, Site 8).

However, at Monte Tronador (41� S; Fig. 1b, Site 12),volcanics, lahars and pyroclastic flows of the TronadorFormation (long ago K/Ar dated at 3.2 and 2.0 Ma, thoughother much younger ages were obtained as well; Greco,1975; Gonzalez Dıaz and Nullo, 1980, p. 1131) appearin-filling deep valleys, possibly of glacial origin, carryingstriated and faceted, volcanic boulders and cobble-sizedclasts (Rabassa et al., 1986). These units should be redatedwith more modern techniques, but it is primarily acceptablethat this part of the northern Patagonian Andes was alreadycovered by at least local ice during the Middle Pliocene.

The relative chronology of tills and basalt flows hasbeen compared with the global climatic variabilityobtained from the oceanic isotopic sequences (Rabassaet al., 2005). This analysis indicates that several coldclimatic events and their consequent glacier advances

took place between the Middle and Late Pliocene inthe Buenos Aires, Viedma and Argentino lake regions(Fig. 1b, Sites 24–26). The first event would have takenplace around 3.5 Ma, during MIS MG6, Gauss normalpolarity, the second one, during the MIS 100, 96, 92 and88, Matuyama reversed polarity. Tills are found in over-and underlying positions of the lava flows dated at3.20 Ma (Lago Argentino) and 3.45 Ma (Lago Viedma;Mercer, 1976), respectively, and they are enclosing coldpeaks found at MIS KM4, KM6, M2 and MG2.

Glaciations of the Late Pliocene and EarliestPleistocene

Feruglio (1944) described the glacigenic sequences atCerro del Fraile (50�330 S, Fig. 1b, Site 28), interbeddedbetween volcanic flows, and considered them as of Plio-cene age. These flows were K/Ar dated by Fleck et al.(1972), Mercer et al. (1975) and Mercer (1976) between2.08 and 1.03 Ma, during the Matuyama Chron. Mercer(1976) identified six piedmont glaciations during thisperiod. Recent studies by Rabassa et al. (1996), Guillouand Singer (1997), Singer et al. (1999, 2004b) and Ton-That et al. (1999) have allowed to redate this sequence by40Ar/39Ar incremental-heating techniques and to providea precision magnetostratigraphy (Figs 9 and 10). In these

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Fig. 9. Cerro del Fraile, Santa Cruz Province, Argentina(Fig. 1b, Site 28). (a) Sequence of interbedded till andlava flows (Photo by Bradley Singer, 1996); (b) striatedglacial boulder in till (Photo by J. Rabassa AU6, 1996).




studies, a minimum of seven glaciations have been recog-nized, and probably a glaciofluvial deposit at the base ofthe profile, all of which would have taken place between2.16 and 1.43 Ma. These glaciations would have devel-oped during MIS 82 to 48 (Matuyama Chron; Ton-That,1997; Ton-That et al., 1999). Finally, a younger glacia-tion covered the uppermost lava flow, dated at 1.08 Ma,thus probably equivalent to the GPG (MIS 30–34).

Strelin (1995) and Strelin et al. (1999) described mor-aines beyond the position of the GPG (Mercer, 1976) alongthe Rıo Santa Cruz valley, overlying the Condor Cliffbasalts (2.66 Ma; Mercer et al., 1975; Fig. 1b, Sites 23,41), which he considered to be possibly correlated with theglacial units at Cerro del Fraile. These moraines are olderthan a basalt flow dated by 40Ar/39Ar at 0.675+ 0.56 Ma.No information about the applied technique is given, norabout the meaning of the very large statistical error of thisdate; see, for example, Ton-That et al. (1999) and Singeret al. (2004a). Likewise, they suggested that the unitsknown as Chipanque Moraines in Lago Buenos Aires byMalagnino (1995) could be correlated with the Santa Cruzvalley units. However, Malagnino (1995, p. 80) suggestedinstead that the Chipanque Moraines could be olderthan 2.3 Ma and younger than 3.5 Ma, followingMercer’s (1976) chronology. Thus, in this interpretation,

the Chipanque Moraines would be older than even thebasal glacigenic unit at Cerro del Fraile.

The Origin of the Earliest Patagonian Glaciations

It is very important to consider that the definitive glacier-ization of Western Antarctica took place in the EarlyMiocene. The glacierization of eastern Antarctica hadstarted in the Early Tertiary, when this continent achievedits present polar position (Kennett, 1995), but the glacier-ization of Western Antarctica and the Antarctic Peninsuladid not occur until the Drake Passage opened (Fig. 1a).

The Drake Passage is the consequence of the dis-membering of both continents due to the continuouseastward movement of the Scotia plate since the EarlyTertiary. This movement generated the huge bend ofthe Fuegian Andean axis from a N–S to an E–W posi-tion, the displacement of the southern Georgias Archi-pelago away from the South American continent andthe formation of a volcanic, oceanic insular arc at thesouthern Sandwich Islands, where the Scotia platesubducts under the Atlantic oceanic plates. The envir-onmental consequence of this new geographic config-uration was the installation of the AntarcticCircumpolar Current in the Early Miocene, perhapsca. 23 Ma (Mercer, 1983). This current isolated theAntarctic Peninsula from the temperate oceanic cur-rents coming from lower latitudes and contributed tothe lowering of the Antarctic oceanic water tempera-tures. This new environmental scenario allowed therapid and definitive cooling of the polar and subpolarair masses, generating the glacierization of the Antarc-tic Peninsula (Ciesielski et al., 1982) and, subse-quently, of the Fuegian and Patagonian Andes.

In addition to the astronomical forcing (Shackleton,1995), other causes of climatic deterioration and subsequentoccurrence of Patagonian mountain glaciations should alsobe considered. The tectonic processes that slowly elevatedthe Patagonian Cordillera and originated the Scotia Arc(Ramos, 1999a and b) should not be moved aside in thisanalysis. The Patagonian Andes would have started itselevation process, at least partially, in the Late Oligoceneor the Early Miocene (Gonzalez Bonorino, 1973; Rabassa,1975). The great pyroclastic eruptions that produced thetuffs and ignimbrites of the Collon Cura Formationin northern Patagonia (ca. 15 Ma; Rabassa, 1975) are indi-cators of such tectonic processes. An incremental-heating40Ar/39Ar dating on ignimbritic pumice overlying thePilcaniyeu Ignimbritic Member of the Collon Cura Forma-tion (Rabassa, 1975) has provided an age of 10.85+0.033 Ma (B. Singer, personal communication; Rabassaet al., 2005). This date may be interpreted as the age of thelast pyroclastic episodes of the Miocene cycle, which wouldbe representing the final emplacement of the PatagonianAndes at elevations comparable to its present position.

The summit accordance line of the northern Patago-nian Andes is located today around 2200 m a.s.l., whereasthe regional, permanent snowline is placed at around2000 m a.s.l., allowing the persistence of many smallcirque glaciers and snow fields, even during the presentInterglacial (Rabassa et al., 1980). It may be assumed

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Fleck et al. (1972)Elevation1200 m

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This studyFlow Age (Ma) Polarity Flow Age (Ma) Polarity

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1.61 – 1.43(10)

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162 Jorge Rabassa

that the regional snowline would have descended signifi-catively during all Late Cenozoic cold episodes at leastsince the Late Miocene, thus favouring the formation oflarger mountain glaciers, and even perhaps, extendingbeyond the mountain piedmont.

4.3. Quaternary Glaciations in Patagonia

During the Early Pleistocene, the Patagonian Ice Sheetwas fully developed, probably for the first time in the LateCenozoic, when the orbital eccentricity forcing signalbecame dominant (Fig. 11). The lower time boundary ofthe Quaternary used in this chapter is the top of the Old-uvai normal polarity event of the reversed Matuyama

Chron, that is, ca. 1.8 Ma. Glacial climatic episodesbecame then long enough to allow the formation of asingle, continuous mountain ice sheet that extended foralmost 2500 km, at least between 36� and 56� S, thatcovered almost completely the Patagonian Andean rangesand extended over the piedmont areas to the east (and tothe present submarine platform south of the Rıo Gallegos;Fig. 1b, Site 27) and to sea level in the Pacific side.

Glaciations of the Early Pleistocene

At the base of Monte Tronador (41� S, Fig. 1b, Site 12),northern Patagonia, Rabassa et al. (1986) and Rabassaand Clapperton (1990) identified glacigenic deposits

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Fig. 11. Map of Patagonia with theposition and distribution of thePleistocene Patagonian Ice Sheet andrelevant tectonic features. From Singeret al., 2004a.




interbedded with volcanic flows. These rocks were origin-ally K/Ar dated at 1.36 and 1.32 Ma, assigning them anEarly Pleistocene age, previous to the GPG. However, thevolcanic flow overlying both the Garganta del Diablo tilliteand glacial surfaces eroded on the Cretaceous granites hasbeen redated by 40Ar/39Ar incremental-heating techniquesby B. Singer (personal communication; sample TR-01;Rabassa et al., 2005; Fig. 12) at 1.021+ 0.102 Ma. There-fore, at least part of these glacigenic deposits could be muchyounger and even equivalent to the GPG (Mercer, 1976).

The GPG represents the maximum expansion of theice in extra-Andean Patagonia. Its geographical distribu-tion was correctly mapped by Caldenius (1932) and cor-responds to his ‘‘Initioglacial’’ event, but considered byhim as an initial phase of the last Pleistocene glaciation,as stated above. The morainic arcs pertaining to the GPGare well preserved, though somewhat lesser than the latersequences. In northern Patagonia, the GPG correspondsto the ‘‘Pichileufuense’’ (Feruglio, 1950) or PichileufuDrift (Flint and Fidalgo, 1964, 1969), or at least to itsoutermost expansion. Most likely, the GPG representsmore than one glacial advance and in the type area ofthis glacigenic unit, the Rıo Pichileufu valley east of SanCarlos de Bariloche (41� S; Fig. 1b, Site 16), at leastthree clearly defined morainic arcs have been observed.Flint and Fidalgo (1964) had considered this drift unit ascorresponding to an earlier phase of the LG, followingCaldenius’ model but ignoring Feruglio’s (1950) pioneercorrelations, and only later (Flint and Fidalgo, 1969) theyaccepted the possibility that it could correspond to anearlier glaciation. Much later, Kodama et al. (1985,1986) and Rabassa et al. (1986, 1990a) defined a pre-LG age for these deposits, and most likely, an Early–Middle Pleistocene age, based on 40Ar/39Ar (whole rock)dating and paleomagnetic studies. Rabassa and Evenson(1996) suggested that the Pichileufu Drift could be com-posed of at least the deposits of three different iceadvances which may correspond to one or, perhaps,

several glaciations, all of which preceded a fluvial can-yon cutting event during the Early Pleistocene (Rabassaand Clapperton, 1990).

South of San Carlos de Bariloche, the region of Esquel islocated (Fig. 1b, Site 22). This area was studied by Flint andFidalgo (1969), where they extended their threefold glacialmodel from the Nahuel Huapi area. Caldenius (1932)described a four moraine sequence, plus several ‘‘post-Finiglacial’’ (e.g. the age equivalent to the ‘‘Younger Dryas’’(YD) moraines of Scandinavia) units inside the mountains.Miro (1967), Gonzalez Dıaz (1993a, b) and Gonzalez Dıazand Andrada de Palomera (1995) basically followedCaldenius’ classical four-moraine system. The longitudinalvalley of El Maiten (42�–42�300 S; Fig. 1b, Site 20) isconsidered as of pre-Andean age (Martınez, 2002). Thisvalley had been glaciated in several episodes by two majorice lobes, the Epuyen and Cholila valley lobes (Fig. 1b, Site19). Smaller transversal valleys, crossing the El Maitendepressions, were occupied by the ice during ‘‘Initioglacial’’times, reaching its maximum extent at ca. 70�400W. AtPortezuelo de Apichig (Fig. 1b, Site 22), all cited authorshave identified two or more morainic ridges, with abundanterratic boulders and faceted and striated cobbles, assignedto the GPG. Further south, Gonzalez Dıaz (1993b) mappeda well-preserved moraine belt at Arroyo Pichico, at1090 m a.s.l. The same author has also identified anothermoraine belt of the same age at Canadon Blancura, 20 kmfarther to the SE. At Portezuelo de Leleque (71�430W;Fig. 1b, Site 22), Gonzalez Dıaz (1993b) described twomorainic arcs of ‘‘Initioglacial’’ age, at 700–800 m a.s.l. It isvery important to mention that Gonzalez Dıaz (1993a, b) andGonzalez Dıaz and Andrada de Palomera (1995) haveproposed a glacifluvial origin for the Blancura Formation(previously considered as of piedmont origin by Volkheimer,1963), one of the most important units of the Patagoniangravels in northern Patagonia. Feruglio (1950) advanced asimilar opinion half a century before.

In the Chilean Lake District, at Lago Llanquihue andneighboring basins (Fig. 1b, Site 13), Mercer (1976)described and mapped three drift units older than theLlanquihue Drift or LG, which he named Rıo Frıo, Cole-gual and Casma drifts. The intensely weathered nature ofthe Rıo Frıo Drift would suggest a GPG age for this unit(Mercer, 1976; Clapperton, 1993). Porter (1981) mappedthe drift sequence in the same area, suggesting a newstratigraphy, composed of the Caracol, Rıo Llico, SantaMarıa and Llanquihue drifts. The drift units were identi-fied in terms of their mappable features, including weath-ering rate, pedogenetic characteristics and landformpreservation. Caracol, the oldest Drift, occurs along thebottom of the central valley and also in certain localitiesalong the eastern slopes of the Cordillera de la Costa(Clapperton, 1993; Fig. 1a). However, this glaciationwas probably less extensive than the following advanceof the ice, indicated by the Rıo Llico Drift. The CaracolDrift is fully weathered and its age is most likely corre-sponding to the GPG. This unit is not exposed at IslaChiloe (Fig. 1a) and probably covered by the youngerdrifts (Clapperton, 1993).

A precise, reliable correlation between the EarlyPleistocene glacial events on both sides of the Andes atthis latitude is still lacking, mostly due to the problems of

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Fig. 12. Glacial boulder within a tillite, interbedded inEarly Pleistocene lava flows at Garganta del Diablo,Monte Tronador, northern Patagonia, Argentina(Fig. 1b, Site 12). (Photo by J. Rabassa, 1988).



164 Jorge Rabassa

accurate dating of these units. However, a glaciationmodel of four major events, the GPG and three youngerepisodes, the youngest being the LG of Late Pleistoceneage, seems to be sustainable.

Within the Lago Buenos Aires Basin (46�300 S; Fig. 1b,Site 24), at least 19 terminal moraines, all of them ofPleistocene age, have been described by Morner and Sylwan(1989), Ton-That et al. (1999), Singer et al. (2004a) andKaplan et al. (2004, 2005). These units were deposited bypiedmont glaciers advancing eastward from the Patagonianice cap during the last 1.2 Myr. 40Ar/39Ar incremental-heating and unspiked K/Ar experiments (Guillou andSinger, 1997; Singer et al., 2004a) on four basaltic lavaflows interbedded with the moraines provide a chronologicframework for the entire glacial sequence. The 40Ar/39Arisochron ages of three lavas that overlie till 90 km east ofLago Buenos Aires strongly suggest that the ice cap reachedits greatest eastward extent ca. 1.1 Ma, during the GPG.At least six moraines were deposited within the 256 kyrperiod bracketed by basaltic eruptions at 1016+ 10 and760+ 14 ka (Singer et al., 2004a; Fig. 13). Six otheryounger, more proximal moraines were deposited during a651 kyr period bracketed by 760+ 14 and 109+ 3 kabasalt flows.

Recently, Douglass and Bockheim (2006) have stu-died the relationships between the glacial landforms,particularly moraine belts, of the Lago Buenos Airesregion with the soils developed on them. These authorsused distinct parameters such as accumulation rates oforganic matter, pedogenetic carbonate and clay, to show

that they decreased with decreasing age of the moraines.A lack of changes in soil redness, and preservation ofminerals that should have been weathered in the oldestsoils indicates that chemical weathering is almost absentin these environments. According to Douglass andBockheim (2006), measured dust input explained theaccumulation of both clay and carbonate, and a carbonatecycling model describing potential sources and calciummobility in Patagonia has been presented. These authorsstated that calibration of rates of soil formation wouldprovide a powerful correlation tool for soils developed ondifferent Patagonian glacial deposits.

A complementary point of view has been presentedby Gaiero et al. (2004), who stated that fluvial- and wind-borne materials transferred from Patagonia to the SWAtlantic show a very homogeneous rare earth element(REE) signature. The REE composition is compatiblewith recent tephra from Volcan Hudson (46� S; Fig. 1b,Site 35). This would imply a dominance of materialsupplied by this source and other similar Andeanvolcanoes. Due to the trapping effect of drainage basins,Patagonian streams deliver to the ocean a suspended loadwith a slightly modified Andean signature, showing anREE composition depleted in heavy REEs. These authorsconsidered Patagonia a sedimentary source distinguish-able from other sources in southern South America.Quaternary sediments deposited in the Scotia Sea, andmost dust in ice cores of east Antarctica would have REEcompositions very similar to Buenos Aires Province loessand to Patagonian eolian dust. The REE compositions of

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Fig. 13. Map of the Pleistocene glaciations at Lago Buenos Aires, Santa Cruz Province, Argentina (Fig. 1b, Site 24).From Singer et al., 2004a.




most sediment cores of the Scotia Sea and Antarcticawould reflect a distal transport of dust with an admixedcomposition from two main sources: a major contributionfrom Patagonia, and a minor proportion from sourceareas containing sediments with a clear upper crustalsignature (e.g. western Argentina) or from the BolivianAltiplano. However, the evidence presented by theseauthors indicates that Patagonian materials were theundisputable predominant sediment source to the south-ern latitudes during the LGM only.

During the GPG, the ice tongues reached the Atlanticcoast in the area north of the Magellan Straits and southof the Rıo Gallegos valley (Fig. 1b, Site 27) for the firsttime in the Cenozoic, and expanded deeply over thepresent submarine platform. It is not clear whether theice margin was effectively calving into the AtlanticOcean, perhaps as far as 200 km east of the presentcoast. The expansion of the ice over the present submar-ine platform was clearly mapped already by Caldenius(1932), as shown by his glacial map of Tierra del Fuegoand the Magellan Straits (Fig. 14).

Mercer (1976) estimated the age of the GPG, basedon K/Ar dating of lava flows underlying glacigenicdeposits in different localities south of the Rıo Gallegosvalley, between 1.47+ 0.1 and 1.17+ 0.05 Ma.Meglioli (1992) obtained total fusion, whole-rock40Ar/39Ar ages of 1.55+ 0.03 Ma at the Bella VistaBasalt, Rıo Gallegos valley (Fig. 1b, Site 27), which iscovered by glacial erratics, thus providing a basal limit-ing age for the GPG. Ton-That et al. (1999) and Singeret al. (2004a) redated the Bella Vista Basalt by incremental-heating 40Ar/39Ar techniques at 1.168+ 0.007 Ma, con-sidering that the observed discrepancy with Meglioli’s

(1992) date is given by the higher precision of the lattertechnique. Likewise, Ton-That et al. (1999) and Singeret al. (2004a) provided for the first time a reliable upperlimit for the GPG by means of the incremental-heating40Ar/39Ar date of 1.016+ 0.005 Ma for the TelkenBasalt, which covers the ‘‘Initioglacial’’ ( = GPG) depos-its at Lago Buenos Aires (Fig. 1b, Site 24).

According to the morphological and chronostrati-graphic evidence of the till deposits, the Fuegian Andes

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Fig. 14. Glacial map of Tierra del Fuego and theMagellan Straits (Caldenius, 1932).

Fig. 15. Glacial map of the Magellan Straits, by Meglioli (1992). This is a portion of the still unpublished map of hisrenowned dissertation, showing the distribution of the different Pleistocene moraines. The outermost morainecorresponds to the GPG (Bella Vista Drift), the two following ones to the latest Early Pleistocene and earliest MiddlePleistocene (Cabo Vırgenes and Punta Delgada drifts, respectively), and the two innermost, to the Late MiddlePleistocene (Primera Angostura Drift) and the Late Pleistocene (Segunda Angostura Drift).



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ice sheet would have had a different extent and thicknessin each of the known glacier advances, reaching thepresent submarine platform and the Fuegian lowlands tothe north and the Drake Passage waters to the south atseveral occasions, receding to the summits and highlandsduring the interglacial periods.

The absolute number of glacier advances that tookplace in southernmost South America is still a matter ofdebate. North of the Magellan Straits at least six majorglacier advances have been described on the basis ofterminal moraines (Meglioli, 1992; Fig. 15), whereas inTierra del Fuego the morphological evidence points to twoice advances in the southern part and five in the north.

Enigmatic, isolated boulders, as well as small till rem-nants, have been found in the Rıo Grande city area and theRıo Chico valley (Meglioli, 1992; Coronato et al., 2004b;Figs 1c and 16) at various elevations along the largetriangular zone between the Bahıa Inutil–San Sebastianand Fagnano ice lobes and the ice margins along thehigh mountains of western Tierra del Fuego (Fig. 1c).This area had been considered unglaciated by Nordensk-jold (1899) but implicitly totally covered by ice at the‘‘Initioglacial’’ stage by Caldenius (1932). These boulders(of undoubtedly glacial origin, based on their size andshape) and the surviving till patches were named the RıoGrande Drift by Meglioli (1992), who estimated its agebetween 2.05 and 1.86 Ma, by stratigraphic and geomor-phological correlation with drifts and radiometric dated

basalts in southern Patagonia. Therefore, Meglioli (1992)interpreted these glacigenic remnants as older than theGPG, of latest Pliocene or earliest Pleistocene age. Thoughthe issue of full glaciation of the island is still open, theseglacigenic remnants are strong evidence in that sense.

Caldenius (1932) recognized the extension of his afore-mentioned four glacial events north of the Magellan Straitsbut only three on the southern coast. He located the easternlimit of the two oldest glaciations beyond the coast, ontothe present Atlantic submarine platform. According to hisinterpretation, both of these glaciations covered the entireisland. His field mapping was extremely detailed, and quitecorrect most of the times, in spite of the serious difficultiesin doing fieldwork, which in some cases even preventedhim of reaching some areas. The Quaternary glaciations ofTierra del Fuego were very extensive. Large outlet glaciersof the Darwin Cordillera (2000 m a.s.l.; 55� S–69� W;Figs 1c and 17) ice cap flowed north and eastward toreach the present Atlantic submarine platform (Porter1989; Meglioli et al., 1990; Isla and Schnack, 1995) fol-lowing large, deep valleys known today as the MagellanStraits, Bahıa Inutil–Bahıa San Sebastian Depression, LakeFagnano, Carbajal–Tierra Mayor valley and the BeagleChannel (Fig. 1c). Several glaciations have been recog-nized in the northern part of the island (Meglioli et al.,1990; Meglioli, 1992) and at least two along the BeagleChannel (Rabassa et al., 1992, 2000).

Meglioli (1992; see also Coronato et al., 2004a) mappedin great detail a very large area (over 25,000 km2) ofthe Magellan Straits and surrounding areas. He identi-fied five or six large glacial events that he gave localnames for each major valley, and which he correlatedwith the GPG and subsequent glaciations. The GPG wasnamed the Bella Vista Drift in the Rıo Gallegos valley andSierra de los Frailes Drift in the straits area and northernTierra del Fuego.

A drumlin and megaflute field of GPG or Bella Vistaglaciation (Meglioli, 1992) age has been recentlydescribed by Ercolano et al. (2004), in the Rıo Gallegosvalley of southernmost Patagonia (52� S; Fig. 1b, Site 27;

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Fig. 16. Large glacial erratic boulder, originated in theDarwin Cordillera (Fig. 1), and found today isolated ontop of Tertiary marine sediments, together with smallremnants of till. Estancia Marıa Behety, 20 km west ofRıo Grande, Tierra del Fuego, Argentina. This bouldercorresponds to the elusive ‘‘Rıo Grande Drift’’, asdefined by Meglioli (1992). (Photo by J. Rabassa, 2004).

Fig. 17. Landsat image (1996) of Cordillera Darwin,western Tierra del Fuego, Chile (Fig. 1), showing theFuegian Ice Sheet, and large discharge glaciers flowingin all directions, including the Beagle Channel and theMagellan Straits.




Fig. 18). Several tens of streamlined landforms, some ofthem several kilometers long that clearly appear even insatellite images, have been identified in a 30 km longsection. Uncovered since perhaps 1 Ma, these landformsare beautifully preserved, thanks to the very dry climateand the lack of surface runoff during most of the Quatern-ary, showing only the incision of meltwater channelrelated to the ice recession during the GPG. Likely, thesefeatures are some of the oldest, well-preserved glacial land-forms in the world, outside Antarctica.

The Magellan Strait lobe was the largest and mostimpressive glacier that covered this region. It originatedin the Darwin Cordillera and flowed to the north and easttoward the Atlantic Ocean. Several discharge tonguescame out in several directions.

The highest (and oldest) drift in the Magellan Straitlobe, in northern Tierra del Fuego (Sierra de los FrailesGlaciation; Meglioli, 1992), extends over the high plains(100 m a.s.l.). This is a wide flat surface, with poor fluvialdrainage, but which underwent an intense deflation.Although the superficial morphology does not showclear glacial landforms in this area, the till, as seenalong the marine cliffs, forms the sedimentary core ofthe high plains. On this surface, large volcanic clastsshow a similar weathering degree to those of the corre-sponding till unit in the northern coast of the Straits. Thisdrift unit occurs as remnants between the MagellanStraits and Bahıa Inutil–Bahıa San Sebastian Depressionlobes, probably representing a piedmont-type glaciationthat would have covered the southern end of the continentand a large portion of Isla Grande de Tierra del Fuego.

The Bahıa Inutil–Bahıa San Sebastian Depressionlobe, emerging from the main body of the Magellan StraitsGlacier and the northern slope of the Darwin Cordillera,reached the Atlantic Ocean Platform and the inner portionsof Isla Grande de Tierra del Fuego at various occasions(Fig. 1c). The oldest tills of this lobe are exposed on top ofthe flat and high surfaces that form the Pampa de Beta.There are no volcanic flows in this area to be dated.However, the Pampa de Beta Drift is thought to corre-spond to the GPG, by correlation with the Magellan Straits

and Rıo Gallegos valley lobes. Its type locality is found inthe marine cliffs between Rıo Cullen and Cabo EspırituSanto, along the Atlantic coast, where it can be observedthat the till forms the high plains and mesetas.

The GPG would thus have developed sometimebetween 1.168 and 1.016 Ma, during MIS 30–34, andeven maybe MIS 36, most likely including more thanone glacial advance.

Glaciations of the Latest Early–Middle Pleistocene

After the GPG, the deposits corresponding to the followingPatagonian glaciations (‘‘Daniglacial’’ and ‘‘Gotiglacial’’,according to Caldenius, 1932; or post-GPG 1, 2 and 3, inthe sense of Coronato et al., 2004a and b) are located atlower elevations in the landscape, and sometimes nestedinside the GPG limits but very far from them.

This circumstance is different to what may be seen inthe Northern Hemisphere Scandinavian and Laurentianice sheets, where the younger ice expansions in mostcases reached the outer positions of the older glaciationsand even extended beyond them. These conditions couldbe due to (1) a smaller intensity of the Southern Hemi-sphere cold episodes after the MIS 30–34 or (2) localphenomena. Concerning the first hypothesis, the South-ern Hemisphere oceanic isotopic sequences do not showsignificant deviations from their equivalents of the North-ern Hemisphere and they suggest similar intensities andchronology. Therefore, the circumstances may be inves-tigated through phenomena of local nature. The evidencesuggests that episodes of valley deepening took placeover most of the Pleistocene, particularly the Middleand Late Pleistocene. The most important would havetaken place immediately after the GPG, forcing the laterglaciations to develop a morphology of discharge glaciersentrenched in their valleys, whereas the dominant glaciermorphology during the GPG would have been of largepiedmont lobes, of great extension but relative reducedthickness. This characteristic would have been favoredby the preexisting landscape, with little incision of thepiedmont valleys. This event has been named the ‘‘can-yon-cutting event’’ by Rabassa and Clapperton (1990)and Rabassa and Evenson (1996), in comparison withsimilar episodes that occurred in the Rocky Mountains.This valley deepening event may have been caused by(1) increased erosion related to a larger discharge duringthe interglacial periods, (2) increased erosion related totectonic ascent of the Patagonian Andes, (3) differentglacier behavior, with large areas of cold ice on thedivides separated by temperate ice in the valleys, or(4) a combination of some or all of these processes. Themuch larger magnitude of the deepening between theGPG deposits and the later events, compared to thatexisting in between the latter, suggests that alternative(2) would probably be correct. The cited tectonicuplift would have taken place perhaps between 1.0 andca. 0.8–0.7 Ma, since in the next post-GPG the glacierswere already entrenched (Ton-That et al., 1999). Thisevent would have possibly contributed even more inten-sively to the development of ‘‘rain-shadow’’ conditions in

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Fig. 18. Drumlins and megaflutes of the Bella Vista Drift,Early Pleistocene (Meglioli, 1992; Ercolano et al., 2004), inthe Rıo Gallegos valley, Santa Cruz Province, Argentina(Fig. 1b, Site 27). The central megaflute is 2.5 km long.Oblique aerial photograph on a stormy day by BettinaErcolano and Elizabeth Mazzoni, 2002.



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extra-Andean Patagonia, but its effective influence in theextra-glaciated Pampean Region is still unknown.

The glacial event immediately after the GPG isknown as ‘‘Daniglacial’’, according to Caldenius (1932) or‘‘post-GPG 1’’, in the sense of Coronato et al. (2004a, b).This unit is characterized by conspicuous and well-preserved morainic arcs that are located in inner positionsrespective to the GPG and entrenched in the valleysyounger than this glaciation. In northern Patagonia, thisunit was complexively named as ‘‘El Condor Drift’’ inthe San Carlos de Bariloche type area (Fig. 1b, Site 9) byFlint and Fidalgo (1964, 1969), including also the ‘‘Goti-glacial’’, but considering it of Late Pleistocene age. Later,Schlieder (1989), Rabassa et al. (1990a) and Rabassa andEvenson (1996) proposed the subdivision of the‘‘El Condor Drift’’ in two allostratigraphic units, La Fra-gua Drift and Anfiteatro Drift, in the type area of the RıoLimay valley (41� S, Fig. 1b, Site 3), or their equivalents,the San Huberto and Criadero de Zorros drifts, furthernorth, in the Rıo Malleo valley (39� S, Fig. 1b, Site 17).This subdivision was based on detailed mapping in bothvalleys, where the aforementioned units occur clearlyseparated both in distance and in elevation. The La Fraguaand Anfiteatro drifts appear also along the dirt road east ofthe San Carlos de Bariloche Airport (Fig. 1b, Site 9),where Flint and Fidalgo (1964) defined the ‘‘El CondorDrift’’. Possibly, if these authors had concentrated theirwork in the Limay valley, their glaciation model wouldhave been different, since the obvious drift elevation dis-tribution in that valley is indicative of significant relativeage differences. However, in the Estancia El Condor area,the differentiation of the drift bodies is more difficultbecause of ice-contact glaciolacustrine sediments andseveral coastlines of proglacial lakes. The La FraguaDrift has been assigned to Caldenius’ (1932) ‘‘Daniglacial’’event (Schlieder, 1989; Rabassa and Evenson, 1996;Rabassa et al., 2005).

In the Esquel region (Fig. 1b, Site 22), the ‘‘Danigla-cial’’ moraines are found immediately west of the outer-most GPG terminal systems, as entrenched sedimentarybodies at lower topographical levels. These units havebeen named the ‘‘post-GPG 1’’ glaciations by Martınez(2002) and Coronato et al. (2004a). These ridges acttoday as local continental water divides, bounding thePacific slope basins. At Portezuelo de Apichig, Caldenius(1932), Gonzalez Bonorino (1944) and Gonzalez Dıazand Andrada de Palomera (1995) have identified a mor-ainic arc of this age, which is physically related to theglaciofluvial gravels of the Fita Michi Formation(Volkheimer, 1963). Thus, the morainic ridges of ‘‘Dani-glacial’’ times are clearly linked to the ‘‘Patagonian grav-els’’ in northern Patagonia. At Portezuelo de Leleque, atleast three frontal moraines highly degrade by mass-movement processes have been mapped behind the‘‘Initioglacial’’ ridges. Frontal moraines of assumed‘‘Gotiglacial’’ age (post-GPG 3; Martınez, 2002; Coro-nato et al., 2004a) occur at Portezuelo de Apichig. Themoraines have later been eroded by spillways from gla-cial lakes.

At the latitude of Lago Epuyen and the heads of RıoChubut (Fig. 1b, Site 19), the best-preserved morainicarcs are found. These would correspond to the

‘‘Gotiglacial’’ (post-GPG 3) and they merge eastwardwith glaciofluvial plains and toward the west with large,varved glaciolacustrine deposits.

In the Lago Buenos Aires region (Fig. 1b, Site 24),Ton-That (1997), Ton-That et al. (1999) and Singer et al.(2004a) obtained limiting ages for the ‘‘Daniglacial’’ drift,by means of incremental-heating 40Ar/39Ar dating of twolava flows associated to glacial deposits. The already-mentioned Telken Basalt is the first of them(1.016+ 0.005 Ma), which covers the ‘‘Initioglacial’’deposits or GPG, and predate the ‘‘Daniglacial’’ or ‘‘post-GPG 1’’ deposits. Moreover, the Telken Basalt presents atransitional paleomagnetic polarity, which corresponds tothe upper portion of the Jaramillo Subchron. The second isthe Arroyo Page Basalt, dated at 0.760+ 0.007 Ma, ofnormal magnetic polarity, which covers the recessionaloutwash deposits of the ‘‘Daniglacial’’ stage (see Fig. 13).Thus, the ‘‘post-GPG 1’’ or ‘‘Daniglacial’’ event wouldhave taken place possibly around MIS 18–20, immediatelybefore the Early–Middle Pleistocene, indicated by theMatuyama–Brunhes paleomagnetic transition, dated at0.78 Ma (Singer and Pringle, 1996).

In the Chilean Lake District (Fig. 1b, Site 13), Porter(1981) considered that at least one of the glaciationscould have been developed in this period. The RıoLlico Drift is clearly older than the Santa Marıa Drift,based on weathering criteria and other field evidence.Since the Santa Marıa Drift is pre-Late Pleistocene inage (Porter, 1981; Clapperton, 1993), a Daniglacial agefor the Rıo Llico Drift is acceptable.

On Isla Chiloe, south of Puerto Montt and southwest ofthe lake district (42� S; Fig. 1a), Heusser and Flint (1977)recognized a three-glaciation sequence of which the oldestunit, the Fuerte San Antonio Drift, has been correlatedwith the Rıo Llico Drift (Porter, 1981) and it is consideredto be of early Middle Pleistocene age, overlying a lavaflow dated at 0.75 Ma (K/Ar) (Clapperton, 1993).

In the Magellan Straits area, Meglioli (1992;Coronato et al., 2004a) mapped two glacial units thatcan be correlated with Caldenius’ (1932) ‘‘Daniglacial’’event: the Cabo Vırgenes and the Punta Delgada drifts. Innorthern Tierra del Fuego, these units are known as theRıo Cullen and Sierra de San Sebastian drifts, whereasonly one unit, the Glencross Drift, has been mappedwithin the Rıo Gallegos valley (Fig. 1b, Site 27).

These glacial advances expanded within deeper val-leys, following the ‘‘Canyon cutting event’’. Abovetheir highest reach, the GPG deposits are formingalmost relief lacking high plains. The oldest of theseadvances (the Cabo Vırgenes Glaciation; Meglioli,1992) is represented by well-defined moraine arcs,though with subdued, planar summits (100 m a.s.l.),which reach the cape where the drift is defined. Themoraines are represented on both sides of the straits;the terminal position is not visible, though it is inferredthat it may be submerged on the present Atlantic plat-form in the eastern entrance of the Magellan Straits, orthat its moraines have been eroded by the fluvioglacialstreams of later glaciations.

An inner moraine belt is developed on both marginsof the Magellan Straits up to Bahıa Posesion (northernmargin) and Punta Catalina (southern margin, Fig. 1a, c).

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These moraines have been interpreted as a second advanceof the ice lobe (the Punta Delgada Glaciation), which isseparated from the Cabo Vırgenes Drift, due to differencesin morphology, soil development, periglacial features andweathering rinds of the clasts incorporated in the till. Boththe Cabo Vırgenes and the Punta Delgada drifts, and theirFuegian equivalents, are considered to be of Gotiglacialage and probably pertaining to the latest Matuyama andthe earliest Brunhes paleomagnetic chrons.

In the Bahıa Inutil–Bahıa San Sebastian depression(Fig. 1c), a post-GPG advance is evident on both itsmargins, represented by the Rıo Cullen moraines, withSW–NE orientation, forming a wide, ample relief ofplanar summits and continuous landforms. On the south-ern margin, the Rıo Cullen Drift covers the slopes andhigh plains of the Sierras de Carmen Sylva (350 m a.s.l.),in northern Tierra del Fuego, extending toward the Atlan-tic Ocean in the shape of a flat moraine, with elongatedridges formed by glaciofluvial deposits and an extensiveerratic boulder field at Punta Sinaı (Fig. 1c; Coronatoet al., 1999; Coronato et al., 2004b). Recent cosmogenicnuclide measurements and paleomagnetic studies haveindicated a Middle Pleistocene age for these deposits(Kaplan et al., 2007; Walther et al., 2007; Fig. 19). Onthe basis of submarine morphology investigations (Islaand Schnack, 1995) the terminal position of the morainearc is located 40 km into the sea.

Toward the center of the cited depression and at bothof its margins, the San Sebastian moraines are located,forming the core of the mountain ranges of this name onthe northern margin, up to 60 m a.s.l. (Fig. 20). Its well-preserved, kettle-hole topography represents disintegrationice stages along the highest plains. The type locality isCabo Nombre, on the Atlantic coast (Fig. 1c), where agray, compact till, with abundant fragments of fossil

shells, wood and coal derived from the preexisting sedi-mentary rocks, has been identified. The frontal position ofthis moraine would be located below present sea level, atapproximately 20 km into the sea (Isla and Schnack 1995).

Finally, based on paleomagnetic and absolute dating,the Daniglacial event would have developed between1.01 and 0.76 Ma, most of this unit being of latestMatuyama age, perhaps during MIS 21–25, perhapseven MIS 19 (Shackleton, 1995). These drifts areprobably equivalent to the younger units of the‘‘pre-Illinoian’’ glacial deposits of Midwestern UnitedStates (Stiff and Hansel, 2004).

However, recent paleomagnetic work on the till unitsat Bahıa San Sebastian have indicated a Brunhes age forall sampled deposits (Ana Walther, personal communica-tion), showing that these deposits have an age � 0.78 Ma(Singer and Pringle, 1996). These new data suggest that atleast part of those stratigraphic units in Tierra del Fuegoand the Magellan Straits corresponding to Caldenius’‘‘Daniglacial’’ stage are clearly Mid-Pleistocene in age.

Glaciations of the Middle Pleistocene

The most important glacial event of the end of the MiddlePleistocene is the ‘‘Gotiglacial’’ period (Caldenius,1932), though in more southern localities like LagosBuenos Aires, Viedma and Argentino (Fig. 1b, Sites24–26), in the Skyrring and Otway Sounds, in the Magel-lan Straits region and Tierra del Fuego (Fig. 1b, Sites 38,39), a previous glaciation defined as post-GPG 2 has beenrecognized (Coronato et al., 2004a, b).

The ‘‘Gotiglacial’’ event corresponds to the youngerportion of the ‘‘El Condor Drift’’ (Flint and Fidalgo,1964), to the Anfiteatro Drift, of the Upper Rıo Limayvalley (Rabassa and Evenson, 1996; Fig. 1b, Site 3;Fig. 21) and to the Criadero de Zorros Drift, of the RıoMalleo valley (Fig. 1b, Site 17; Rabassa et al., 1990a), inNeuquen, northern Patagonia.

The ‘‘Gotiglacial’’ moraines or their stratigraphicequivalents appear in all studied localities as very

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Fig. 19. Erratic boulder field on top of the Rıo CullenMoraine, latest Early–earliest Middle Pleistocene, PuntaSinaı, Bahıa San Sebastian, Tierra del Fuego, Argentina(Fig. 1c). The boulders are composed of one singlelithology, granodiorites coming from the DarwinCordillera, most likely a rock avalanche on top of theglacier. Some of the boulders have been exposed at themoraine surface at least for more than 200,000 years(Kaplan et al., 2007). (Photo by J. Rabassa, 2004).

Fig. 20. Giant erratic boulder on top of the SanSebastian Moraine, early Middle Pleistocene, LosChorrillos, Bahıa San Sebastian, Tierra del Fuego,Argentina (Fig. 1c). Calvin J. Heusser, the author andNat Rutter for scale. (Photo by A. Meglioli, 1988).



170 Jorge Rabassa

well-preserved morainic arcs, located on valley sidesabove the altitudinal range of the LG and at several tensof kilometers downvalley from its terminal moraines. Itsstate of preservation is excellent, which clearly explainswhy Caldenius (1932) and Flint and Fidalgo (1964) hadmistaken them for deposits of a phase of the LG. Theassignation of these deposits to this glaciation was pos-sible, thanks to geomorphological studies and radio-metric dating of associated volcanic rocks.

At the Rıo Malleo valley (39� S, Fig. 1b, Site 17), thePino Santo Basandesite was originally dated by K/Ar at0.207 Ma (Rabassa et al., 1990a). This flow is infilling aglacial valley excavated in post-Criadero de Zorros Drifttimes. This basandesite was redated later by B. Singer(Sample PSA-01; personal communication; Rabassaet al., 2005) at 0.089+ 0.004 Ma by incremental-heating40Ar/39Ar techniques. In both cases, these dates confirm thepre-Late Pleistocene age of the Criadero de Zorros Drift(= ‘‘Gotiglacial’’). In the Rıo Limay valley (Fig. 1b, Site3), the Anfiteatro Drift is correlated with the Criadero deZorros Drift (Rabassa and Evenson, 1996; Rabassa, 1999)and, thus, to a pre-LG event, based on their surficial mor-phology and their respective altitudinal positions withrespect to the LG deposits. However, a TL date performedon glaciofluvial sands incorporated in the Anfiteatro Mor-aine (Fig. 21) yielded an age of 0.065 Ma (Amos, 1998),thus implying that the Anfiteatro Drift would have formedduring MIS 4 (Early Late Pleistocene). However, this TLdate should be considered as a ‘‘minimum age’’, unlessvery local, unknown conditions have operated in the area,since in no other site in Patagonia MIS 4 morainic arcs arefound so far downslope from and altitudinally above theLG moraines (Kaplan et al., 2004, 2005).

In the Chilean Lake District (Fig. 1b, Site 13), Porter(1981) has identified a large glacial event, well beyondthe boundaries of the LG, represented by the Santa MariaDrift. This unit is located around 7–14 km east of theEarly Pleistocene glacial deposits and more than 20–30 kmwest of the LG Llanquihue moraines forming arcuateridges interpreted as moraines by Laugenie and Mercer(1973). This drift has been 14C dated at 57.8 ka, whichshould be considered as a minimum age due to conta-mination with modern rootlets (Clapperton, 1993), and itsdegree of weathering, relative to older and younger mor-aines (Porter, 1981).

In the Lago Buenos Aires region (47� S, Fig. 1b, Site24), a lava flow of normal magnetic polarity, whicherupted from the Cerro Volcan, postdates the post-GPG2 and post-GPG 3 (‘‘Gotiglacial’’) deposits and predatesthose of the LG (Coronato et al., 2004a). This flow wasdated by whole-rock K/Ar by Mercer (1976) at0.177 + 0.056 Ma. Ton-That et al. (1999) obtained adate by 40Ar/39Ar plateau age of 0.123+ 0.005 Ma andan unspiked K/Ar age of 0.128+ 0.002 Ma was pre-sented by Guillou and Singer (1997) and Singer et al.(2004a). These dates were later confirmed by cosmo-genic isotopes (3He) exposure dates from pyroxene con-centrates, which provided an average age of0.128+ 0.003 Ma (Ackert et al., 1998; Singer et al.,1998, 2004a; Fig. 13), as a weighed mean of four sitesand two locations. These ages confirm also that the 3Heproduction rates at 47� S are constant for the last 100 kyr.Later, in situ cosmogenic surface exposure ages ofboulders in the Moreno moraines (Kaplan et al., 2005;Fig. 22) together with the 109 ka 40Ar/39Ar age of CerroVolcan (Singer et al., 2004a) imply that the morainesdeposited during the penultimate local glaciation corre-spond to MIS 6. These ages have been challenged byWenzens (2006b), who claimed that cosmogenic datesare useless in these environments and that the dated CerroVolcan flow is in fact redeposited basalt, suggestinginstead MIS 2 ages for these units based on 14C dating.However, Kaplan et al. (2006) have rejected these objec-tions, particularly those concerning the primary nature ofthe volcanic flows and confirmed their glacial chronol-ogy for the area.

In the Magellan Straits area, Porter (1989) identifieddifferent drift units based on weathering criteria. Mor-aines older than the LG were described at PrimeraAngostura and marine shells included there were datedat >47 ka, confirming a pre-LG age for these deposits.The significantly higher degree of weathering of thesemoraines suggested a pre-LG age as well. Meglioli(1992) defined local glacial units corresponding to theMiddle Pleistocene in the different ice lobes. In the RıoGallegos valley (Fig. 1b, Site 27), the Rıo Turbio Driftwas defined, whereas in the Straits area the prominentPrimera Angostura moraines have been assigned to thisperiod. In northern Tierra del Fuego, Meglioli (1992)assigned the moraines in the middle portion of theBahıa Inutil–Bahıa San Sebastian depression (Fig. 1c)to the Lagunas Secas Drift, at an elevation of180 m a.s.l., of Middle Pleistocene age. The LagunasSecas Drift is composed of deeply dissected morainearcs, with small E–W elongated lakes, probably ancient

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Fig. 21. Stratified drift, glaciotectonically deformed,Anfiteatro Moraine of Gotiglacial age (= Late Illinoianage), Rıo Limay valley, Neuquen Province, Argentina(Fig. 1b, Site 3). (Photo by J. Rabassa, 1988).




outwash channels, strongly eroded and deepened by defla-tion. Both Porter (1989) and Meglioli (1992) haveaccepted an MIS 6 age for these moraines, but otherprevious glacial events (i.e. MIS 8–12) may be present aswell. Clapperton (1993) suggested that the moraines at theStraits of Magellan are most likely of composite origin andthus presenting a large range of glacial events, not onlyequivalent to MIS 6, but throughout the entire Pleistocene.

In central Tierra del Fuego, the Lago Fagnano lobewas built by many different glaciers merging to form alarge, outlet valley glacier in the present Seno Almiran-tazgo, a branch of the Magellan Straits (Fig. 1c). The lakebasin is in fact a tectonic depression crossed by the first-order Magellan fault, the boundary between the SouthAmerican and Scotia plates. An ice thickness of morethan 1500 m favored its eastward spreading, with addi-tional ice supply from local glaciers at Sierra de Beauvoirand Sierra Alvear, on both sides of Lago Fagnano.

Caldenius (1932), Auer (1956) and Meglioli (1992) sug-gested that the eastern limit of the ice lies between theIrigoyen and Noguera ranges, east of Lago Fagnano, oreven along the Atlantic coast. However, Caldenius(1932) had indicated in his map the possibility that theentire island had been covered by the ‘‘Initioglacial’’(= GPG) glaciers, with their terminal moraines lyingsomewhere on the submarine platform. Thus, Caldenius(1932) is clearly referring to post-GPG events. Betweenthe Atlantic shore and the eastern end of the lake, amoraine belt is found, in which Laguna del Pescado(20 km east of Lago Fagnano) and a number of peatbogs are situated. This belt was mapped by Caldenius(1932) as the outermost extent of the ice in this area,corresponding to his ‘‘Gotiglacial’’ glaciation.

In the Lago Fagnano lobe, the moraines at the easternend of the lake are thought to correspond to glacieroscillations during the Middle Pleistocene. Meglioli

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(b)

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Aires200 m a.s.l. Fenix moraines

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10Be26AI3He

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Samples 7368525150484611111068666662133731302725 65134

Deseado I

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172 Jorge Rabassa

(1992) identified two drift units predating the LG: (i) theRıo Valdez Drift, along the southern coast of Lago Fag-nano, believed to be of Illionian/Riss or pre-Illinoian/Rissage; and (ii) the Lago Chepelmut Drift, beyond the north-ern lake coast, which is referred to Late Illinoian glacia-tion. These units are covered by LG deposits along themargins of the lake and several kilometers beyond itseastern end. A proglacial deltaic sequence assigned tothe Rıo Valdez Drift develops along the eastern lakemargin, next to Hosterıa Kaiken, at the easternmost endof Lago Fagnano (Bujalesky et al., 1997; Fig. 23). Thegravel and lacustrine sequence overlies basal till andother glacial deposits and underlies till and other glaci-genic units related to the frontal moraines at the easternmargin of Lago Fagnano. Two peat beds, dated at 39,00014C yr BP and >53,000 14C yr BP (Bujalesky et al.,1997; Rabassa et al., 2000), interbedded in the upperlacustrine levels, suggest that the delta was formed bydeglaciation processes during an Early Wisconsinan/Wurm or pre-Wisconsinan/Wurm interstadial, when cli-mate was colder and drier than today. Recent findings(November 2005) of thin organic layers in between tillunits much farther west along the lake cliff have con-firmed pre-LGM radiocarbon ages for these units (unpub-lished data). The pollen content of these layers lacksarboreal (Nothofagus spp.) pollen, confirming an impo-verished tundra environment associated to glacial condi-tions (J.F. Ponce, personal communication). Thus, theglacigenic materials outcropping along the southern mar-gin of the lake were formed either during a very earlyphase of the LG or, most likely, during the final phase ofthe penultimate glaciation (MIS 6, Late Illinoian/Riss).

The Beagle Channel (Fig. 1c) is a drowned glacialvalley, formerly occupied by a large outlet glacier, theformer ‘‘Beagle Glacier’’, from the Darwin Cordillera.This valley was repeatedly glaciated, at least in twomajor episodes. Caldenius (1932) described glacialdeposits in the Beagle Channel and on the Nueva, Len-nox and Navarino islands (Fig. 1c). These are the oldest

known glacial deposits in the southern Fuegian Andes:the so-called Lennox Glaciation. Evidence from previousglaciations was certainly eroded by the ‘‘Beagle Glacier’’during successive events.

The oldest recognizable glaciation has been namedthe Sloggett Glaciation, which is considered of Illinoian/Riss age, MIS 6 and older (Rabassa et al., 2004). Duringthis event, the ice occupied the entire channel basin, asfar away as Bahıa Sloggett (Fig. 1c), depositing the PuntaJesse and Punta Argentina moraines (Rabassa et al.,2004), located east of the LG moraines or Moat Glacia-tion (see below). A thick sequence of glaciofluvial grav-els along the bay head would represent ice meltingepisodes of perhaps both pre-Moat and Moat age. Innermoraines, closer to the mountain front, would representthe maximum development of a local glaciation of Moatage, with local cirque and valley glaciers, independentfrom the Fuegian mountain ice field. Fieldwork hasestablished the existence of a drumlin field beyond theMoat moraines (David Serrat, pers. comm.). Whetherthese drumlins were formed during MIS 6 (correspondingto the Slogget Glaciation) or 4 (earlier phase of the LG) isstill a task of future research.

Based on the presented evidence, it is possible toconfirm a pre-LG age for the ‘‘Gotiglacial’’ period andits equivalent units (post-GPG 3 and post-GPG 2;Coronato et al., 2004a). It is most likely that the glacialdeposits included in this unit would have been formedduring MIS 6, but they could also have been originated inother previous Middle Pleistocene cold periods, such asthose between MIS 8 and 16. Thus, the ‘‘Gotiglacial’’event are only partially coeval to the ‘‘Illinoian Stage’’ ofMidwestern United States or the Riss Glaciation of theEuropean Alps, since it includes this stage but most likelyextends beyond MIS 10, perhaps comprising some of theso-called pre-Illinoian deposits in the United States.

Glaciations of the Late Pleistocene

The glacigenic deposits of the LG in Patagonia and Tierradel Fuego are those that were formed after the Last Inter-glacial, that is, MIS 5e, 125 ka (Panhke et al., 2003 AU7). TheLGM was reached during the last major glacial event ofthe Late Pleistocene, during MIS 2, after a relativelywarmer period identified with MIS 3. The age of theglacigenic deposits of the LG may be estimated startingperhaps at a maximum of 85 ka, since the process offormation of the Patagonian Andes ice sheet was undoubt-edly slow and took at least 30 ka after the maximum of theLast Interglacial. A maximum concentration of d18O hasbeen identified in Atlantic marine records as well as aminimum of dD has been recognized at the Vostok icecore, both around 70–75 ka BP (Panhke et al., 2003 AU8),suggesting the most probable age of the largest tempera-ture depression in the Southern Hemisphere during MIS 4.Therefore, the ice expansion could have taken place onlyat an advanced stage of MIS 4. In most available marineand ice isotopic records, MIS 4 temperature depressionwas significant, but not as large as that of MIS 2.

The LG was named as ‘‘Finiglacial’’ by Caldenius(1932), and as Nahuel Huapi Drift by Flint and Fidalgo

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Fig. 23. Glacigenic sediments at the cliffs of LagoFagnano, Hosterıa Kaiken (Fig. 1c), Tierra del Fuego.Glacial delta beds overlying basal till. Lacustrine andpeaty deposits overlying the sequence have infiniteradiocarbon ages, suggesting that these deposits maycorrespond to a glacial advance during MIS 4, or morelikely, MIS 6. (Photo by J. Rabassa, 2004).




(1964). This denomination has been preserved by laterauthors. The LG in the Chilean Lake District is known asthe Llanquihue Glaciation (Mercer, 1976; Porter, 1981;Clapperton, 1993).

Clapperton (1993) proposed to extend the nameof ‘‘Llanquihue Glaciation’’ to the LG in the wholeSouth American continent, with its type area in theLago Llanquihue, Chilean Lake District (40�–41� S, Fig.1b, Site 13; Lowell et al., 1995), being represented by theNahuel Huapi Drift on the eastern slope of the Andes.Clapperton (2000) summarized the knowledge about theLG in the southern Andes, in the most significant areas,such as the Chilean Lake District, the Patagonian icefields, the Magellan Straits and the Beagle Channel. Thisauthor recognized a minimum of five glacial advancesduring the LG in the southern Andes, around 30, 27,22.5, 15 and 12–9.3 14C ka BP.

The LG deposits form moraines of extremely well-preserved morphology, very fresh appearance, abrupt slopesand abundant erratic boulders on their surface. The morereliable chronological dates for the LG are coming preciselyfrom the Lago Llanquihue area. There, successive studiesby Mercer (1976), Porter (1981), Lowell et al. (1995) andDenton et al. (1999a, b) have provided an adjusted chron-ology based on radiocarbon dates. According to theseauthors, there were ice expansions during the MIS 4 andrecession during the MIS 3 (Laugenie, 1984; see Rabassaand Clapperton, 1990; Clapperton, 1993). Based on anextremely detailed radiocarbon chronology, Lowell et al.(1995) have identified later readvances during MIS 2,which peaked at 13,900–14,890, 21,000, 23,060, 26,940,29,600 and more than 33,500 yrs BP. Revised chronologyof these areas, including Isla Chiloe (Fig. 1a), by Dentonet al. (1999a, b), indicated that full-glacial or similar envir-onmental conditions were maintained between 29,400 and14,550 14C yr BP, with major glacial advances at 29,400,26,670, 22,295–22,570 and 14,550–14,805 14C yr BP.Cooling events, suggested by pollen data from Isla Chiloe,would have taken place at 44,520–47,110, 32,105–35,762,24,895–26,019, 21,430–22,774 and 13,040–15,200 14C yrBP. The maximum expansion of the ice in the northern partof the studied area occurred at 22,295–22,570 14C yr BP,whereas in the southern portion it took place at14,805–14,869 14C yr BP (Denton et al., 1999b). This out-standing reconstruction of glacial events in a large, pied-mont area, showing variable glacier behavior in differentparts of the ice front, is very important to evaluate andunderstand apparent discrepancies in moraine chronologyover extended areas. The facts exposed in the cited papersshould be carefully taken into consideration when discuss-ing chronology of terminal moraines, in relationship withglobal climate episodes.

The external positions of MIS 4 ice were generallyreached and even surpassed by MIS 2 readvances. Thereare perhaps exceptions at certain areas of Lago Llanqui-hue, where the outermost moraine, Llanquihue I (Porter,1981) was formed more than 39,000 14C yr BP, and inLago Ranco (north of Lago Llanquihue), where it wasdeposited more than 40,000 14C yr BP (Laugenie andMercer, 1973). Mercer (1983) suggested that the Llan-quihue I outer moraines would be ca. 73,000 yrs old, andcorrelated them with MIS 4.

Lowell et al. (1995) and Denton et al. (1999a, b)concluded that the glaciers of the Chilean Lake Districtfinally collapsed ca. 14,000 14C yr BP and suggested thatthe ice advances in this region were coeval with ice-raftingpulses of the North Atlantic Ocean, and that the lasttermination was suddenly and simultaneously initiated inboth hemispheres before the modern termohaline circula-tion was restarted. These authors concluded that interhe-mispheric coupling implied a global atmospheric signalforcing rather than regional climatic changes.

The extent of the LG on the Argentinian slope of theAndes at this latitude, the San Carlos de Bariloche–LagoNahuel Huapi area (Fig. 1b, Site 9; Fig. 24), has beenstudied by Feruglio (1950), Flint and Fidalgo (1964),Gonzalez Bonorino (1973), Rabassa (1975), Schlieder(1989) and Rabassa and Evenson (1996), among manyothers. The LG is represented by the Bariloche moraines,wrapping around the eastern edge of the lake. Equivalentmoraines in similar positions can be found near most oflakes in the region. At least two well-defined moraines ofthis age (Nahuel Huapi I and II) have been identified,each of them around 1 km wide, though so far no absolutechronology is available (Rabassa and Clapperton, 1990).These two moraines are separated by a depression filledby outwash, tephra and eolian deposits. Radiocarbondates on these moraines are lacking due to the absenceof organic matter in the tills, probably because of theextreme aridity of the area during the maximum expan-sion of the ice. Contrary to what happened on the Chileanside, where the ice advanced into the northern PatagonianNothofagus forest, on the Argentinian side the forest wasdisplaced eastward and northward or just supressed,trapped in between the ice front and the 500 mm-yrisohyeth on the dry Patagonian tablelands. A few excep-tions exist, as in the Rıo Malleo valley (Fig. 1b, Site 17),where Rabassa et al. (1990a) found organic layers on topof the Criadero de Zorros Drift (Penultimate Glaciation)and covered by the LG outwash dated at more than30 ka BP. Schlieder (1989) and Rabassa et al. (1990a)presented K/Ar data on volcanic flows in the Rıo Malleoand neighboring valleys, which provided limiting agesfor the LG, which is younger than 0.126+ 0.019 Ma. Ascited above, Rabassa et al. (2005) quoted 40Ar/39Ar

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Fig. 24. Lago Nahuel Huapi, northern Patagonia(Fig. 1b, Site 9), a glacial piedmont lake. (Photo byJ. Rabassa, 1979).



174 Jorge Rabassa

redating of the Pino Santo Andesite at Rıo Malleo(B. Singer, personal communication; 0.089+ 0.004Ma) providing a closer upper age for the LG in thearea. Ongoing cosmogenic dating research may for thefirst time provide absolute dates for the northern Patago-nian ice advances on the eastern side of the Andes(A. Hein and O. Martınez, personal communication,2006). Glacial deposits of the LG have been studied byvan der Meer et al. (1992) in San Martın de los Andes, inthe northern Patagonian Andes (Fig. 1b, Site 18) from asedimentological and glaciotectonic point of view (Fig. 25).

In Volcan Copahue, northern Neuquen Province (37� S;Fig. 1b, Site 11), Gonzalez Dıaz (2003) has recognizedonly one major glaciation on the eastern slopes of thisactive volcano. Previous works had identified two glacia-tions, but the older one is reinterpreted as a Pleistocenegiant slide from the volcano slopes. The confirmed glacialevent is considered of Late Pleistocene age, followed by avery rapid recession.

The LG in the area south of San Carlos de Bariloche(Fig. 1b, Site 9) was studied by Caldenius (1932) whorecognized extensive ‘‘Finiglacial’’ and ‘‘post-Finiglacial’’moraines, the latter of assumed post-LGM age, i.e.Late Glacial. Flint and Fidalgo (1969) extended theirrespective four- and threefold glaciation model southward,being also unable to obtain an absolute chronology of theirNahuel Huapi Drift. Similar conditions were encounteredby Gonzalez Dıaz and Andrada de Palomera (1995) andMartınez (2002). Miro (1967) mapped two morainic arc of‘‘Finiglacial’’ (LG) age in the Epuyen valley (43� S;Fig. 1b, Site 19). Lapido et al. (1990) described the MallınGrande Drift at 43� 300 S, forming two well-preservedmorainic arcs with their corresponding glaciofluvialplains, and adjacent glaciolacustrine deposits, assigning itto the LG. Martınez (2002) proposed to consider only theinner of the Mallın Grande moraines as of LG age(Coronato et al., 2004a).

In the Lago Buenos Aires region (Fig. 1b, Site 24),recent work by Kaplan et al. (2004) has confirmed theage of the LGM by means of cosmogenic isotope dating,allowing the differentiation of five glacial episodes, ofwhich the outermost corresponds to the LGM. The

respective ages, expressed in calendar years, extendbetween 25 ka for the outermost Fenix V Moraine and16 ka for the innermost Fenix I Moraine (Fig. 26). AnAMS radiocarbon age of 15.3+ 0.3 ka BP in post-LGMglaciolacustrine deposits confirms the validity of theseexposure ages and provides an upper limiting age for theLGM in this region. On top of these glaciolacustrinedeposits, the Menucos Moraine corresponds to a post-LGM (early Late Glacial) advance, dated at 13.8 ka bycosmogenic isotopes. The whole set of moraines isyounger than the Cerro Volcan Basalt flow(0.109+ 0.003 Ma; Singer et al., 2004a). Surface expo-sure dating of boulders on these moraines, combined withthe 14C age of overlying varved lacustrine sediment,indicates deposition during the LGM (23–16 ka).

Although Antarctic dust records signal an importantPatagonian glaciation as their most likely source ataround 60–40 ka, moraines corresponding to MIS 4 arenot preserved at Lago Buenos Aires, or elsewhere insouthern Patagonia. Most likely, the MIS 4 moraineswere overrun and obliterated by the younger (MIS 2)ice advance (Singer et al., 2004a). The LGM ages forthe Fenix moraines have been recently discussed byWenzens (2006b) and Kaplan et al. (2006).

Wenzens (2000, 2002) suggested that the presentseparation of the northern and southern Patagonian icesheets is not just a consequence of Holocene meltingaway of the Pleistocene Ice Sheet, but a feature thatwas already established during the LG. The depressionbetween both ice sheets is related to a tectonic depres-sion, which already in the Pleistocene oriented ice drai-nage toward the Pacific Ocean. The eastern margin of theice at this latitude (47�45–48�150 S) would then be theresult of moraine accumulation by local valley glaciers,isolated from the major ice cap. The expansion of theLGM glaciers south of this latitude was much reducedwhen compared with the Northern Patagonian Ice Sheeteastern margin. This has been interpreted as a result ofthe northward displacement of the Pacific precipitationbelt during glacial times (Wenzens, 2000).

The Patagonian glaciations are progressively smallerduring successive glacial advances after the GPG. Thesecircumstances were explained by Rabassa and

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Fig. 25. Clastic dykes in Late Pleistoceneglaciolacustrine sediments, San Martın de los Andes(van der Meer et al., 1992; Fig. 1b, Site 18). (Photo byJ. Rabassa, 1988).

LGM in Lago Buenos Aires Area

Moraine age (ka)1 Age (ka)2

Fénix I

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Fénix III

Fénix V

Mean ±1σ

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Fig. 26. Cosmogenic ages of the LGM moraine systems,Lago Buenos Aires area, Santa Cruz, Argentina (Fig. 1b,Site 24). From Kaplan et al., 2004. 1Means based onboulder 10Be/ 26Al ages that include propagation of alluncertainties except for production rate. 2Means basedon boulder 10Be/ 26Al ages that include propagation of alluncertainties including production rate. For explanationsee Kaplan et al., 2004.




Clapperton (1990) as the effect of tectonic uplift duringthe Early Pleistocene after the GPG, which induced thedowncutting of fluvial valleys (the ‘‘canyon-cuttingevent’’), modifying the original pattern of piedmontlobes into subsequent development of discharge glaciersnested in deep valleys during later glaciations. However,the total volume of ice would have remained almostconstant. Alternatively, Singer et al. (2004a) hypothe-sized that tectonically driven uplift of the PatagonianAndes, which began in the Pliocene, yet continued intothe Quaternary, in part due to subduction of the Chile risespreading center during the past 2 Myr, maximized theice accumulation area and ice extent by 1.1 Ma. Subse-quent deep glacial erosion has reduced the accumulationarea, resulting in less extensive glaciers over time.

Marden (1994) has discussed the volume and prove-nance of the glacigenic deposits in Torres del Paine (51� S,73� W; Fig. 1b, Site 30) and other areas of the south-ernmost Andes, concluding that the sediment budget ofthe last ice sheet was low, with very little supraglacialdebris input and limited older drift reworking, becausemost glaciers advanced over drift-free terrain, the depo-sits of earlier ice sheets being confined to areas beyondthe extent of the Last Glaciation. For this author, glacialerosion in the southern Andes seemed to be decreasingwith successive glaciations. Alternatively, in the opinionof the present author, once the upper portions of theTertiary weathered surface (a subtropical planation sur-face) had been denudated, the lower, unweathered mate-rials were not removed easily, thus modifying the totalsediment budget. One of the interesting features of theglacigenic deposits of the southernmost part of thisregion is that gold particles, supposedly coming fromthe Darwin Cordillera, at the core of the ice sheet, andwhich were exploited intensively in Tierra del Fuegoduring the early twentieth century, occur only in theEarly and Middle Pleistocene drifts, being almost absentin the youngest drifts. This fact may be related to totalerosion of the original gold veins in the accumulationarea of the ice sheet or a radical change in rockaccesibility.

The innermost moraine arc in the Magellan Straitsregion (Fig. 1a) is located at Segunda Angostura, a nar-row pass in the Straits, representing the youngest glacia-tion. These moraines present little erosion, angularridges, nonfilled depressions, and abundant, slightlyweathered metamorphic clasts. Soils have very poordevelopment. The regional glaciation model proposedby Meglioli (1992) includes the Segunda AngosturaDrift as the local equivalent of the LG, composed ofseveral moraines, in tightly packed belts. The BahıaInutil Drift is the local equivalent of the LG along thedepression of this name. The heads of the bay and itsmargins are surrounded by these moraines, composed ofa clayey–silty till, with scarce clast content, glaciolacus-trine structures and abundant, large erratic boulders,aligned over the surface. This moraine is in fact a land-form complex, representing different glacial advances.Clapperton (1989) described an LG drumlin field atCabeza de Mar, along the northern shore of the MagellanStraits (Fig. 1c), in between the two older moraine ridgesbelonging to this glaciation.

Clapperton et al. (1995) mapped inner moraine arcs(in relation to the Segunda Angostura moraines) betweenIsla Santa Isabel and Penınsula Juan Mazıa, centralMagellan Strait (Fig. 1c). The five mapped morainearcs have been interpreted as glacial advances that tookplace during the Last Glacial cycle and Late Glacialevents. A similar model has been suggested by theseauthors for the Bahıa Inutil ice lobe.

McCulloch and Davies (2001) discussed climaticevents in the Magellan Straits, based on pollen and dia-tom sequences at Puerto del Hambre, south of the city ofPunta Arenas (Fig. 1b, Site 31). They recognized that theice receded from the site sometime before 14,470 14C yrBP (17,330 cal. yr) and that a significant glacier read-vance took place between ca. 12,000 and 10,300 14C yrBP. After this date, a very dry period started, which theyassociated with a high rate of forest fires. A differentapproach had been presented by Heusser (2003), whoconsidered the charcoal accumulation as an indicationof human arrival at the area. Note that the original basaldate at this site of 16,590 14C yr BP (Porter et al., 1984)had been recalculated to 14,455+ 155 14C yr BP, due tolignite contamination (Heusser et al., 2000; McCullochand Davies, 2001).

Sugden et al. (2005) presented extensive informationabout the paleoclimatic and paleoenvironmental evolu-tion in the Magellan Straits area during the LateGlacial–Holocene transition. These authors have sug-gested that there is a ‘‘blend’’ of Northern Hemisphere(e.g. North Atlantic Ocean) and Southern Hemisphere(e.g. Antarctic) climatic signals during this period, suchas ice advances at LGM times (ca. 25–23 ka) and againat 17.5 ka (both calendar years). They have also recog-nized a readvance of the ice during the ‘‘Antarctic ColdReversal’’ (ACR), ca. 15.3–12.2 ka, with the beginningof the deglaciation in the middle of ‘‘YD’’ times. Sug-den et al. (2005) estimated that these conditions impliedthat during the Last Glacial–interglacial transition theregional climate was determined by a strong Antarcticsignal. They concluded that during deglaciation, theconditions are more related to oceanographic changes,such as thermohaline circulation, than to astronomicalforcing.

A careful geomorphological mapping of the Strait ofMagellan and neighboring regions has been attempted byBentley et al. (2005). These authors have stated that theLGM moraines and other landforms can be certainlyseparated from those of the older glaciations, on thebasis of geomorphological features, mostly weatheringand drainage development. Likewise, it was possibly toseparate different ice margins during LGM times, basedon discontinuous moraine belts and meltwater channelsthat run along their margins. These geomorphologicalunits have been considered as a very important supportto fully understand the radiocarbon chronology ofthe area.

The chronology of the LG in the Magellan Straitshas been presented in great detail by McCulloch et al.(2005) AU9. Several moraine belts, associated with indivi-dual glacial advances, have been recognized. The ageof the outermost advance, named as ‘‘A’’, has not beenclearly established. It could be related to a pre-LG

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176 Jorge Rabassa

advance (older than 90 ka, based on amino acid data) andsynchronous with the Moreno moraines of the Lago Bue-nos Aires region (Kaplan et al., 2004) of Late Illinoian/Riss age. The following stage B corresponds to the LGMin the area, with ages of ca. 25,200–23,100 cal. yr. Iceadvanced again in stage C (sometime before ca. 21,700–20,300 cal. yr) and in stage D (before ca. 17,500 cal. yr).Finally, the ice readvanced again in between ca. 15,507–14,348 and 12,587–11,773 cal. yr (12,638 and 10,314 14Cyr BP; see also McCulloch et al., 2005). This lateradvance is considered to be in phase with the ACR, asidentified in the Vostok ice core record, though it alsooverlapped with the onset of the YD period of the North-ern Hemisphere. The beginning of rapid warmingand final retreat of the Magellan glaciers took placesometime before 10,315 14C yr BP (11,770–12,580 cal. yr;McCulloch and Davies, 2001), which seems to be coincidentwith the coolest portion of the YD event. These findingssuggested to McCulloch et al. (2005) that there would bea clear antiphase behavior between the two hemispheresduring the Late Glacial–Holocene transition.

Benn and Clapperton (2000a) studied the glacial sedi-ments and landforms preserved in the Strait of Magellanarea. The available record showed repeated advances ofoutlet glaciers of the Patagonian Ice Field during andfollowing the LGM (25,000–14,000 14C yr BP). Theice-marginal landform assemblages are composed ofthrust moraine complexes, kame and kettle topographyand lateral meltwater channels. When analyzed togetherwith other forms of paleoenvironmental evidence, thelandform complex showed that, during the LGM andLate Glacial time, permafrost occurred near sea level insouthernmost South America, indicating that meanannual temperatures were ca. 7–8�C lower than at pre-sent, somewhat lower than those reconstructed by currentglacier–climatic models. In comparison with precipita-tion–temperature relations for modern glaciers, precipita-tion levels would have been lower than today.Precipitation during glacials would have been lower,forced by precipitation shadow conditions induced bythe Patagonian Ice Field, as well as an equatorwardmigration of the average position of westerly cycloniccenters.

The significant role of neotectonics in the develop-ment of local conditions and disturbances in the geologi-cal and geomorphological record has been discussed byBentley and McCulloch (2005), particularly in referenceto the classical site of Puerto del Hambre, MagellanStraits. Late Pleistocene and Holocene movementsalong regional faults have affected the sedimentary accu-mulation and generated drainage diversion, affecting gla-cial and sea level reconstructions. Several annomalies ofthe Puerto del Hambre record can be explained by post-glacial neotectonic activity.

Along the eastern margin of Lago Fagnano (Fig. 1c),Caldenius (1932) mapped ‘‘Finiglacial’’ moraines wrap-ping around the lake. Meglioli (1992) defined the LagoCami Drift as represented by the moraines at the eastern-most end of the lake. Further studies are under way inorder to establish the latest glacial stades and glaciola-custrine sequences of the Lago Fagnano Basin, whichextend toward the Atlantic coast along the valleys of

the San Pablo, Lainez and Irigoyen rivers (Fig. 1c;Coronato et al., 2005).

Although precise 14C dating is still lacking for theLGM in Argentinian Tierra del Fuego, the most extensiveexpansion of the ice in the eastern Beagle Channel, Tierradel Fuego, was probably attained between 18 and 2014C ka BP, but ice recession from its maximum positionhad already started before 14.7 14C kyr BP AU10(Heusser andRabassa, 1987; Rabassa et al., 1990b). The Moat Glacia-tion is represented by a complex system of terminalmoraines at Punta Moat (Fig. 1c). These depositsand landforms have been correlated to Meglioli’s(1992) Segunda Angostura and Bahıa Inutil drifts(Wisconsinan/Wurm Glaciation; Rabassa et al., 1990b).The position and extent of the ice field during the LGMhas been reconstructed from various lines of evidence(Coronato et al., 1999) and it is presented in Fig. 27.

Coronato et al. (2004b) defined the Moat Glaciationas the maximum expansion of the ice in the BeagleChannel during the Late Pleistocene. Unfortunately,only minimum 14C ages at the base of peat bogs grownon top of the moraine have been obtained so far, whichare clearly younger than the assumed ages for the LGM,and even younger than the basal date at the Harbertonpeat bog, located 50 km to the west (Fig. 1c). Furtherwork is needed to attain a precise, absolute chronology ofthe LGM in the Beagle Channel.

At least five, still undated moraine arcs have beenrecognized for the LGM, with very fresh morphology,extending between sea level and 150–200 m a.s.l. A strik-ing feature is the development of a drumlin field on IslaGable (Fig. 1c and Fig. 28; Rabassa et al., 1990c).Drumlins on Isla Gable are part of a larger field thatextends along the Beagle Channel from Estancia Harber-ton to Bahıa Brown, and Puerto Williams (Chile). Calde-nius (1932) and Kranck (1932) misinterpreted theselandforms as terminal moraines, but Halle (1910) hadalready suggested that these landforms could be drumlinsor drumlinoid features. Sedimentary structures reveal thatthese landforms would have been formed during the finalphases of the Moat Glaciation. No absolute dating has yetbeen obtained for the LGM in this area. A minimumradiocarbon age of 14,640+ 260 yrs BP for the glacierretreat from the Punta Moat moraines is given by a basal14C date at the Puerto Harberton peat bog (Heusser1989a, b; Rabassa et al., 1990c; Heusser, 2003; Fig. 29a, b).The lateral complexes that extend to Estancia Moat,100–150 m a.s.l., are considered to correspond to the LGM(Fig. 30). These moraines appear again on Isla Picton and thenon Isla Navarino (Chile; Fig. 1c). At the same time, the uppersurface of the Beagle Glacier at Ushuaia (110 km Westof Punta Moat, Fig. 1c) reached over 1200 m a.s.l., as shownby glacially eroded surfaces and the occurrence of erraticsinside the major cirques.

According to abundant and relevant evidence, the LGin Patagonia and Tierra del Fuego is equivalent to theWisconsin or Wurm glaciations of the Northern Hemi-sphere, spanning over MIS 4, 3 and 2. Evidence for asignificant expansion of the ice during MIS 4 is availableprobably only in the Chilean Lake District and it has beennamed as the Llanquihue 1 AU11event. The LGM is repre-sented by the Llanquihue 2 moraines in Chile and the

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Nahuel Huapi Drift in northern Patagonia, and the Fenixmoraines in the Lago Buenos Aires Basin. The LGM wasattained around 25 cal. ka and ended around 15 cal. ka,probably corresponding to the Heinrich 2 and 1 events ofthe North Atlantic Ocean, respectively.

Late Glacial Expansions of the Ice

The Patagonian glaciers reached their maximum expan-sion around 23 ka (calendar years), but several read-vances took place until the definitive recession startedaround 18–17 ka, based upon basal radiocarbon ages inpeat bogs and cosmogenic dates on recessional moraines(Kaplan et al., 2004), though other smaller advancesoccurred during the Late Glacial. The Late Glacial periodconventionally extends between 15 and 10 14C ka BP, butice fluctuations may have started before the olderboundary.

Caldenius (1932) was the first to clearly identifymoraine systems younger than the LGM. He mappedthese units along the bottom of the glacial valleys, in anintermediate position between the Finiglacial (= LGM)moraines and the present glacier margins or, instead, thesource cirques. He labeled them ‘‘Post-Finiglacial mor-aines’’, implying also a timely concept. Most of thesemoraines have been found to have Llate Glacial ages.

Porter (1981) described a recessional phase of the icein the Lago Llanquihue lobe (Fig. 1b, Site 13), which henamed the ‘‘Llanquihue III’’ event, indicating that therewere no defined moraines assigned to this phase, but ageneral ice-disintegration terrain complex, due to stag-nant ice, around between 14 and 12.2 14C ka BP.

Mercer (1976) did not support the idea of Late Glacialadvances of the Patagonian Andes. He concluded that thewarming trend initiated around 13 14C ka BP had con-tinued without interruption until Holocene climates wereestablished. Clapperton (1983) challenged this point ofview, following Caldenius (1932) mapping, and Rabassa

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Fig. 28. Last Glaciation drumlin field at EstanciaHarberton, Beagle Channel, Tierra del Fuego,Argentina (RabassaAU12 et al., 1990; Fig. 1c). This drumlinis seen from its up-ice end, looking downslope, in adrowned portion of the drumlin field. Note the MiddleHolocene marine terrace around the drumlin base,slightly above present sea level, and the wave cuterosional features on both sides of the drumlin. (Photoby J. Rabassa, 2004).

MAGELLANSTRAITS

CAPE HORN

Beagle

200

0 50 100 km

Fggogno

Bahia inutil

Skyring

Bru

nsw

ick

Otway

STEPPE

PACIFIC OCEAN

ATLANTIC OCEAN

FOREST REFUGE

GLACIOFLUVIAL PLAINS65° O71° O

53° S

N

200

TUNDRA

DARWIN CORDILLERA

Fig. 27. Paleogeomorphological and paleoecological map of Tierra del Fuego during the LGM (from Coronato et al.,1999).



178 Jorge Rabassa

(1983) proposed also that some of the inner moraines ofLago Nahuel Huapi (Fig. 1b, Site 9) could be of LateGlacial age, because they were younger than peat basinswith basal ages of ca. 14 14C ka BP, but located fardownslope from the Neoglacial moraines in the samevalley.

Glasser et al. (2004) presented evidence for Late Gla-cial glacier fluctuations of the Patagonian ice fields. Theseauthors considered that glaciers still covered large areas ofPatagonia at approximately 14,600 14C yr BP, but uniformand rapid warming took place after 13,000 14C yr BP.There has been no agreement about evidence for climatefluctuations equivalent to those of the Northern Hemisphere

YD cooling event (the YD Chronozone), dated to 11,000–10,000 14C yr BP (12,700–11,500 cal. yr).

Singer et al. (2004a) and Kaplan et al. (2004) haveidentified a significant advance of the Lago Buenos Airesice lobe (Fig. 1b, Site 24), cosmogenic isotope dated atca. 14.4+ 0.9 ka, which they have called the Menucosmoraine, when the ice was overriding its own glaciola-custrine deposits. No other ice expansions have beenrecorded here until the early Holocene.

Hajdas et al. (2003) have reported high-resolutionAMS 14C chronologies from the San Carlos de Barilocheand Chilean Lake District areas (Fig. 1b, Sites 9, 13) thatsuggest the development of a cool episode between

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(a)

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Fig. 29. (a) Harberton Bog, Beagle Channel, Argentinian Tierra del Fuego (Fig. 1c; photo by J. Rabassa, 2004);(b) absolute pollen rain data. Note the basal age of 14,640 – 260 14C yr BP. From Heusser, 2003.




11,400 and 10,200 14C yr BP, which they named the‘‘Huelmo/Mascardi Cold Reversal’’, that would have pre-ceded the onset of the Northern Hemisphere YD coldevent by at least 550 calendar years. However, theseauthors estimated that both events occurred during aradiocarbon-age plateau at ca. 10,200 14C yr BP. Thus,the Huelmo/Mascardi Cold Reversal and the YD wouldhave been a couple of short-term cool-warm oscillationsthat immediately preceded the onset of the latter in theNorth Atlantic region. These observations partially agreewith the discussion by Sugden et al. (2005) about theACR, an Antarctic climatic signal affecting southernPatagonia during Late Glacial times.

Bennett et al. (2000) had denied the existence of a YDcooling event in southern Chile, based on chronological,sedimentological and paleoecological records from sedi-ments of small lakes in the coastal zone, which is con-trolled by a heavily oceanic climate. Thus, these authorshave suggested that there was little or no cooling in thesouthern Pacific surface waters, and therefore, indicatingthat the YD cooling in the North Atlantic Ocean was aregional, rather than global, phenomenon. However, itshould be noted that the climate of the studied region isvery different from the rest of Patagonia. In this area, theavailable moisture brought from the ocean onto the con-tinent would have probably been constant, regardless oftemperature changes, thus not affecting the distributionof local species. Perhaps plants and insects do not react to1–2�C changes, whereas glaciers actually do so to ELAmodifications at the same scale. It sounds very extreme toextend paleoenvironmental conclusions to all of Patago-nia, based on findings of a rather unique area.

Though working on a global scale, Blunier and Brook(2001) have found a close relationship between similarevents in both hemispheres. They studied the methaneand isotopic content in Greenland and West Antarctic icecores, confirming that the onset of seven major millen-nial-scale warming events in Antarctica preceded theonset of equivalent periods in Greenland by 1500–3000yrs. In general, Antarctic temperatures increased gradu-ally, while Greenland temperatures were decreasing orconstant, and the termination of Antarctic warming wasapparently coincident with the onset of rapid warming inGreenland. This pattern provides further evidence for the

operation of a ‘‘bipolar see-saw’’ in air temperatures.However, an oceanic teleconnection between the hemi-spheres on millennial timescales can be proposed, thuslinking the precedent ACR with the subsequent YD oversuch a delay.

Ariztegui et al. (1997) stated that several high-resolution continental records have been reportedrecently in sites in South America, but the extent towhich climatic variations were synchronous betweenboth hemispheres during the Late Glacial–Holocene tran-sition, and the causes of the observed climatic changeshave not been solved yet. According to these authors, eastof the Andes, the middle and high latitudes of SouthAmerica warmed uniformly and rapidly from 13,00014C yr BP, with no indication of subsequent climatefluctuations, equivalent, for example, to the YD cooling.They presented a multiproxy continuous record, 14Cdated by accelerated mass spectroscopy, from proglacialLago Mascardi (Fig. 1b, Site 15), which indicates thatunstable climatic conditions, comparable to thosedescribed from records obtained in the Northern Hemi-sphere, dominated the Late Glacial–Holocene transitionin Argentina at this latitude. They suggested that a sig-nificant advance of the Monte Tronador local ice cap(Fig. 1b, Site 12), which feeds Lago Mascardi throughthe Upper Rıo Manso, occurred, however, during the YDChronozone. These circumstances suggested a climatichistory that reflected a global, rather than a regional,forcing mechanism. These authors indicated that theLago Mascardi record provides strong support for thehypothesis that ocean–atmosphere interaction, ratherthan global ocean circulation alone, led interhemisphericclimate teleconnections during the last termination.

McCulloch et al. (2000) noted the uncertainty aboutthe interhemispheric timing of climatic changes duringthe Last Glacial–interglacial transition. They discussedvarious hypotheses, according to different lines of evi-dence, which suggest either that the Northern Hemi-sphere climatic changes were leading the SouthernHemisphere ones, and vice versa, or alternatively thatboth hemispheres acted in synchrony. The location ofsouthern South America is considered appropriate totest the various alternatives using both glacial andpaleoecological evidence. These authors estimated that,from varied sources of evidence, there was a sudden risein temperature that initiated deglaciation simultaneouslyover more than 16� of latitude at 14,600–14,300 14C yrBP (17,500–17,150 cal. yr). There was also a secondwarming episode in the Chilean Lake District at13,000–12,700 14C yr BP (15,650–15,350 cal. yr), whentemperatures almost achieved modern values. A thirdmajor warming step occurred at ca. 10,000 14C yr BP(11,400 cal. yr), reaching Holocene temperature levels.Following the initial warming, there was a laggedresponse in precipitation as the westerlies, after a delayof ca. 1.6 kyr, migrated from their northern glacial loca-tion to their present latitude, which took place ca. 12,30014C yr BP (14,300 cal. yr). According to these authors,the latitudinal contrasts in the timing of maximum pre-cipitation are reflected in regional contrasts in vegetationchange and in glacier behavior. A large, 80 km glacieradvance in the Strait of Magellan at 12,700–10,300

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Fig. 30. LGM moraines at Punta Moat, Beagle Channel,Argentina (Fig. 1c). (Photo by J. Rabassa, 1989).



180 Jorge Rabassa

14C yr BP (15,350–12,250 cal. yr), a period that includesboth the ACR and the earlier part of the YD, was influ-enced by the southward return of the westerlies. Thedelay in the migration of the westerlies would be perhapscoincident with the Heinrich 1 iceberg event in the NorthAtlantic. Thus, the suppressed global thermohaline circu-lation at that time may have also affected sea-surfacetemperatures in the South Pacific, modifying the positionof the westerlies, which returned to their present south-erly latitude only after oceanic conditions achieved theirpresent interglacial mode.

Marden (1997) presented evidence of two Late Gla-cial advances of the South Patagonian Ice Field at Torresdel Paine (51� S, 73� W; Fig. 1b, Site 30), Chile, whichchallenge the concept raised by others that climate insouthernmost South America was characterized by unin-terrupted warming after LGM termination. Theseadvances are marked by moraines and other ice-marginaldeposits, 18–20 km and 10–16 km from the modernlimits of two outlet glaciers, whereas older full glaciallimits are indicated by other sets of moraines ca. 50 kmfrom the modern glaciers. Pumice clasts included in theglacial deposits are related to an eruption of VolcanReclus (Fig. 1b, Site 34) at ca. 11,880 14C yr BP, whichprovided a close limiting age for the older Late Glacialevent, whereas the younger advance occurred during theinterval 11,880–9180 14C yr BP. This author supportedthe idea that deglaciation occurred slowly in the studiedarea because initial warming was accompanied byincreased moisture as precipitation belts migrated south-ward. As the climate cooled, the outlet glaciers advanced.The temperature depression was estimated to have beennot more than 2�C below current values, since LateGlacial moraines at some local glaciers lie within 200 mof the modern ice margins. This idea of twofold, lateglacial expansions had been previously supported bypalynological evidence (e.g. Heusser, 1987; Heusserand Rabassa, 1987; Clapperton, 1993; Heusser, 1987,1993, 2003).

Fogwill and Kubik (2005) have presented preliminarycosmogenic 10Be data from a former ice limit in Torresdel Paine. The offered data indicate a stillstand or areadvance of Patagonian glaciers culminating at around12–15 ka with a mean age of 13.2+ 0.8 ka. The glacierextended some 40 km beyond the present ice margin andwas within 15 km of the presumed LGM boundary in thisarea. This glacier stage is interpreted as partially coin-cident with the ACR (14.5–12.9 ka). According to theseauthors, the data implied that glaciers at these latitudeswere out of phase with those in the Northern Hemisphere,but instead, followed an Antarctic climatic signal duringLate Glacial times. The Puerto Banderas moraine at LagoArgentino (Fig. 1b, Site 26) was mapped by Caldenius(1932) as one of his ‘‘Post-Finiglacial’’ moraines, anddescribed by Mercer (1976). Strelin and Malagnino(2000) proposed a Late Glacial age for a system ofthree moraine belts, with a maximum age of 15.5+ 2.4cal. yr. Recently, Strelin and Denton (2005) have sug-gested a new chronology of these units, following theprevious scheme of using radiocarbon ages of organicmaterials found at the marginal moraines. Becker et al.(2005) discussed the problem of the ACR and YD

problem in relation to this unit. They have mapped anddated the Puerto Banderas I and II moraines, and 36Cl and10Be dated these moraines with an average age of11.2+ 05 ka from 18 samples. Mercer (1976) had pre-viously obtained a radiocarbon age of 11.7+ 0.3 ka BP.Becker et al. (2005) concluded that the Puerto BanderaMoraine is younger than previously thought, and it is notrelated to the termination of the LGM, neither depositedduring the ACR. This ice advance would be consistentwith the expansion of the South Patagonian Ice Fieldduring, or shortly following, the YD period. Thus, theseauthors have stated that the proposal by Sugden et al.(2005) that the ACR is more prominent in southernmostPatagonia may be premature. Thus, not all of SouthAmerica south of the Chilean Lake District seemed tobe in phase with the Antarctic climate, and as the polarfront and westerlies migrate, the boundary between‘‘northern’’ and ‘‘Antarctic’’ response may be latitudin-ally displaced as well.

Mercer (1976) found no evidence around the Patago-nian ice fields that glaciers had advanced during the LateGlacial interval at 12–10 14C ka BP. But because peatolder than 11 14C ka BP lies beneath Neoglacial morainesin some places, Mercer concluded that since the intervalof deglaciation at ca. 13 14C ka BP, Patagonian glaciershad not been any more extensive than they are now untilca. 5000 14C yr BP. Consequently, Mercer (1976) sug-gested that the Holocene most probably began in south-ern South America at around 13 14C ka BP and that thelate glacial cooling known as the Younger Dryas in theNorthern Hemisphere had been restricted to north westEurope.

This opinion was supported by studies of fossil bee-tles in the Chilean lake region by Hoganson and Ash-worth (1992) and by pollen studies in Patagonia east ofthe Andes by Markgraf (1991, 1993). These authors alsoconcluded that the so-called Hypsithermal warming trendhad begun at about 13 14C ka BP and was not followed orinterrupted by any significant cooling. These views arequite the reverse of those determined from palynologicalstudies by Calvin Heusser who, in a number of articles(Heusser, 1974, 1984, 1987; Heusser and Streeter, 1980;Heusser et al., 1981; Heusser and Rabassa, 1987), hadargued strongly that significant climatic cooling occurredduring not only the last 5 kyr, but also at 11–10 14C kaBP. The high precipitation and low temperatures esti-mated by Heusser and Streeter (1980) for the Late Glacialinterval, if valid, should have caused glaciers in theregion to advance.

Clapperton (1993) has argued, however, that in areaswhere the Andes are low and presently ice-free, as alongmuch of the crest east of the Chilean Lake District, theLate Glacial temperature depression may have beeninadequate to have caused glaciers to form again. Hehas also suggested (Clapperton, 1983) that if there areno Late Glacial moraines around the Patagonian icefields, it is because this area had become almost ice-free by 13 14C ka BP. Late Glacial cooling might haveinitiated the regrowth of glaciers, but these wererestricted to the ranges now buried by the (Neoglacial)ice field systems. Thus, any Late Glacial moraines thatwere deposited lie beneath the present ice cover.

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Alternatively, John Mercer may have been wrong in hisobservation that Late Glacial moraines do not existbeyond the limits of those he dated to be part of theNeoglacial interval.

It is particularly crucial to understand what happenedaround the Patagonian ice fields during this intervalbecause evidence of late glacial advances has beenfound in other parts of the southern Andes. Also, in theSan Carlos de Bariloche area, several moraines have beenobserved inside the limits of the LGM (Nahuel Huapimoraines). As they appear well down-valley from thoseof the Neoglacial interval in the Rıo Manso valley, it ispossible that they formed during the Late Glacial inter-val. They were mapped by Caldenius (1932) as ‘‘Post-Finiglacial’’ moraines. Those observed by Rabassa(1983) at Lago Moreno, Puerto Blest and Divisoria deAguas, near San Carlos de Bariloche (Fig. 1b, Site 9), areworthy of more detailed study as candidates for LateGlacial limits in this region. They are believed to beyounger than ca. 14 14C ka BP, the age of basal peat ina bog lying between these moraines and Lago NahuelHuapi. Moraines east of the South Patagonian Ice Fieldyounger than those of the LGM were mapped by Calde-nius (1932) and Feruglio (1950). Particularly striking arethose at Punta Bandera, Punta Ciervo and Marıa Antoniaalong the southern shore of Lago Argentino (Fig. 1b, Site26). They are situated at distances of 22, 24 and 50 kmbeyond the present outlet glaciers. These distances sug-gest that the moraines are more likely to represent limitsof a significant glacial stadial at ca. 15–1414C ka BP.

Along the Beagle Channel, several glacial advancesor stabilization periods took place during Late Glacialtimes (Rabassa et al., 1992, 2000). A first ice retreatphase probably took place before 14.7 14C ka BP. Amodel of a calving glacier front, in either the adjacentsea or a proglacial lake, has been favored. A 1–2 kyr longstabilization phase could have occurred when the icefront reached the Isla Gable rise (Fig. 1c). This is sug-gested by the basal 14C age of the Caleta Robalo peatbog (near Puerto Williams, Isla Navarino, Chile, Fig. 1c;12,700+ 90 yrs BP), a minimum age for ice retreat fromIsla Gable. During the initial recession period, the glacierthickness decreased at Ushuaia by a minimum of 550 m.Then, the main Beagle Glacier receded from the cirques,allowing their glaciers to expand downslope. Two large,extensive lateral moraines have been mapped aroundUshuaia and named the Pista de Ski and Ushuaia mor-aines. Radiocarbon dating of basal peat at 12,060+ 60 yrsBP at Pista de Ski Moraine (300 m a.s.l.) suggested thatthis retreat phase probably peaked ca. 12 ka 14C yr BP,when a relative maximum of arboreal pollen is reachedaround 11,780+ 110 14C yr BP, at Puerto Harberton bog(Fig. 1c; Rabassa et al., 1990b).

Morphological evidence of stabilization occurs alsobetween Punta Segunda (35 km West of Isla Gable) andArroyo Fernandez (Fig. 1c), building up a four-stagefrontal moraine complex that extends into the BeagleChannel, below present sea level. These morainesdevelop from 100 m a.s.l. at the mountain sides, ina discontinuous shape, as till pockets preservedagainst erosional bedrock remnants or as low moraines(<75 m a.s.l.) in the city of Ushuaia. Notwithstanding, the

basal ages of the break point (80 m a.s.l.; 12,430+ 8014C yr BP) and San Salvador (10 m a.s.l.; 12,100+ 5014C yr BP) peat bogs in Ushuaia show that the ice wouldhave already disappeared from these sites allowing theformation of lacustrine environments (Heusser, 1998).The similarity of the peat bog basal ages between 300and 10 m a.s.l. in Ushuaia suggested that the ice recessionfrom Isla Gable to Ushuaia had taken place during a shortperiod. Although the pollen profiles show evidence ofcooling between 11 and 10 ka and subsequent vegetationchanges (McCulloch et al., 1997), perhaps the climaticconditions had been not harsh enough so as to alter theBeagle Glacier dynamics and to allow ice stabilizationand interrupt the general headward recession. Futurestudies will probably lead to a discussion of the chronos-tratigraphy of the lowest moraine arcs in Ushuaia, pre-viously defined as Ushuaia Drift (Rabassa et al., 1990b).Radiocarbon ages of the terminal moraine complex atPunta Segunda are still needed to adjust the chronologyin this region.

The 10 ka glacial retreat was definitive: basal peatlayers of Punta Pinguinos in Ushuaia (20 m a.s.l.) andLapataia (20 km westward, 18 m a.s.l.; see Fig. 1c)showed ages of 10,080 14C yr BP (Rabassa et al., 1986;Heusser and Rabassa, 1987), a condition observed alsofor the glaciers that were tributaries to the glaciation axislocated in the eastern end (66� W, Bahıa Aguirre, Penın-sula Mitre, Fig. 1c), where basal peat layers have yieldedan age of 10,920+ 70 14C yr BP (UTC-5402). The rapiddisappearance of the ice within the eastern portion of theBeagle Channel was probably due to the collapse of afloating ice snout, as sea level invaded the valley androse to almost present positions around 8.7 14C ka BP(Gordillo et al., 1993).

A glacierization model in mountain valleys of theFuegian Andes, tributaries to the Beagle Channel valley,was proposed by Coronato (1995 a, b) and Coronato et al.(2004b). The transversal and longitudinal valleys of theFuegian Andes show the effect of extensive Pleistoceneglacier erosion. The tributary valleys were occupied bymultiple valley glaciers, ranging from 20 to 30 km inlength, though smaller, single-valley glaciers were alsopresent.

These valleys probably underwent the same sequenceof glacial events as the rest of Tierra del Fuego, but suchepisodes are not represented in the existing geologicalrecord. This is probably due to erosion during the LGM.Moreover, the entire study area was mostly ice-coveredand well above the ELA, impeding the formation oflateral moraines. As in all interdependent ice system,glacial activity in the tributary valleys was controlledby the behavior of the main ice stream and regionalclimatic variations. Several phases that took placebetween the Late Pleistocene and the Early Holocene inthe Andean valley glaciers have been established: (i) theLGM, (20–18 14C ka BP); (ii) ‘‘Individualization’’, as theMoat Glaciation was decaying (18–14 14C ka BP); (iii)‘‘Stabilization’’, when the ice bodies achieved their max-imum positions during Late Glacial times (14–12 14C kaBP); and (iv) ‘‘Deglaciation’’ (10–9 14C ka BP), whenglaciolacustrine environments were dominant in themountain valleys.

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182 Jorge Rabassa

The erosive landforms are recognizable in the rockyaretes in which glacial peaks, horns and cirques abound,some of them still bearing mountain glaciers of signifi-cant magnitude. Glacial accumulation landforms are rare,although some depositional features have been modeledin subglacial, supraglacial and marginal ice environ-ments. The presence of latero-frontal moraine arcs,basal moraines, kame terraces, glacial plains and peatbogs has been clearly defined and mapped in the valleysinvolved, as well as the remains of glaciolacustrinebodies (Coronato, 1990, 1995a, b). The frontal morainearcs of the Andorra and Canadon del Toro valleys, nearUshuaia, are separated by glaciolacustrine deposits thatare also present in the Pipo valley. In that valley, ice-marginal landforms related to the general glacier reces-sion prevailed (Coronato, 1993; Fig. 31).

The Carbajal–Tierra Mayor valley (Fig. 1c) is anotherimportant glaciation axis in the Fuegian Andes, tributary ofthe Beagle Channel at Bahıa Brown, 50 km east of Ushuaia(Fig. 1c and Fig. 32). During the maximum of the LG atrunk glacier established here, flowing from W to E alongthe tectonic alignment Carbajal–Tierra Mayor–Lasiparshak(Fig. 1c), with minor tributary glaciers, coming from lateralcirques. Due to glacial diversion, an overflowing ice tonguewould have displaced southward along the Rıo Olivia

valley down to its confluence with the Beagle Glacier,E of Ushuaia (Coronato, 1995a, b). The Late Glacial–EarlyHolocene depositional sequence is presently under studyconcerning the palynological and paleoclimatic aspects.

Geomorphological evidence found in the FuegianAndes indicates that the definitive deglaciation processwould have started after 10 14C ka BP. In the innervalleys, a lacustrine phase has been characterized, inlake sedimentary sequences or at the base of the presentpeat bogs (Coronato, 1991; Gordillo et al., 1993;Coronato, 1995a, b). The existence of paleolakes in dif-ferent relative positions in between confluent glaciershas been dated ca. 10–9 14C ka BP (Coronato, 1993).

Moraines situated close to the cirque basins aboveUshuaia appear to be late glacial in age, but precisedating remains to be done. Planas et al. (2002) mappedthe geomorphological units within the Martial Glaciercirque, near Ushuaia (Fig. 1c), with at least two morainelevels of Late Glacial age developed on both valley sidesand Holocene moraines occurring next to the ice front,the latter still lacking plant colonization (Planas et al.,2002; Figs 33 and 34). Some support for a Late Glacialage cold interval came from the interpretation of pollendiagrams obtained for this area (Heusser, 2003). Theserecords showed that a significant reduction in Nothofaguspollen occurred during the interval ca. 13–10 14C ka BP(Heusser, 1989a; Rabassa et al., 1990a). Such a declinehas been associated with a significant climatic deteriora-tion, perhaps coeval with glacier advance.

The Late Glacial history of Patagonia and Tierra delFuego is now much better known than only two decadesago, but much precise dating is still needed if definitivecorrelation with Northern Hemisphere climatic events isintended.

4.4. Holocene Glaciation

Mercer (1965, 1968, 1970, 1976, 1982) published pio-neer studies on Patagonian Holocene ice advances andtermed them ‘‘Neoglaciations’’. Mercer (1976) foundevidence that by ca. 13 14C ka BP deglaciation hadcleared ice from the Rıo Baker valley, which separatesthe two Patagonian ice fields (Fig. 1a), and concludedthat glaciers in the region did not readvance again untilabout 5 14C ka BP. Investigations of moraines lyingseveral kilometers from the present glaciers on the east-ern and western sides of the two Patagonian ice capsled Mercer (1968, 1970, 1976) to conclude that three

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Fig. 31. Kame and glaciolacustrine deposits at the mouthof the Rıo Pipo valley, Beagle Channel, near Ushuaia,Tierra del Fuego, Argentina (Fig. 1c). These were formedas the valley glacier receded and sediments poured fromthe main Beagle Glacier, still occupying the valley duringLate Glacial times. (Photo by J. Rabassa, 2003).

Fig. 32. Panoramic view of the Carbajal valley, heads of the Rıo Olivia, near Ushuaia, Tierra del Fuego, Argentina(Fig. 1c). Erosional glacial landscape, carved on metamorphic rocks during the LG. (Photo by J. Rabassa, 2004).




advances had occurred during an interval of Neoglacialcooling that spanned the last 5 kyr. The first was at4700–4200 14C BP, the second at 2700–2000 14C BP andthe third has taken place during the last three centuries(seventeenth to twentieth centuries). Most of the organicmaterial from which radiocarbon dates were obtained gaveonly minimal ages for the advances, however, and nonehave been closely bracketed. Nevertheless, there is agree-ment with the ages of Neoglacial fluctuations determinedin other parts of the Andes and in the Northern Hemi-sphere (Clapperton, 1993). Clapperton (1993) studied allof Mercer’s data on Neoglacial glacier fluctuations andnoted that an interesting pattern exists. During the firstadvance at 4700–4000 14C BP glaciers in the west weremore extensive than during the advance at 2700–200014C BP; glaciers in the east were less extensive at4700–4200 14C BP than at 2700–2000 14C BP. A preli-minary hypothesis is that an eastward migration of the

iceshed occurred as the Patagonian ice fields built upover two mountain ridges separated by an intermontanedepression. Clapperton (1993) has also suggested that thePatagonian ice fields may not have existed until theNeoglacial cooling and that only small glaciers restrictedto the (now-buried) mountain ridges survived during thepreceding interval of Hypsithermal warmth.

Palynological studies of cores taken in the ChileanLake District (Heusser, 1974; Heusser and Streeter,1980; Heusser et al., 1981; Fig. 1b, Site 13) alsoindicate three cooling intervals during the last 5 kyr.Radiocarbon dating of major vegetational changes thatindicate cooler conditions suggested that the climatereversals occurred at 4950–3160 14C BP, sometimebetween 3160 14C BP and 890 14C BP, and during thelast 350 yrs. The intervals of relatively low temperatureappear to have coincided with periods when precipitationwas significantly higher than now, with total annualrainfall as much as 150% above the present mean(Heusser and Streeter, 1980).

Bertani et al. (1986) recognized at least two Neogla-cial advances in addition to that of the Little Ice Age(LIA) at the Castano Overo Glacier (Monte Tronador,northern Patagonia; Figs 1b, Site 12; Figs 35 and 36), andRabassa et al. (1984) and Brandani et al. (1986) notedthat the Rıo Manso Glacier had advanced at least oncebefore the LIA (Fig. 37). However, none of these earlierevents has been precisely dated yet. The LIA extendedbetween middle seventeenth and middle nineteenth cen-turies, based on dendrochronological analysis of the treescolonizing the successive moraine ridges (Rabassaet al., 1984; Brandani et al., 1986). Rabassa et al.(1981) described in-transit moraines on the Casa PangueGlacier (western slope of Monte Tronador, Chile), whichsupported forest colonization on the active glacier inthose times (Fig. 38).

Although the earliest Holocene has generally beenconsidered an interval of ameliorating climatic condi-tions, Rothlisberger (1987) and Rothlisberger and Geyh(1985) concluded that glaciers advanced at least twicebetween 8600 and 8200 14C yr BP. These events were

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Fig. 33. View of the lateral, cirque moraines of LateGlacial age, Martial Cirque, Fuegian Andes. The citybelow the cirque is Ushuaia, and an ample view of theBeagle Channel (Fig. 1c). The cirque moraines areactively covered by periglacial fans and talus, comingdown from the summit areas. (Photo by J. Rabassa, 2004).

Fig. 34. Holocene moraines, Martial Cirque, Ushuaia(Fig. 1c). (Photo by J. Rabassa, 2004).

Fig. 35. Castano Overo Glacier, Monte Tronador, northernPatagonia, Argentina (Fig. 1b, Site 12; photo by J. Rabassa,1975). This regeneration cone, reconstructed below a veryhigh ice fall, melted away in recent years due to regionalwarming (see Bertani et al., 1986).



184 Jorge Rabassa

suggested on the basis of volcanic ashes covering appar-ent Neoglacial moraines and should be considered asminimal ages only; the moraines could be much older,possibly even of Late Glacial age.

Glacial advances in the earliest Holocene have beendiscussed for a long time. There has been a general agree-ment that ice readvances suggested for this interval either

have been wrongly dated or have been misinterpreted; forexample, some were probably associated with glaciersurges, kinematic waves or other oscillations related tointernal glacier dynamics independent of climatic changeand do not reflect global or regional cooling.

Palynological interpretations in southern Chile byHeusser (1974) suggested that the warmest climate inter-val following the Late Glacial cooling occurred betweenca. 8500 and 6500 14C yr BP, when temperatures aver-aged about 2�C warmer than now. This corresponds wellwith data from other parts of the world indicating thatHolocene Hypsithermal conditions had peaked before ca.6000 14C yr BP; but a subsequent study by Heusser andStreeter (1980) suggested that the maximum warmth hadoccurred earlier, between 9410 and 8600 14C yr BP.

Porter (2000) extensively discussed the nature andage of the Patagonian Holocene glaciations. Evidenceof early Neoglacial expansion of glaciers in the Andeswas primarily located within a belt extending between46� and 52� S. The glaciers of this area included land-terminating alpine glaciers as well as tidewater- and lake-calving glaciers that drain the north and south Patagonianice fields (Fig. 1a). On the Chilean side of the southernAndes, the San Rafael Glacier is a large tidewater glacier,flowing from the northwestern sector of the North Pata-gonian Ice Field (Warren et al., 1995a). The Tempanosmoraines (Muller, 1959; Heusser, 1960) border theLaguna de San Rafael beyond the glacier margin. Akettle (Lago 1) west of Laguna San Rafael on the outer-most moraine was cored by Heusser (1960), whoobtained an age of 3610+ 400 14C yr BP (later recalcu-lated at 3740+ 400 yrs BP; Heusser, 1964) for peat at adepth of 2.1 m that was overlying laminated silt layers.Based on this date, Heusser inferred that the earliest(Tempanos I) advance culminated ca. 4000 14C yrs ago.Mercer (1982) subsequently suggested that the initialTempanos advance likely dates to ca. 4700–4200 14C yrBP, consistent with then available evidence in the

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Fig. 36. Art Bloom (Cornell University) posing as scaleon a striated glacial boulder in front of Castano OveroGlacier, Monte Tronador, Argentina (Fig. 1b, Site 12;photo by J. Rabassa, 1982).

Fig. 37. Rıo Manso Glacier, Monte Tronador, Argentina(Fig. 1b, Site 12). This is a debris covered glacier thathas prominent Holocene and LIA moraines, which can beseen on both sides of the glacier (Rabassa et al., 1978).(Photo by J. Rabassa, 1983). This glacier is presentlyundergoing very rapid retreat due to regional warming.

Fig. 38. Casa Pangue Glacier, Monte Tronador, Chile(Fig. 1b, Site 12). This is the largest glacier in northernPatagonia. This is a debris covered, reconstructedglacier which supported in-transit moraines with soils.Vegetation was growing on top of the active ice (Rabassaet al., 1981). (Photo by J. Rabassa, 1979). In the 1990s,regional warming forced the collapse of the underlyingice, and with it, the soils and trees.




southern Andes. ClappertonAU13 and Sugden (1989) indicatedthat the date provides only an upper limiting age for amoraine that could be much older. A second limiting datewas obtained from a site inside the limit of nineteenth-century moraines, where 75 cm of unweathered till over-lies sand and peat. Compressed wood in the peat has anage of 6850+ 200 14C yr BP. Heusser (1960) consideredthat this date was providing a lower limiting age for theinitial Tempanos advance, the beginning of which heplaced at ca. 5000 14C yr BP. Muller (1959) inferredthat the overlying till at this locality dated to the nine-teenth century, and that the date for the wood representedan upper limiting age for the Tempanos advance. Hesuggested that late glacial recession from the Tempanosmoraines took place ca. 9000 14C yrs ago. Mercer (1982)did not agree with this chronology, noting that it did notexplain why organic sedimentation would only have fol-lowed deglaciation 5000 yrs later. Porter (2000) sug-gested that further studies are needed on this glacier.However, as San Rafael is a calving glacier, a majoradvance of its terminus may not correlate with regionalclimatic events (Warren, 1993; Warren et al., 1995b).

Porter (2000) summarized also the knowledge forother glaciers in the area. The Ofhidro Sur Glacier is anoutlet glacier of the South Patagonian Ice Field (Fig. 1a)with a grounded snout. However, during Late Glacialtimes, its terminus calved into a fjord. Mercer (1970)stated that its snout was located at 2500 m from thefjord head and was bordered by a series of recent mor-aines, the innermost of which dated to the eighteenthcentury. At the time when the outermost moraine wasconstructed, the terminus was perhaps in contact withtidewaters. Basal peat from the crest of the second mor-aine was dated at 4060+ 110 14C yr BP; a similar sam-ple from the sixth moraine had an age of 3740+ 11014C yr BP. Mercer (1970) estimated that the second mor-aine was built no later than ca. 4200 14C yr BP, but theoutermost moraine was not dated.

The Tempano Glacier is an outlet glacier of the SouthPatagonian Ice Field, ending at the Tempano fjord alonga calving front. Mercer (1970) dated basal peat from abog lying between the outer moraine and the adjacenthillside at 4120+ 105 14C yr BP, providing a minimumage for moraine construction. Probably, being a tidewaterglacier, this advance might correspond to local conditionsunrelated to regional climatic variations. At the LosCipreses Glacier, Rothlisberger (1987) described fourlateral moraines lying inside deposits more than 670014C yr BP old, also postdating a paleosol with a date of5180+ 295 14C yr BP. Other outer moraines have notbeen dated.

On the Argentinian side of the southern Andes, theSan Lorenzo Este Glacier is a large land-terminatingglacier on the eastern side of Cerro San Lorenzo (Fig.1b, Site 32), ca. 100 km northeast of the South Patago-nian Ice Field. Mercer (1968, 1976, 1982) described twoend moraines bordering the glacier and three older ones.The outermost moraine dammed a small lake, where arooted tree stump was dated at 4590+ 115 14C yr BP,the tree presumably having been drowned by a glacieradvance. It is possible that local factors may have influ-enced the behavior of this glacier.

The Narvaez Glacier is located about 50 km east ofthe South Patagonian Ice Field. It shows a proglacial lakeand three moraines. Mercer (1968) inferred that the innermoraine dated to the nineteenth century and the outer tothe seventeenth century. Basal peat on the outermostmoraine had been dated at 4320+ 110 14C yr BP, provid-ing a minimum age for that moraine. Wenzens (1999b)studied moraines of the Rıo Manga Norte Glacier, on theeastern slope of the Precordillera between Lago Viedmaand Lago Argentino (Fig. 1b, Sites 25, 26), inferred to dateto the LG and postglacial times. Terminal Neoglacialmoraines were dated as older than 4280+ 100 14C yrBP, but this may not be close to the real age. Dates of8350+ 50 and 8694+ 45 14C yr BP were obtained fromdeposits downvalley from the late Neoglacial limit innearby Arroyo Guanaco, and another date of 7370+ 7014C yr BP was obtained ca. 12.5 km downvalley from lateNeoglacial moraines in the adjacent valley of RıoGuanaco. According to Porter (2000), further studies areneeded to determine whether these moraines could beof pre-Neoglacial age, either Early Holocene or LateGlacial.

The famous Moreno Glacier (Fig. 1b, Site 29) is alarge outlet glacier of the South Patagonian Ice Cap,calving into Lago Argentino and Brazo Sur, and whichhas been permanently advancing during the last century(Fig. 39). Mercer (1968) obtained a date of 3830+ 11514C yr BP for basal peat indicating the end of a glacieradvance. A similar age (3860+ 115 14C yr BP) suggestedthat the glacier had retreated by that time. Porter (2000)mentioned that the earliest lake-damming advance of Mor-eno Glacier occurred ca. 4850–5050 14C yr BP ago, and adate of 4640+ 40 14C yr BP for wood in a moraineprobably provides a close age for the maximum of themost extensive Neoglacial advance. Warren (1994)pointed out that the unusual behavior of Moreno Glacierhas been more closely controlled by calving dynamics andtopography than by regional climate trends.

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Fig. 39. Moreno Glacier (Fig. 1b, Site 29). A large outletglacier of the Southern Patagonian Ice Field, Argentina,which has advanced almost constantly over the last twocenturies across Lago Argentino (Fig. 1b, Site 26),blocking drainage and causing the lake level to rise.The pressure of the water finally breaks the ice wall,bursting the snout of the glacier in a remarkable,recurrent natural event. (Photo by J. Rabassa, 2004).



186 Jorge Rabassa

The Frıas Glacier, further south, shows Neoglacial mor-aines along the southwestern shore of Brazo Sur of LagoArgentino (Fig. 1b, Site 26). Mercer (1968, 1976) describedthree moraines dating to recent centuries, obtaining a date of3465+ 130 14C yr BP from wood on top of the outermostmoraine. He assigned the moraine to an early Neoglacialage (i.e. ca. 4600–4200 14C yr BP), but the moraine may beactually younger than this event. The O’Higgins Glacier is alake-calving glacier flowing from the Southern PatagonianIce Field (Fig. 1a). Rothlisberger (1987) obtained radio-carbon dates for in situ wood fragments between4675+ 120 and 6020+ 75 14C yr BP.

Porter (2000) concluded that, due mostly to a lack ofmultiple and precise dating, the hypothesis that SouthernHemisphere glaciers advanced more or less synchronouslyin the Middle Holocene, and in concert with NorthernHemisphere glaciers, has not yet been rigorously proven.

Glasser et al. (2004) presented evidence for Holoceneglacier fluctuations of the Patagonian ice fields. Theseauthors considered that during the early Holocene(10,000–5000 14C yr BP) atmospheric temperatures eastof the Andes were about 2�C above modern values in theperiod 8500–6500 14C yr BP. The period between 6000and 3600 14C yr BP appears to have been colder andwetter than present, followed by an arid phase from3600 to 3000 14C yr BP. From 3000 14C yr BP to thepresent, there is evidence of a cold phase, with relativelyhigh precipitation. West of the Andes, the available evi-dence points to periods of drier than present conditionsbetween 9400–6300 and 2400–1600 14C yr BP. Holoceneglacier advances in Patagonia began around 5000 14C yrBP, coincident with a strong climatic cooling around thistime (the Neoglacial interval). Glacier advances can beassigned to one of three time periods following a‘‘Mercer-type’’ chronology, or instead, four time periods,following an ‘‘Aniya-type’’ chronology (Aniya, 1995).The ‘‘Mercer-type’’ chronology has glacier advances4700–4200 14C yr BP; 2700–2000 14C yr BP and duringthe LIA (seventeenth to twentieth centuries). The ‘‘Aniya-type’’ chronology has glacier advances at 3600 14C yr BP,2300 14C yr BP, 1600–1400 14C yr BP and during the LIA.These chronologies are best regarded as broad regionaltrends, since there are also dated examples of glacieradvances outside these time periods. Possible explanationsfor the observed patterns of glacier fluctuations inPatagonia include changes related to internal characteristicsof the ice fields, changes in the extent of Antarctic sea-icecover, atmospheric/oceanic coupling-induced climaticvariability, systematic changes in synoptic conditions andshort-term variations in atmospheric temperature andprecipitation.

Douglass et al. (2005) used cosmogenic nuclide sur-face exposure dating to show that at least one glacier onthe Chilean side of Lago Buenos Aires (46� S; Fig. 1b,Site 24) advanced ca. 8.5 and 6.2 14C ka BP. These dataon the so-called Fachinal moraines suggest that the iceadvanced most likely as a result of a northward migrationof the southern westerlies, which caused an increase inprecipitation and/or a decrease in temperature at thislatitude. The older advance is 3000 yrs older than theaccepted beginning of Holocene glacial advances insouthern South America (Mercer, 1976). According to

these authors, these events are temporally synchronouswith Holocene climate oscillations that occurred in otherparts of the world. If there are causal links between theseevents, then rapid climate changes appear to be eitherexternally forced (e.g. solar variability) or are expandedshortly all over the surface of the Earth by atmosphericprocesses.

After 10 14C ka BP, ice persisted only as cirque glaciersand small valley glaciers in the eastern Fuegian Andes, andas remnants of a mountain ice sheet in the Darwin Cordillera(Fig. 1a; Rabassa et al., 1992; McCulloch et al., 1997).

In the Andorra and Canadon del Toro valleys, nearUshuaia (Fig. 1c), the cirques are dominantly orientedtoward the S, SE and SW. The ice occurrence/orientationrelationship shows concordant aspects with the hemi-spheric insolation and the regional climatic conditions,because ice relicts are still present facing toward the SE(45.1%) in the Andorra valley, to the S (18.5%) in theCanadon del Toro, to the SW (16.1% and 33.3%) in theAndorra valley and Canadon del Toro (Coronato, 1995a).

Recession followed the late glacial maxima and evi-dence for several Neoglacial readvances are observed inthe cirques. Three moraine arcs have been mapped at theVinciguerra Glacier, near Ushuaia, the largest cirqueglacier still existing in the Argentine Fuegian Andes(Fig. 1c). The oldest moraines reach 600–650 m a.s.l.and have been largely colonized by the Fuegian forest.The youngest one lies well above the timberline. Thismoraine was apparently formed during the LIA; olderreadvances are represented by complex moraine systems,but all of them remain undated.

The occurrence of ice bodies within the glaciatedvalleys is restricted to a minimum elevation of700–800 m a.s.l. The topography of the glacier valleysclearly shows the Holocene events. These were definedas (a) the Vinciguerra I phase (8.5–5.0 14C ka BP), whenthe glacier receded continuously without evidence ofstabilization; (b) the Vinciguerra II phase (5.0 14C ka BP)represented the stabilization of the glacier, generatinglateral moraines at 500–540 m a.s.l., and the erosional land-forms on the first threshold at 480–600 m a.s.l., and (c) thelast phase, or Vinciguerra III (LIA), corresponded to asecond stabilization event with two well-developedpulsations, depicted by moraines formed within the presentforefield (Rabassa et al., 1992).

4.5. Glaciation of Islas Malvinas/Falkland Islands

Quaternary studies on the Islas Malvinas/FalklandIslands (Fig. 1a) started as early as the mid-nineteenthcentury, when Darwin (1846) described the presence ofdistinctive periglacial features like block streams or‘‘stone rivers’’.

The existence of Pleistocene glaciers in the archipe-lago was demonstrated by Clapperton (1971), Clappertonand Sugden (1976), Roberts (1984) and Clapperton AU14andRoberts (1986), among others, and summarized byClapperton (1990, 1993). During the Quaternary, onlyconditions for marginal glaciation had developed,whereas at the same latitude, very large ice fields existedin Patagonia. Roberts (1984) identified 76 nivoglacial

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features, made up of 8 nivation crests, 41 nivation hollows,7 nivation cirques and 20 glacial cirques. Erosional anddepositional characteristics identified on the islands sug-gest that there were at least two intervals of cirque glacia-tion. Three glacial cirques are larger than the rest, but theyalso show the development of cirque-in-cirque glaciation.A few glaciers expanded beyond the cirque basins anddeposited till and terminal moraines in the valleys. Roberts(1984) suggested that the Pleistocene glacial history of thearchipelago is restricted to only three events: an earlycirque phase, a valley phase and late cirque phases. Thelatter probably represents the LGM.

The entire archipelago is affected by periglacial masswasting, suggesting that Quaternary cold periods arelargely responsible for landscape development. Accord-ing to Clapperton (1990), a radiocarbon date of26,060 þ 400/–380 yrs BP, for a podsol buried by1.5 m of solifluction debris, suggests that the last inter-val of solifluction probably coincided with the LGM.

4.6. Modeling the Late Pleistocene Ice Sheetand Glacier Behavior

Sugden et al. (2002), Hulton et al. (2002) and Hubbardet al. (2005) have tried to model the expansion and reces-sion of the Patagonian Ice Sheet during the LGM. Hultonet al. (2002) used a coupled ice sheet/climate numericalmodel with empirical evidence, simulating the ice sheet atthe LGM and at different stages of deglaciation. UnderLGM conditions, an ice sheet with a modeled volumeslightly in excess of 500,000 km3 built up along the south-ern Andes. There is a marked contrast between the mar-itime and continental flanks of the modeled ice sheet. Themodel is most sensitive to variations in temperature andthere is good agreement between modeled ice extent andempirical evidence. This was achieved by applying anestimate of a 6.1�C temperature decrease with constantwind. Assuming a stepped start to deglaciation, modeledice volumes declined sharply, contributing 1.2 m to globalsea level, of which 80% occurred within only 2000 yrs.The empirical record suggested that such a stepped warm-ing occurred around 17,500–17,150 cal. yrs ago.

Hubbard et al. (2005) presented a time-dependentmodel to investigate the interaction between climate,extent and fluctuations of Patagonian ice sheets between45� and 48� S during the LGM and the deglaciation thatfollowed. The model was applied at 2 km resolution andenabled ice thickness, lithospheric response and icedeformation and sliding to interact freely. Relativechanges in sea level and ELA were considered as well.Experiments implemented to identify an LGM configura-tion compatible with the available empirical record indi-cated that a stepped ELA lowering of 750–950 m wasrequired over 15,000 yrs to fit the Fenix I–V moraines atLago Buenos Aires (Fig. 1b, Site 24). However, 900 m ofELA lowering yielded an ice sheet that best matches theFenix V moraine (ca. 23,000 14C yr BP) and Caldenius’reconstructed LGM limit for the entire modeled area.According to these authors, this optimum LGM experi-ment yielded a highly dynamic, low aspect ice sheet, witha mean ice thickness of ca. 1130 m drained by numerous

large ice streams to the western, seaward margin and twolarge, fast-flowing outlet lobes to the east. Forcing thisscenario into deglaciation using a rescaled Vostok icecore record resulted in a slowly shrinking ice sheet thatwas only 25% of the LGM volume by ca. 14,500 14C yrBP, after which it collapsed rapidly, with a loss of 85% ofits volume in only 800 yrs. It is interesting to note that itsmargins stabilized during the ACR, after which it recededto near present-day limits by 11,000 14C yr BP.

Wenzens (2004) has strongly criticized Sugden et al.(2002) models, and implicitly, his criticism applied alsoto Hubbard et al. (2005) later work. Wenzens (2004)estimated that the boundaries depicted in the models donot fit with any actual ice margin of comparable age, thatthe considerations did not apply to both Patagonian icesheets and that the Andean topography had not beenproperly considered in the models.

Another point of view is presented by Benn andClapperton (2000b), who described proglacial and sub-glacial glaciotectonic sediments and landforms aroundthe margins of the Strait of Magellan. These depositsrecorded the advance and retreat of outlet glaciers ofthe Patagonian ice cap during the Last Glacial cycle.The glaciotectonic landforms in the area would havebeen the result of advancing ice lobes with cold-basedmargins due to permafrost regional conditions, but withwet-based inner portions. As the ice advanced, subglacialbasins would have been dug underneath the glacier mar-gins and the eroded material was pushed up into thrustmoraines, probably because frozen-bed conditionsformed a thermal dam against the free drainage of sub-glacial meltwaters. Later, these ice-marginal glaciotec-tonic landforms would have been overridden andstreamlined into drumlins and flutes when thicker, wet-based ice advanced over these areas. Evidence for per-mafrost near sea level in Patagonia during the LastGlaciation suggests that mean annual temperatures wereseveral degrees lower than indicated by recent modelingstudies. The results indicate that future modeling experi-ments should incorporate more realistic basal boundaryconditions, particularly the presence of a weak deforminglayer at the glacier bed, to improve climatic reconstruc-tions of southern South America.

Concerning interhemispheric links, in the opinion ofBlunier et al. (1998), a main aspect of climate dynamicsis to understand if the Northern and Southern hemi-spheres are effectively coupled during climate events.The fast and strong temperature changes observed inGreenland (the Dansgaard–Oeschger events) during thelast glaciation have a certain analogue in the temperaturerecord from Antarctica. A comparison of the globalatmospheric concentration of methane as recorded inice cores from Antarctica and Greenland allowed estab-lishing a phase relationship of these temperature varia-tions. Greenland warming events between ca. 36 and45 ka ago have a delay in relation to their Antarcticcounterpart by more than 1 kyr. On average, Antarcticclimate change precedes that of Greenland by 1–2.5 kyrover the period 47–23 ka. However, it should be consid-ered that the observed delay is usually within the opera-tional error of the dating techniques, and improved dataare needed to confirm these opinions.

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188 Jorge Rabassa

5. Discussion

5.1. Environmental Changes in Southern SouthAmerica Following the Glacial Events

The Patagonian glacial sequence provides a reasonableframework to understand the environmental evolution ofsouthernmost South America from the latest Miocene tothe Pleistocene–Holocene boundary. Particularly, therelative lack of other long terrestrial records gives aparamount importance to the glacial evidence for pre-Late Pleistocene times.

Clapperton (1993), Heusser (2003) and Rabassa et al.(2005) have discussed the climatic and environmentalchanges in southern South America that followed theestablishment and development of the Late Cenozoic Pata-gonian glaciations, which may be summarized as follows.

First, global sea level changes forced by glaciationpartially exposed the Argentinian submarine platform,which enhanced the climatic continentality. Significant

eustatic movements took place, with sea level loweringof at least several tens of meters during cold events, andup to 100–140 m during full glacial episodes. Climaticcontinentality of the surrounding areas increased, withrising extreme temperatures, precipitation diminution andlack of the sea moderating effect as the coastline movedeastward. This process occurred both in Pampa and Pata-gonia, with almost a doubling in size of the continentalareas and subsequent strong continentalization. Note thatfor the Latest Pleistocene, this is an important factregarding the environmental conditions and availablepathways and space for human colonization in thePampean and Patagonian Regions (Fig. 40).

Sea-surface temperatures were lowered up to 4�C inthe tropical areas during MIS 2, with at least a lowering of5–6�C in southern South Africa (30�–32� S; Tyson andPartridge, 2000), with increased lowering toward thepoles. This lowering in mean sea-surface temperature(MSST) had certainly influence on the evaporation andmobility of marine currents, with a consequent diminution

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62 Fig. 40. Map of South America during the LGM. Partly modified from Clapperton, 1993AU15 .




of mean annual temperature in all continental areas, whichin northern Patagonia would have been of at least 5–6�Cand perhaps much more in the southern regions (Heusser,1989b; Clapperton, 1993).

These conditions increased the influence of the Mal-vinas/Falkland Current (Fig. 40), which today reaches upto southern Brazil. Most likely, this current reached amuch more northerly position during the glacial winters,and stayed there for longer portions of the year. As aconsequence of the coastline mobility, the position of thelittoral marine currents, both the Brazil and Malvinas/Falkland currents were affected. During the glacialepochs their meeting front was displaced northward,modifying the Pampean winter storm pattern, and prob-ably, diminishing the oceanic influence and increasingwater deficit during these periods.

Moreover, sea level lowering provoked a strong low-ering of marine depth between the Patagonian coast andthe Malvinas/Falkland Archipelago, forcing an eastwarddisplacement of the Malvinas/Falkland Current, with afurther increase in climatic continentality along the pre-sent littoral zones.

The climatic conditions during glacial episodes hadan influence on the displacement of the oceanic antic-yclonic centers, both in the Pacific and in the Atlanticoceans. The South Pacific anticyclonic center was dis-placed northward during the glacial periods, increasingthe effect of the ‘‘Pampero’’, cold-dry winds that dom-inate the weather and eolian sedimentation in the Pampasof eastern Argentina, Uruguay and southern Brazil. Thenorthward movement of this anticyclonic area deter-mined that those regions previously free of the cold anddry ‘‘westerlies’’ were progressively affected by thesewinds. The increasing eolian action leads to the develop-ment of intensive deflation processes, with the genesis ofhydroeolian depressions, salt lakes and endorheic basins,and also the dune field formation in northern Patagoniaand western Buenos Aires Province (Clapperton, 1993;Iriondo, 1999; Fig. 41). This eolian activity was alsoresponsible for loess accumulation in the PampeanRegion, Uruguay and southern Brazil, beyond the dunebelts, where the Pampean vegetation, though thinner thanin interglacial times, was capable of retaining the finesand-coarse silt fractions. A similar role had the RıoSalado of Buenos Aires Province (35� S, Buenos AiresPampean Region, Fig. 1a), which acted as a sand trap,originating the La Chumbiada (Dillon and Rabassa,1985) and Guerrero (Fidalgo et al., 1975) members ofthe Lujan Formation, during the Late Pleistocene (MIS 4to 2). Similar conditions would have taken place in most,if not all, glacial events of the rest of the Pleistocene andbefore, since the Rıo Salado has long been occupying avery ancient tectonic basin (Rabassa et al., 2005). More-over, it is also probable that a northward displacement ofthe anticyclonic centers generated changes or at least ahigher variability in the eolian sediment supply contribut-ing to the Pampean loess formation, incorporating epi-clastic products coming from western Argentina and thecentral Andes (Iriondo, 1999; Muhs and Zarate, 2001).

As a consequence, deflation was strongly dominantduring all glacial events, with formation of sand dunesand loess mantles in the Pampas, excavation of endorheic

hollows and depressions, and genesis of salt lakes inareas that are wetter today.

Climatic changes forced changes in the plant cover,with large latitudinal displacements of the major ecosys-tems during glaciations.

Tundra, which is restricted today to mountain sum-mits above the treeline, developed all over southern Pata-gonia, and perhaps up to 42–44� S. Tundra conditionsincluded permanent or transient frozen ground, at leastaround the ice margins, though its eastward expansioncould have been larger (C. Heusser, in Bujalesky et al.,1997). Tundra paleoenvironment, inferred from palyno-logical records of fossil peat at Lago Fagnano, nearTolhuin (Fig. 1c; Bujalesky et al., 1997), was character-ized by the absolute lack of arboreal (Nothofagus spp.)pollen, while it was dominant during a glacial phaseof the penultimate glaciation (MIS 6) and, most likely,was also present during the LG. Still unpublished paly-nological studies of fossil peat layers interbedded withtills in the area have confirmed these environmentalconditions (J.F. Ponce, personal communication). Seealso Coronato et al. (2004c) and Trombotto (Chapter12), for ice-wedge cast development in northern Tierradel Fuego AU16during the Last Glaciation.

In Late Glacial times as the glaciers receded, thistundra environment was probably rapidly replaced by apark vegetation, with isolated Nothofagus spp. forestpatches in a grassy steppe environment. These conditionsare particularly evident in the Harberton peat bog (Fig. 1c)pollen profile (Heusser, 1989 AU17), where the recession of the‘‘Beagle Glacier’’ from its outermost LGM positionsallowed the partial recovery of the Fuegian forest asearly as 14.8 14C ka BP. At that moment, and for severalhundred years, the forest started its slow but steady

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Fig. 41. Sand sea and loess accumulation in the Pampasand surrounding lowlands during the LGM. FromIriondo, 1999.



190 Jorge Rabassa

recovery, advancing from (still) theoretical refuges locatedat the present submarine platform or perhaps at Isla de losEstados (Staaten Island; Fig. 1a), as suggested by thepollen record (Coronato et al., 1999). However, in atleast two opportunities, around 13 and 11 14C ka BP,respectively, the arboreal pollen content practicallydisappears from the record, being entirely replaced byGramineae and Empetrum, indicating the return to coldregional conditions, which forced perhaps a new east–northeastward recession of the Fuegian forest, toward itsPleistocene refugia. But toward 10.2 14C ka BP, the contentof the pollen records indicates that the forest restarted itsexpansion on Isla Grande de Tierra del Fuego, reachingpresent-day conditions in the first millenium of the Holo-cene, though the present conformation of the forest wasachieved only toward ca. 8 14C ka BP. These cold LateGlacial episodes (herein named as ‘‘late glacial I’’ and ‘‘lateglacial II’’) may be comparable both in chronological andin intensity terms with their Northern Hemisphere equiva-lents, the ‘‘Oldest?/Older? Dryas’’ and ‘‘Younger Dryas’’.Alternatively, a strong influence of the ‘‘Antarctic ColdReversal’’ event has been proposed (Sugden et al., 2005).Nevertheless, the pollen record undoubtedly indicates thatthe ‘‘late glacial II’’ event was more intense and extremethan the previous one, but its environmental consequenceson the forest are still unknown.

The Patagonian forest became isolated from otherSouth American forest formations perhaps already in theMiddle Miocene. On the Chilean side, as the ice reachedthe Pacific Ocean waters south of 44� S, the forest wasprobably completely suppressed, perhaps with isolatedrefuges on small, remote islands or uncovered coastalpeaks. On the eastern slopes, the forest was concealed inbetween the glacier front and the 0�C annual isothermtoward the west, and the shrubby steppe environmentsand the 300 mm annual isohyeth eastward, which wouldhave bounded its eastern expansion. These ecosystemswere severely damaged and the forest was disrupted infragmented populations, in remote and restricted refuges,from 36� southward. In Tierra del Fuego, the forest wasprobably displaced toward the present submarine platform,northwest and north of Penınsula Mitre and Isla de losEstados (Fig. 1a, c; Coronato et al., 1999).

The Pampean grassy prairies were spatially reducedand pushed north and northeast during glacial events.Thus, the Patagonian steppe expanded northward intothe Pampean domain and perhaps, even into Uruguayand northeastern Argentina, thus becoming an importantfactor in loess accumulation. The Patagonian equivalentsof the Pampean prairies, which had developed since theEarly-Middle Miocene, disappeared as well, beingreplaced by the north- and eastward expanding Monteand steppe ecosystems (Rabassa et al., 2005).

These ecosystem changes were closely followed by asignificant terrestrial faunal replacement, with northwardexpansion of Patagonian faunas during cold events,reaching up to southern Brazil. Likewise, the Brazilianfaunas invaded the Pampas and even northernmost Pata-gonia during interglacial periods. This has been provedfor the Late Pleistocene in the Pampean vertebrate fossilrecords (Chapter 13AU18 ) and it was probably in effect duringeach major climatic cycle.

Under drier-colder climate, Patagonian faunas predo-minated in the Pampas during glacial epochs, andwarmer-wetter Brazilian faunas during the interglacials.This faunal replacement, clearly observed during thePleistocene–Holocene transition and, more recently, inthe Late Holocene, would have probably taken placewith similar characteristics during each glacial ‘‘termina-tion’’, that is, for at least 15 times since the GPG, andperhaps even 50 or more times since the early Plioceneand even more than 100 times since the Late Miocene.The consequences that the high frequency of these dis-placements would have had on the Pampean faunas, bothfrom a taxonomic and a biogeographical point of view,remain still in the hypothetical domain, but they shouldnot be let aside in paleobiological, paleogeographical andpaleoenvironmental reconstructions in southern SouthAmerica (Rabassa et al., 2005).

It is clear then that the climatic cycles identified in theglobal oceanic isotopic sequences have been confirmed bythe terrestrial Patagonian glacial record. These changes havebeen very important and they should have had a significantinfluence in the development of the Pampean and otherSouth American ecosystems, perhaps up to southern Brazil.

These paleoenvironmental modifications would havehad severe consequences in the entire studied region,although it is understandable that their characteristics andintensity would have not been identical over the hugePatagonian and Pampean geography. But, undoubtedly,they should have played an important role in the processof early peopling of Pampa and Patagonia. It is highlypossible that the human expansion in southern SouthAmerica would have started immediately after the LGM(ca. 25 cal. ka) and most certainly, after the last phase ofmorainic construction, ca. 16 calendar ka (Lago BuenosAires, Fig. 1b, Site 24; Kaplan et al., 2004). The south-heading human groups, probably looking for regions witha higher density of surviving Pleistocene megamammals,underwent not only the progressive environmental changestypical of the Last Termination, but they should havesuffered as well the two Late Glacial cold episodes,which affected them and the regional biota in a similarmanner (Rabassa et al., 2005).

The Pleistocene–Holocene transition, the timing ofthe human occupation of Patagonia, was an epoch ofhigh environmental instability. There was a varied envir-onmental mosaic which, together with locations closer tothe sea and under its influence, would have offeredappropriate, though perhaps different, routes for humanpeopling. In those times, environments and thus, faunalresources would have been equivalent in both Pampa andPatagonia. These faunas are characteristic of grasslandenvironments or, at least, grassy steppes of cold, dry tosemiarid climates (Cione and Tonni, 1999). The changesleading to definitive Holocene environments took placeonly after 9 14C ka AP, toward a shrubby steppe, with thefinal disappearance of the Pleistocene faunas and increas-ing abundance of Lama guanicoe (Miotti and Salemme,1999; Rabassa et al., 2005).

When the Holocene environments were finally estab-lished, the glaciers were reduced to their present condi-tions, thus allowing for full occupation of most of thePatagonian lands, including the Andean piedmont and the

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Magellan Straits. Faunas changed from those of drierenvironments to others corresponding to higher relativemoisture. The Brazilian faunas occupied the Pampasduring the Late Holocene, and the Patagonian faunashave been similar to the extant ones throughout theHolocene (Tonni and Cione, 1999AU19 ).

5.2. The Buildup of the Patagonian Ice Sheet

The variations in length and frequency of the cold-warmclimatic cycles have determined that the intensity of theextreme isotopic content peaks of the global oceanicrecord became larger toward the Early Pleistocene. Thus,

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MaBiozones

3.703.75 EARLY3.803.85 Neocavia

depressidensCHAPADMALALAN

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LBA

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5.35.45.55.65.7 TG20?5.8 TG22?5.96 C3a

n16.1

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6.3

5C3

T

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4.7

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HUAYQUERIAN

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EN

E

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Inferred glaciation

Overlying basalt

Underlying basalt

Basalts layers

Glaciofluvialdeposits

?

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MagneticPolarity

IOS Age South American Locali-ties

SourceStages

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LBA 3

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LBA 1

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Fig. 42. Patagonian glaciations during the latest Miocene and Early Pliocene (from Rabassa et al., 2005). The dark bandscorrespond to radiometric dated lava flows; the dotted bands correspond to individual tills and the vertical line bandsrepresent inferred glacial events. Black triangles depict whether the lava flow is used as an upper or lower limiting age fora certain glacial episode. The columns at the left of the figure represent the chronological global scale in million years, theglobal paleomagnetic scale and the global marine isotope stage sequences. The Pampean biostratigraphic units, theirestablished biozones and stages and their time boundaries have been taken from Verzi et al. (2002).



192 Jorge Rabassa

climate became more extreme and pleniglacial conditionswere gradually achieved at lower latitudes since theAntarctic Peninsula was glaciated during the MiddleMiocene and piedmont glaciers occurred in Patagonia atthe Latest Miocene. This is due to the fact that, withshorter and milder cycles, the glacial conditions were notfunctional during such periods long enough so as to allowthe building and persistence of extensive ice fields in thePatagonian Cordillera. Only in the Late Pliocene wouldappropriate conditions have been reached so as to developa continuous mountain ice sheet, since latitudes from 36� Sto Cape Horn (56� S; Fig. 1c), which would have recur-rently grown in each subsequent glacial cycle during thewhole Pleistocene up to the Last Glaciation.

The high climatic variability recorded since the LateMiocene in Pampa and Patagonia was a consequence ofchanges in the astronomical, orbital parameters. These para-meters would have been predominant in different times:(a) equinoctial precession from the Late Miocene to theMiddle Pliocene, developing cycles of ca. 23–19 kyr duringthis period; (b) obliquity from the Late Pliocene to the EarlyPleistocene, with cycles of ca. 41 kyr and (c) eccentricityfrom the Middle to Late Pleistocene, with cycles of 100 kyr(see, for example, Ruddiman et al., 1986; Berger andLoutre, 1991; deMenocal and Bloemendal, 1995; Opdyke,1995). The shorter cycles would have impeded the forma-tion of the Patagonian mountain ice sheet from the LateMiocene to the Middle Pliocene, favoring instead the devel-opment of local glaciers, of which the sedimentary record isstill scarce (Rabassa et al., 2005).

5.3. The Correlation of the Patagonian Glaciationsand the Pampean Land Mammal Stages

A correlation of the Patagonian glaciations and paleoenvir-onmental conditions and the Pampean stratigraphy since theLate Miocene was proposed by Rabassa et al. (2005). ThePampean biostratigraphic units have been known sinceAmeghino (1889) and thoroughly described by Tonni andCione (1995), Alberdi et al. (1995), Pascual et al. (1996),Tonni et al. (1999a, 1999b) and Verzi et al. (2002), amongmany others. The geomagnetic sequence of the continentalPampean sequences, as presented by Orgeira (1990) andNabel et al. (2000), among others, has been the basic toolfor the correlation with the Patagonian glacial sequences.The correlation results have been shown in Figs 42–44.

The oldest Patagonian glacigenic deposit was formedduring the Late Miocene, in the Montehermosan SouthAmerican land mammal (SALMA) stage (Tonni andCione, 1995), although it is not yet clear if this corre-sponds to a glacial event during the colder events MISTG 20–22, in the Latest Miocene, or even somewhat laterduring MIS Si 4–Si 6 (Earliest Pliocene). In these periodsthe global temperatures would have been lower thanduring the Early Chapadmalalan (Early Pliocene) landmammal stage. In the Late Chapadmalalan, local glacia-tion would have taken place, at least in the Lago Viedmaarea (Fig. 1b, Site 25; Figs 42, 43).

Colder than present conditions appeared only sinceca. 2.6 Ma, in the Sanandresian land mammal stage.Before 3 Ma, the climatic conditions were always

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SourceStagesBiozones

22.15 CF

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VOROHUEAN

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MARPLATAN

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CF: Cerro del FraileLA: Lago ArgentinoLV: Lago ViedmaLBA: Lago Buenos Aires

Inferredglaciation

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IOS Age South American

Fig. 43.

AU20

Patagonian glaciations during the Middle and Late Pliocene (from Rabassa et al., 2005). See Fig. 42 forexplanation.




warmer than the last interglacials (Holocene, MIS 5 andMIS 7), according to the global isotopic record, with theexception of short events at 3.12, 3.3, 3.35, 4.8–4.9 and5.7–5.8 Ma, during the Chapadmalalan and Montehermo-san land mammal stages.

Loess-like beds have been mentioned in the olderPampean units, at least since the Montehermosan landmammal stage and perhaps even before (see, for exam-ple, Zavala and Quattrocchio, 2001). The Pampean loess/soil sequences are much more poorly developed thanthose that have been described in China (Rutter et al.,1991). This fact is probably due to either (a) the feeble

pedogenetic effect during the integlacials or (b) a power-ful erosional action over the interglacial soils during thecold cycles (Rabassa et al., 2005).

The Early Ensenadan land mammal stage is corre-lated by the recurrent glaciations at Cerro del Fraile(Fig. 43). The Late Ensenadan is characterized by thelargest extension of the Patagonian glaciers, at the GPG,and the subsequent, still important ‘‘Daniglacial’’ events.The Ensenadan stage is the epoch of the astronomicalshift from the 41 kyr to the 100 kyr cycle in the globalrecord, and the establishment of full glacial conditions intemperate areas.

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IOS Age South American Localities SourceBiozones Stages

0.05 E.(Amerhippus)neogeus

LUJANIAN0.1 LBA 10.15 VM,NH,LBA 2, 30.20.25 NH,LBA 4, 20.3 Megatherium

americanum0.350.4 NH 2

0.45 BONAERIAN0.50.550.6 160.650.7 C10.75 LBA 5, 40.80.85

0.90.95

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2

2.05 LA, CF 6,9

2.1 LA 6, 9

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uyam

a

62

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7270

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34

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L E

I S

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8?

6

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ENSENADAAN

18,22

Sources1: Guillou and Singer, 19972 : Rabassa and Evenson, 19963: Mercer, 19824: Ton-That et al., 19995: Sylwan, 1989 6: Mercer, 19767: Mercer, 19838: Meglioli, 19929: Rabassa, 199910: Mercer, 196911: Schellmann, 1998

LBA: Lago Buenos AiresVM: Malleo valleyNH: Lago Nahuel HuapiLA: Lago ArgentinoSO: Otway SoundT: Tronador MountCF: Cerro del FraileRG: Río Gallegos valley

Overlying basalt

Underlying basalt

Basalts layers

Till

J

Old

uva

iB

run

hes

Fig. 44. Patagonian glaciations during the latest Pliocene and the Pleistocene (from Rabassa et al., 2005). See Fig. 42for explanation.



194 Jorge Rabassa

During the Bonaerian stage (Fig. 44), several smallerglaciations and the ‘‘Gotiglacial 1’’ events (Early Illi-noian; MIS 8–16) took place. Finally, the Lujanan stagehosted the ‘‘Gotiglacial 2’’ event (Late Illinoian, MIS 6)and the Last Glaciation (Wisconsinan, MIS 4 to 2).

The analysis of the global isotopic record, the Pam-pean stratigraphy and the Patagonian terrestrial glacialevidence contradicts the Mid-Pliocene Antarctic fulldeglaciation hypothesis (following Bruno et al., 1997),because the climate would have stayed within the meanlevels of the glacial/interglacial cycles in that epoch, andthere is no regional evidence of such a very high sea levelthat could provide scientific support to this theory.

It must be concluded that during the lapse comprisedfrom the Montehermosan to the Lujanian stages, therewould have existed at least 50 complete cold-warmclimatic cycles, forcing the regional development of thePatagonian–Pampean ecosystems. These large-scaleclimatic variations should be taken into considerationwhen discussing paleoecological, paleobiogeographicaland evolutionary characteristics of the Late CenozoicPampean faunas.

6. Final Remarks

When the Antarctic Circumpolar Current became finallyestablished and the Andean ranges reached elevationscloser, or even higher, than their present positions, regio-nal climatic conditions allowed the frequent developmentof glaciation, from the end of the Miocene. However,glacierization could have started even before, perhaps11–12 Ma (Wenzens, 2006a) during the final phase ofthe Santacruzan–Friasan land mammal stage. However,these very early and supposedly extensive glaciationsneed further confirmation.

The high climatic variability that began in the LateMiocene is due to Milankovitch cycles. Equinoctial preces-sion would have been dominant during the Late Miocene andthe Early Pliocene, with cycles of ca. 23 kyr. Axis obliquitywould have been largely influential during the Late Plioceneand the earliest part of the Early Pleistocene, with cycles of41 kyr. Orbital eccentricity would have prevailed during thefinal portion of the Early Pleistocene and later until today,with cycles of ca. 100 kyr. The shorter climatic cycles ofearlier times would have impeded the formation of the Pata-gonian Ice Sheet as one single unit during the Late Mioceneand the Pliocene. Thus, it is assumed that only local ice capsand mountain glaciers would have developed until the EarlyPleistocene when the Patagonian Ice Sheet was finally estab-lished (Rabassa et al., 2005).

Though Pampean loess layers from the Late Mio-cene are known, their occurrence is more frequent sincethe Late Pliocene, during the Marplatan land mammalstage. The Pampean loess/soil sequences are not as wellpreserved as their Chinese counterparts. This is prob-ably due to either (a) a poor ‘‘Brazilian’’ (i.e. warmerclimate) effect on the Pampa environments during theinterglacial periods, when pedogenesis should havetaken place, or (b) intense erosion (deflation) duringthe colder, arid glacial periods. The thicker loess unitswould have been formed only during the 100 kyr cycles,

that is, the last 1.2 Ma, when the Patagonian Ice Sheetwas fully developed.

The ecological, faunal and floral, mobility of thePatagonian and Pampean regions, as well as of othermidlatitude areas in South America, would have beengreater also in these periods. The glacioeustatic move-ments would have been smaller during the Pliocene, witha smaller exposure of the present submarine platform, amore reduced climatic continentalization and lessextreme climatic events.

Colder-than-today environmental conditions would havebeen frequent only after ca. 2.6 Ma ago. Before 3 Ma,climatic conditions would have been warmer than even thelast interglacial periods, that is, the Holocene, MIS 5 andMIS 7, with possible exceptions at 3.12, 3.3, 3.35, 4.8–4.9and 5.7–5.8 Ma, according to the Southern Ocean 18O record.

The environmental and biogeographical changes thattook place during the LG and the last Termination wouldhave taken place at least 15 times during the last 1 Myr,since the GPG. Perhaps with smaller intensity, they couldhave occurred up to 100 times since the beginning of thePliocene. The ecological consequences of such climaticchanges are hard to quantify, but they must have beenhighly significant. They should not be ruled out whenstudying the Late Cenozoic biogeography, paleontologyand paleoenvironments of the Pampas and similar areasof Uruguay and southern Brazil.

Much more is known today compared with what hadbeen proved only three decades ago. See, for instance,Mercer (1976), or the discussion during the INQUA ‘‘TillCommision’’ Patagonian Regional Meeting in March–April 1982 (Rabassa, 1983; Fig. 45), or even the sum-mary in Rabassa and Clapperton (1990).

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Fig. 45. Group photo of the participants of the ‘‘INQUATill Commission South American Regional Meeting’’, SanCarlos de Bariloche, Argentina, during postmeetingfieldtrip in front of Castano Overo Glacier, MonteTronador (Fig. 1b, Site 12; photo taken by a fieldtripassistant in April 1982). The author is accompanied byseveral then graduate students (Andres Meglioli, AndreaCoronato and Elizabeth Mazzoni among them) and manydistinguished visitors, Cal Heusser, Linda Heusser, ErnestH. Muller, Art Bloom, Gerry Richmond, Trevor Chinn,Dirk van Husen, Robert Vivian, Jan Lundqvist, EdwardEvenson and Friedrich Rothlisberger, among others.




Much has to be done yet, but the construction of thePatagonian glacial chronology, the best land-based glacialrecord of the temperate zones of the Southern Hemisphereand one of the most complete in the entire world has slowlybut steadily developed. This has been possible, thanks to theefforts of many Argentinian and foreign scientists who havechallenged the huge Patagonian distances, loneliness andemptyness, lack of logistics, roads or services, just as thegreat Carl C:zon Caldenius did 75 yrs ago.

Much future work is needed to extend and consolidatethis chronology, which will provide a firm base for cor-relation with glacial and paleoclimate records elsewherein the world.

Acknowledgments

The author wants to dedicate this chapter to the memory ofDr Carl C:zon Caldenius, on the 75th anniversary of thepublication of his paramount work. The author is deeplygrateful to Dr John Mercer, who kindly introduced him tothe problem of ancient Patagonian tills in 1972, and Pro-fessor Francisco Fidalgo (Universidad de La Plata), whogenerously cosupervised his doctoral dissertation in 1974and provided him with the basic concepts and methodologyfor the study and correlation of Patagonian glaciations.Professor Felix Gonzalez Bonorino (Fundacion Bariloche),who was the main advisor of his dissertation, oriented himin the study of past and present glacigenic sediments.

The author also wants to thank all those colleaguesand graduate students who over the last 30 yrs havegenerously educated him or kindly worked with him onthe study of Patagonian glaciations: Calvin J. Heusser(deceased), Linda Heusser, Sigfrido Rubulis (deceased),Jorge Suarez (deceased), Edward B. Evenson, Stephen C.Porter, Andres Meglioli, Luis Bertani, Aldo Brandani,Daniel Cobos, Jose Boninsegna, Fidel Roig Junyent,Ricardo Villalba, Guida Aliotta, Gunnar Schlieder,George Stephens, Jim Clinch, David Serrat, CarlesMartı, Jaap van der Meer, Kenneth Kodama, DonaldEasterbrook, Chalmers Clapperton, David Sugden, NickHulton, Bradley Singer, Thao Ton-That, Dave Mickelson,Mike Kaplan, James Bockheim, Matti Seppala, AndreaCoronato, Claudio Roig, Juan Federico Ponce, OscarMartınez, Monica Salemme, Elizabeth Mazzoni andBettina Ercolano, among many others.

Dr Bradley Singer (Department of Geology and Geo-physics, University of Wisconsin at Madison, USA),Dr Robert Ackert (Harvard University, USA) and DrMichel Kaplan (Lamont-Doherty Geological Observatory,USA), generously provided published information andunpublished radiometric and cosmogenic data to supportthe interpretations of this work.

The author is greatly indebted to Professor Jaap van derMeer for his careful and dedicated review of a first draft ofthe manuscript, thus certainly improving this chapter.

This chapter is the result of more than 30 yrs of field-work in different Patagonian regions and lab work atCADIC-CONICET (Ushuaia), Fundacion Bariloche (SanCarlos de Bariloche), Universidad Nacional del Comahue(Neuquen) and other organizations. These investigationswere funded by many grants from CONICET, Agencia

Nacional de Promocion Cientıfica y Tecnologica(ANPCYT, Argentina), Parques Nacionales (Argentina),National Geographic Society (USA) and other institutions.CONICET supported this work with Grant N� 4305/97and other more recent grants.

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204 Jorge Rabassa

AUTHOR QUERY

Chapter 8

AU:1 ‘‘Rabassa, 2006’’ has been changed to ‘‘Rabassa, 2007’’ in order to match with the reference list. Is this OK?

(Here and elsewhere).

AU:2 ‘‘Coronato et al., this volume’’ has been changed to ‘‘Chapter 3’’. Please confirm whether this change is OK.

AU:3 ‘‘Wherli (1899)’’ has been changed to ‘‘Wehrli (1899)’’ in order to match with the reference list. Is this OK?

AU:4 ‘‘Groeber (1956)’’ has been changed to‘‘Groeber (1952)’’ in order to match with the reference list. Is this OK?

AU:5 ‘‘Ton That et al., 1999’’ has been changed to ‘‘Ton-That et al., 1999’’ in order to match with the reference

list. Is this OK?

AU:6 ‘‘Rabassa, 1996’’ is not listed in the reference. Please check.

AU:7 ‘‘Pankhe et al., 2003’’ has been changed to ‘‘Panhke et al., 2003’’ in order to match with the reference list. Is

this OK?

AU:8 ‘‘Pahnke et al., 2003’’ has been changed to ‘‘Panhke et al., 2003’’ in order to match with the reference list. Is

this OK?

AU:9 ‘‘McCulloch et al. (2005a)’’ has been changed to ‘‘McCulloch et al. (2005)’’ in order to match with the

reference list. Is this OK?

AU:10 ‘14.714C ka yr BP’ has been changed to ’14.7.14C kyr BP’. Is this OK?

AU:11 Please clarify whether this should be ‘‘1’’ or ‘‘I’’ in ‘‘Llanquihue 1’’, ‘‘Heinrich 1’’ in all occurences.

AU:12 Please clarify ‘‘Rabassa, et al. 1990a or 1990b or 1990c’’.

AU:13 ‘‘Clapperton and Sugden (1988)’’ has been changed to ‘‘Clapperton and Sugden (1989)’’ in order to match

with the reference list. Is this OK?

AU:14 ‘‘Clapperton and Sugden (1984)’’ has been changed to ‘‘Clapperton and Sugden (1986)’’ in order to match

with the reference list. Is this OK?

AU:15 ‘Map of South America during the LGM. Partly modified from Clapperton, 2003’ has been changed to ‘Map

of South America during the LGM. Partly modified from Clapperton, 1993’. Is this OK?

AU:16 ‘‘this volume’’ has been changed to ‘‘Chapter 12’’. Please confirm whether this change is OK.

AU:17 Please clarify ‘‘Heusser, 1989a or 1989b.’’

AU:18 ‘‘Tonni and Cione, 1995; Tonni and Carlini, this volume’’ has been changed to ‘‘Chapter 13’’. Please confirm

whether this change is OK.

AU:19 ‘‘Tonni and Cione, 1999’’ is not listed in the reference. Please check.

AU:20 ‘‘Schellman, 1998’’ has been changed to ‘‘Schellman, 1998’’ in order to match with the reference list. Is this

ok? (here & elsewhere)

AU:21 Please provide volume number and page range.

AU:22 Please provide publishing details for ‘Pascual, R., Ortiz Jaureguizar, E. and Prado, J. (1996)’.

AU:23 Please provide publishing place for ‘Rabassa, Rublis and Brandani, 1980’.

AU:24 Please provide teh publishing place for ‘Tyson, P. D. and Partridge, T. C. (2000)’.

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Els AMS LPCT 00008_Auq 22-1-2008 14:07 Page:1 Trim:210�297 mm Floats: Top/Bottom TS: Integra, India

8 Late Cenozoic Glaciations in Patagonia and Tierra del...

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