Journal of Paleolimnology20: 353–368, 1998. 353c 1998Kluwer Academic Publishers. Printed in the Netherlands.

Post-Wisconsinian paleoenvironments at Salinas del Bebedero basin, SanLuis, Argentina

Miguel A. Gonzalez1 & Nora I. Maidana21 CONICET, Servicio Geologico Minero Argentino & Carl C:Zon Caldenius Foundation. C.C. 289 – Sucursal 13(B) – 1413 Buenos Aires, Argentina (e-mail: postmaster@funcar.org.ar)2 Universidad de Buenos Aires & CONICET, Dpto. de Biologıa, Fac. de Cs. Exactas y Naturales – Pabellon 2,Ciudad Universitaria, 1428 Buenos Aires, Argentina (e-mail: nim@biolo.bg.fcen.uba.ar)

Received 12 March 1996; accepted 16 December 1997

Key words:Argentina, Holocene, paleolimnology, diatom assemblages, paleoclimatology, paleoenvironments,brackish water

Abstract

We present a climatic reconstruction of Holocene lacustrine episodes in the Salinas del Bebedero basin (Argentina),based on geological and diatom information.

Morphological, sedimentological and diatom evidence between 11 600�140 yr BP and 325� 95 yr BP, allowedus to interpret the paleoenvironments of the basin. Episodes of high energy (sandy levels) are linked to large inflowof meltwater through the Desaguadero River, related to development of glaciers on the Andes. This inflow ischaracterized by peaks of relative abundance of the brackish water diatomCyclotella choctawatcheeanaPrasad.The values ofC. choctawatcheeanadecrease in deposits of low energy (clay levels), where it co-dominates witholigohalobousFragilaria andEpithemiaspp.

To the last two peaks of large inflow of meltwater, radiocarbon dates corrected to sidereal ages, are AD 1280/1420and AD 1443/1656. These ages agree with two cold episodes clearly recorded in dendrological studies from thePatagonian Andes and were correlated to the Little Ice Age. Thus, older Holocene episodes of large inflow of waterto the basin were correlated with the Neoglacial Advances defined by Mercer (1976) for the Andes.

Introduction

Researchers of Buenos Aires University, ServicioGeologico Minero Argentino, Carl C:zon CaldeniusFoundation and CONICET, carried out studies in theSalinas del Bebedero basin (66�450 W, 33�200 S,380 m asl, Argentina; Figure 1). These are part of theproject: ‘Late Pleistocene and Holocene paleoclimaticand paleoenvironmental behaviour of South Americanmid-latitudes’. It is a local contribution to the Pole-Equator-Pole Paleoclimate Initiative, PEP-I, in the PastGlobal Changes Project (PAGES), a Core Project of theInternational Geosphere-Biosphere Project.

Salinas del Bebedero is a tectonic basin that, dur-ing global episodes of cooling had inflow of meltwaterfrom the Andes through the Desaguadero River (Fig-ure 1); it generates high Lacustrine Stages (LS) in the

basin. The highest catchment area of the DesaguaderoRiver lies on Mesozoic marine sediments (mainly ofThitonian Age; Yrigoyen, 1979) which contain largeamounts of halite and gypsum. These salts are dis-solved and transported by this river, whose historicsalinity is ca. 20‰ (Deletang, 1929). In Salinas delBebedero, deposits and landforms of at least 5 latePleistocene LS, were identified (Figure 2). Gonzalez(1981; 1990; 1994) correlated those to glacial ‘pulses’(in the sense of Porter, 1981) which occurred in thehighest Andes between 28� and 34� S. The respectivemean14C ages of that LS, are: LS ‘A’: 20 000 yr BP; LS‘B’: 17 500 yr BP; LS ‘C’: 14 700 yr BP and LS ‘D’:13 700/12300 yr BP (see Geochronology; Table 1).

Gonzalez (1994) described evidence of anotherLS (LS ‘E’) centered around of 11 600�140 yr BP.He tentatively interpreted it as probably reflecting a

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Figure 1. Catchment area of the Desaguadero fluvial system.

posthumous glacial stage in the Andes. New field evi-dence allows the LS ‘E’ to be reinterpreted as beingdue to local rainfall by dominance of Easterly winds,during an early warm episode (Gonzalez, 1993).

Morphological and sedimentological evidence,also indicated two younger LS. The first of them hasan age of 630� 110 yr BP and the second one has amean age of 325� 95 yr BP (Table 2 and 3); they wererelated to the Little Ice Age (Gonzalez, 1993, 1994).

Maidana & Gonzalez (1990) and Maidana (1994),presented results concerning the diatom assemblagesfrom 11 600� 140 yr BP to the Little Ice Age. In thispaper environmental inferences from geological anddiatomological data, to this time span are presented.

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5

Figure 2. Schematic representation of studied outcrops.

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Table 1. Salinas del Bebedero. Paleoenvironmental data.14C dates and correlations of Lacustrine Stages

Sample Age (yr. BP.) Mean age Stages Deep of Lacustrine evidences

N� (see Table 2) water

���� undated 18th/19th high Late + 5/ dead forest; historical

centuries Little Ice Age + 6 m documents

AC-0706 300� 120 325� 95 high paleoberms;

AC-0379 350� 70 Little microcliff;

����� ������� ������� ���� Ice Age + 10 m archaeological

AC-0492 630� 110 630� 110 high settlement

���� undated ����� high Neoglacial ? paleoberms

Advances?(1)

���� undated ����� low warm ? features of erosion

(dried?) by local rainfall

���� undated ����� high Neoglacial ? paleoberms

Advances? (1)

AC-0906 8570� 130 top low Shallow archaeological

(episodic Hypsithermal

AC-0904 9070� 130 base flood?) (dried?) settlement

AC-0711 11 600� 140 11 600� 140 L.S. ‘E’ Early warmth ? deep floor sediments

AC-1183 12 270� 240 12 310� 220 Late

AC-1180 12 355� 205 (sub-set c) Llanquihue III?

����� ������� ������� (1) deep

AC-0708 13 200� 150

AC-0375 12 700� 190 2th ‘Pulse’? floor

AC-0374 12 700� 150 (2)

AC-0373 13 000� 160 13 020� 170 L.S. ‘D’ + 25 m sediments

AC-0371 13 290� 190 (sub-set b) ‘Younger Dryas-

AC-0369 13 260� 200 like’ and

AC-0105 13 000� 140 of

����� ������� ������� Bolivian paleoberms

AC-0710 13 850� 160 13 775� 160 Altiplano?

AC-0709 13 700� 160 (sub-set a) (3)

AC-0707 14 700� 180 14 700� 180 L.S. ‘C’ Llanquihue III + 25 m paleoberms

1th ‘pulse’

AC-0994 17 320� 270 17 410� 285 L.S. ‘B’ Llanquihue II + 25 m paleoberms

AC-0368 17 500� 300

AC-0993 20 140� 370 20 140� 170 L.S. ‘A’ Early + 25 m paleoberms

Llanquihue II?

���� undated ����� ? pre-Wisconsinian > 25 m deep floor sediments

(1) After Mercer (1976, 1982, 1984); (2) After Porter (1981); (3) After Lauer and Frankenberger (1984).

Characteristics of studied outcrops

Samples were taken in two geological sections sepa-rated by ca. 15 m, both on a naturally exposed outcropalong the Bebedero stream channel. Those sectionswere connected on the basis of lithological guide hori-zons.

Sediments are lacustrine silty-clays and fine sands,with two subaerial silty-sandy levels with abundantarchaeological material composed of lithic and food

remnants (Balbuena et al., 1982). CaCO3 increasestowards younger levels and gypsum crystals appear atthe top of the sedimentary sequence; being the productof the normal evaporitic evolution of the basin. Thesediments do not have recorded pollen, probably dueto the alkaline environment of burial dominated by theabundant CaCO3.

The lacustrine bed in the bottom (Figure 3) has11 600� 140 yr BP. The base of the older archaeolog-ical (subaerial) deposit has 9070� 180 yr. BP and its

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Table 2. Methodology to calculate mean ages of sets of dates, considering the respective methodological errorof each date. Dates of youngest berms in Salinas del Bebedero

Sample Age Range Mean extreme values: Mean age to the dated

N� (yr. BP.) of each age Maximum: mev feature (yr. BP.):

a Minimum: mev Ma =Mev + mev

2

maximum minimum Mev = mev =

value value

V = a+ error v = a– error mean (�x) mean (�x)

of V of v

AC-0706 300� 120 420 180

420 230 325� 95 (1)

AC-0379 350� 70 420 280

(1) The value of the error to this mean age is considered as: Mean age minus mev, or, in opposite form, Mevminus Mean age (after Gonzalez, 1992).

Table 3. Sidereal ages of the Little Ice Age at Salinas del Bebedero

Sample 14C age Mean Age Calibrated Ranges from intercepts (2)

N� (yr. BP.) (yr. BP.) (1) ages

AD BP one sigma two sigma

AC-0706 300� 120 325� 95 1523 427 AD 1443/1626 AD 1420/1690

1581 369 BC 507/294 BC 530/260

AC-0379 350� 70 1625 325

1304 646 AD 1280/1420 AD 1210/1450

AC-0492 630� 110 1371 579 BC 670/530 BC 740/500

1384 566

(1) see Methodology in Table 2; (2) after Stuiver & Becker, 1986.

top has 8570�130 yr BP. Younger and still undatedlacustrine deposits overlap those subaerial sediments.Towards the top there is the younger archaeological(subaerial) deposit, finally covered by the youngestlacustrine sediments whose age is 300� 120 yr BP.

Methods

To reduce the risk of contamination, sampling startedfrom the top (sample SB-1) towards the base (sampleSB-41) using sterile PVC pipes (2.5 cm in diameterand 15 cm in length). The pipes were pushed hori-zontally into the previously cleaned vertical front ofthe outcrop. Samples were obtained from each macro-scopically different sedimentological layer. Whereversedimentary uniformity was evident,samples were tak-en at an arbitrary distance of 7.5 cm.

Laboratory methods for diatom studies weredescribed in Maidana (1994). Principal componentanalysis and cluster analysis (‘BMDP statistical soft-ware’; Dixon, 1981) were performed considering only

those taxa whose relative frequencies were above 3%(Table 4) in at least one sample (non-diatomaceoussamples were not included).

Salinity tolerances of the identified taxa were main-ly taken from De Wolf (1982), with the following cat-egories:a) Polyhalobous taxa: forms with an optimum range

of 30‰ or more.b) Mesohalobous taxa: brackish water taxa with their

optimum and lower limit within the range 30‰ and0.2‰.

c) Oligohalobous taxa: taxa living in both brackishand freshwater.

c.1) Halophilous taxa: optimum in slightly brack-ish water.

c.2) Indiferent taxa: optimum in freshwater.

d) Halophobous taxa: exclusively freshwater taxa,strongly averse to chloride ions.According to qualitative and quantitative results of

diatom analysis (Maidana, 1994), samples were divid-ed in three groups (see Geology and diatoms):

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Table 4. Relative frequencies (%) of the main species (FR>3% in at least one sample).

Name Fig. Samples

8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22� 31 32 33 34 35 36 37 38 39 40 41

Achnanthes biasolettianaGrun. 1 a,b 0 0.3 0 0.4 3.4 1.5 1.2 1.8 0.05 0 0.4 6 6.8 4.1 3.8 1.6 0.5 1.3 10.1 0.6 0.05 0.05 – 0 0 0 0.05 0 0.4 2.5 0.5 0.2 0.1 2

Amhora coffeaeformis(Ag.) Kutz. 2 0 1.4 0.7 5.9 2.4 2.3 1 1.2 4.4 0.2 1 2.3 1.8 2.2 1.9 1.7 1.8 5.7 7.4 5.7 9 3.4 – 0 0 0 0 0 0.1 0.1 0 0 0.2 0

Cocconeis placentulavar. lineata(Ehr.) V. H. 3 0.4 2.6 3.6 2.2 14.7 10.7 5.5 12.5 0.1 0.9 1.6 2.5 1.9 3.1 2.5 2.1 2.7 2.3 2.2 1.5 0.8 0.5 – 0.7 0.3 0.7 1.5 1.8 2.3 2.3 1.7 1.8 5 5.2

Cyclotella choctawhatcheeanaPrasad 4 87.4 8.3 10.7 14.2 10.8 24.2 37.9 17.8 7.6 72.4 71.4 34.4 37.6 32 39.7 42.7 42.6 42.5 24.3 33.7 35.9 57.6 – 74.5 82.6 72.7 45.3 19.3 61.3 78.7 59 49.9 33.1 33.9

Cymbella cistula(Ehr.) Kirchn. 5 0 0 0.1 0 0 0 0 0 0.05 0 0 0.05 0 0 0 0 0 0 0 0 0.05 0.05 – 0.4 0.6 0.7 0.9 4.1 0.8 0.1 0.2 1 1.8 0.9

C. pusillaGrun. 6 0 10.3 0.1 0.6 1.5 1.3 4.9 5.8 5.5 5.3 5.1 4.4 6.2 8.9 6.8 10.6 3 0.7 1.1 4.7 1.1 0.5 – 0 0 0 0.1 0.1 0.05 0.3 0.3 0.3 0 0.1

Denticula elegansKutz. 7 0.8 0.1 0.2 0 0 0.3 0 0 0.2 0.6 0.3 1.3 0.4 0.8 0.7 0.4 0.8 1.1 0.6 2.1 3.6 1.8 – 3 2.5 0.9 1.1 1.9 2.3 0.6 1.9 1.8 0.9 1.8

Diatomella balfourianaGrev. 8a,b 0.05 0 0.4 0 0 0 0.2 0 0 0 0 0.1 0 0.05 0.05 0.2 0 0 0 0.05 0 0 – 0.1 0.3 0.1 0.7 0.9 4 1.6 3.9 3 2.9 3.2

Epithemia argus(Ehr.) Kutz. 9 2.9 0.3 0.05 0.05 0 0.1 0 0.1 0.05 0.1 0 0.1 0 0.05 0 0 0 0 0.05 0.1 0.1 0.05 – 3.2 1.8 4.8 5 12 5.3 1.7 8.8 4.5 6.4 2

Fragilaria brevistriataGrun. 10 0.7 0 0.2 0.05 0 0.2 0 0 0 0.2 0 0.05 0.1 0.1 0 0 0 0.05 0 0.1 0.1 0.2 – 2.6 1.5 8.1 27.5 30.7 9.2 3.4 4.9 21.7 29.3 32.9

F. construens(Ehr.) Grun. 11 0.05 0 0 0 0.05 0.1 0 0 0 0 0 0 0.05 0 0 0 0 0 0 0.1 0 0 – 0.8 0.1 0.6 4.2 6.9 0.7 1 1.4 2.5 4.5 8.6

Hantzschia amphyoxis(Ehr.) Kutz. 14 1.9 0.4 0.9 0.3 0 0.9 3.7 0.1 0.6 0.6 0.05 0.1 0.05 0.4 0.1 0.1 0 0.1 0.05 0.2 0.4 0.5 – 1.9 0.3 0.2 0.05 0.2 0.05 0.05 0.1 0.1 0.1 0

Hyalodiscus lentiginosusJohn 13 0.05 0.5 28.2 15.5 2.9 4.7 2.5 5.6 1.7 2.1 2 1.3 0.2 2.6 1.1 0.9 1.5 1.8 0.6 1.2 2.2 1.7 – 0.2 0 0.4 0.4 0.2 0.1 0.05 0.4 0.05 0.2 0.1

Mastogloia brauniiGrun. 25 0.05 15.3 24.1 22.3 10.7 6.2 3.8 17.1 1.3 6 2.2 3.1 1.6 4.5 2.7 2.4 3.9 5.1 2.4 6 6.2 4.9 – 0.05 0 0 0 0 0 0 0 0 0 0

M. elliptica (Ag.) Cl. 24 0.6 0.1 0.6 1.6 1.2 0.9 0.33 1.6 3 1.4 0.2 0.4 0.2 0.2 0.1 0.1 0.1 0.4 0.3 0.4 0.4 0.4 – 2.8 4.5 0.6 0.9 0.8 1.4 0.6 1.5 1.1 1.1 1.3

M. lanceolataThwaites 23 0 34.1 0.05 0.5 4.7 0.6 0.4 4.4 0.05 2.1 0.7 2.5 2.9 4.5 2.6 5.5 0.7 5.1 1.3 1.8 2 1.4 – 0 0 0 0 0 0 0 0 0 0 0

Melosira moniliformis(O. Mull.) Ag. 12 1.5 2.1 14.4 7.5 4.2 3.3 2.8 4.5 0.1 1 1.4 2.8 1 10.4 3.1 3.1 4.7 11.7 0.2 0.2 0.3 0.1 – 0 0.1 1.4 0.4 1.9 2.2 0.9 4.7 2 1.7 0.6

Navicula cincta(Ehr.) Ralfs 16 0 0 0 0 0 0.3 0 0.3 4.1 0.1 0.2 0.7 0.6 0.2 0.3 0.3 0.3 0.4 0.3 0.4 0.3 1 – 0 0 0.1 0.05 0 0 0 0 0 0 0

N. muticaKutz. 17 0.1 0.7 0.2 0.1 0.4 2.3 24.2 1 1 1.7 0 0.6 0.3 0.1 0 0.3 0.2 0.1 0.3 0.7 1.6 0 – 1.7 0.2 0.2 0.1 0 0.1 0 0 0.1 0.1 0

N. peregrina(Ehr.) Kutz. 18 0 0 0 0 0 0 0 0 0.05 0 0.4 0.05 0 0.05 0 0 0 0 0 0.05 0 2.2 – 0.7 0.1 0.3 1.3 3.4 0.3 0.1 0.5 0.6 1.2 1

N. perminutaGrun. 15 0.1 1 0.4 21.7 12.1 8.2 2.7 11.3 0.05 0 2.6 29 31 16.3 24.7 17.2 8.8 15.1 39.5 23.8 4.8 0.1 – 0 0 0 0 0 0 0.1 0.1 0 0 0.1

Nitzschia constricta(Kutz.) Ralfs 19 0.05 1 0.1 0.2 1.8 2.5 1.8 1.2 23.7 0.5 1.3 0.1 0.1 0.1 0.05 0.2 0.1 0.1 0.1 0.3 5.7 0.05 – 0.05 0 0 0.05 0.6 0.2 0.1 0.05 0.1 0 0.2

N. elegantulaGrun. 20 0 0.5 0.2 0.5 2.7 3.9 0.4 1.5 2.2 0.1 0 0.4 0.2 0.05 0.1 0.2 0 0.4 2.4 0.8 0.05 2.6 – 0 0 0 0 0 0.7 1.3 0.05 0.4 0 0.6

N. hustedtianaSalah 21 0.2 16.1 4.3 1.2 14.9 14 7.2 6.5 41.3 3.7 1.3 2.3 1 2.4 1.3 1.2 1.6 0.8 3.3 5.9 5 2.6 – 0.2 0.1 0.6 0.4 0.5 0.9 0.2 0.9 0.4 0.5 0.5

N. tergestina(Kutz.) Ralfs 22 0 1.8 0 0 3.3 0.2 0 1.8 0 0 0.6 0.5 2.1 0.6 2.1 4.1 9 0.1 0.1 0.1 0.1 0 – 0 0 0.05 0 0.1 0 0.05 0 0 0 0

Rhopalodia gibberula(Ehr.) O. Mull. 26 0.1 0.05 5.5 1.7 1.6 1.9 0.3 1.6 0.2 1.1 0.4 0.6 0.1 1.3 0.4 0.4 0.9 0.3 0.2 0.9 1 0.05 – 0.4 0.2 0.5 0.1 0.7 0.7 0.5 0.8 0.5 0.6 0.4

Tabularia fasciculata(Ag.) Williams & Round 27 0 0 0 0.6 2.1 1 0.5 1.1 4.4 0.9 1 0.6 0.3 0.4 0.1 0.05 0.1 0.1 0.1 1.1 6.8 0 – 0.1 0.1 0.5 0.1 0.2 0.05 0.05 0.1 0.1 0 0

� Non diatomaceous samples: SB-23 to SB-30.

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Figure 3. Cluster Analysis of samples SB-01 to SB-41. Non-diatomaceous samples were not included.

Sample Group I (Figure 3). Lacustrine sedimentsincluding the oldest levels (more than 9070 yr BP to11 600 yr. BP: SB-41 to SB-31) and the youngest one(ca. 300 yr BP: SB-01)

Sample Group II (Figure 3). Lacustrine sedimentsyounger than 8570 yr BP (SB-22 to SB-02).

Sample Group III (unshown in Figure 3). Subaerialsediments with archaeological evidences, and withoutdiatoms (9070 yr BP to 8570 yr BP: SB-30 to SB-23).

Geochronology

Geochronological analysis were performed bythe LATYR and INGEIS radiocarbon laboratories(Argentina). Dates and interpretations were publishedmainly by Gonzalez (1982, 1983, 1990, 1993, 1994)and are sumarized in Tables 1 and 2.

Mean ages coming from two or more dates, werecalculated according the methodology in Gonzalez(1992). This takes into account the methodogical errorof each date and is summarized in Table 3, with theexample of the dates to the youngest LS, related to theLittle Ice Age.

Results

Diatom analysis

To the 41 studied samples, diatoms appear only in the33 lacustrine ones. Among these, 115 taxa, includingspecies, varieties and forms, were identified (Maidana,1994). According to principal componentanalysis theyalso were distributed in three groups, as follows:

Diatom Group 1: The main species areEpithemiaargusKutz. (Figure 8–9),Fragilaria brevistriataGrun.

360

Figure 4. (a) Sand percent composition related to silt and clay.; b, c, d and e: Relative frequencies of the main diatom species; (b)Fragilariabrevistriata; (c) Nitzschia hustedtiana; (d) Mastogloia braunii; (e) Cyclotella choctawhatcheeana.

(Figure 4b and Figure 8–10),F. construens(Ehr.)Grun. (Figure 8–11),Navicula peregrina(Ehr.) Kutz.(Figure 8–18),Cymbella cistula(Hemprich) Grun.(Figure 8–5),Diatomella balfourianaGrev. (Figure 8–8) andDenticula elegansKutz. (Figure 8–7). Theyshow the highest relative frequencies in Sample Group1.

Diatom Group 2: The main species areNitzschiaconstricta(Kutz.) Ralfs (Figure 8–19) andN. husted-tiana Salah (Figure 4c and Figure 8–21). They havethe highest relative frequencies in Sample Group 2 andpredominate in SB-09.

Diatom Group 3: This group is composed mainlyof Mastogloia brauniiGrun. (Figure 4d and Figure 8–25),Melosira moniliformis(O. Mull.) Ag. (Figure 8–12),Hyalodiscus lentiginosusJohn (Figure 8–13) andRhopalodia gibberula(Ehr.) O. Mull. (Figure 8–26).

They appear in almost all the lacustrine samples butthey have the highest relative frequency (up to 25%)in the younger ones (Sample Group 2). Their relativefrequency in the other samples is< 0:1%.

Figure 7 shows the differences in salinity betweenthe Sample Group I (oldest: SB-41 to SB-31) and Sam-ple Group II (younger: SB-22 to SB-01). In the oldergroup, two episodes of high salinity (relative frequen-cies of mesohalobous taxa> 80%) alternate with twoepisodes of lower salinity (high relative frequencies ofoligohalobous taxa> 60%). Here the relative frequen-cy of Cyclotella choctawhatcheeanaPrasad (namedC.caspiaGrun. in Maidana, 1994; Figures 4e, 7 and Fig-ure 8–4) is in its minimum value (< 20%). This diatomis associated with brackish environments (Hakanssonet al., 1993). In turn, in the younger group the relativeabundance of mesohalobousand polyhalobous taxa are

361

Figure 5. Seasonal competence between Easterlies and Westerlies. 1. Conditions in summer (January); 2. Conditions in winter (July); 3. In 1000measurements of winds, Easterlies predominate 300 times more than Westerlies; 4. In 1000 measurements of winds, Westerlies predominate300 times more than Easterlies (Note that the influence of Westerlies shifts to the Northeast during cool conditions).

always high (> 65%), andC.choctawhatcheeanafluc-tuates between 7.6% and 87.4% of relative frequency.

Geology and diatoms

Sample Group I (SB-41 to SB-31; Figure 3) is datedbetween 11 600� 140 yr BP (SB-41) and 9070�180yr BP (SB-30: base of Sample Group III). At the base ofthe outcrop (SB-41) pure,fine sand, indicates relativelyhigh depositional energy (Figure 4a). The gastropodChilina parchappiiD’Orb., whose shells are present insamples SB-41 to SB-39, indicate water well aereatedby waves.

In samples SB-40 to SB-37, intercalated clay bedsindicate episodes of relative low depositional energy.That ‘quiet’ environment dominates in samples SB-35and SB-34, composed of pure clay. In samples SB-

33 to SB-30, intercalated sandy beds indicate renewedepisodes of increasing depositional energy.

Oligohalobous diatoms have two high peaks in SB-34/35 and SB 40/41 (Figures 4b and 7) with relativefrequency exceeding 30%. Their relative abundancefluctuates inversely with that ofC. choctawhatch-eeana (which is associated with brackish environ-ments). Those episodes of relatively high depositionalenergy (sandy levels) correspond with higher values inthe relative frequency ofC. choctawhatcheeana.

Sample Group II (SB-22 to SB-01; Figure 3) isdated between 8570�130 yr BP (SB-23; top of Sam-ple Group III) and 300�120 yr BP (SB-1). Severalinterbedded levels of sand and clay indicate a succes-sion of short episodes of high and low energy, respec-tively.

362

Figure 6. Main geomorphological and geological features at Salinas del Bebedero. 1 and 2: Regional faults; 3: Pleistocene sand dunes; 4:Holocene sand dunes; 5: Neotectonic uplift; 6: Microcliff (Little Ice Age); 7: Gypsum-clay dunes; 8: Bebedero creek; 9: Archaeological site;10: Late Pleistocene berms; 11: Holocene berms; 12 to 15: Lacustrine Stages A, B, C, D respectively.

Sample Group III (SB-30 to SB-23) is datedbetween 9070�180 yr BP and 8570�130 yr BP. Ithas abundant archaeologicalmaterial and is sterile withrespect to diatom frustules.

All samples in Group II have signs of soil develop-ment in a flooded environment, showing the proximityof the coast. This agrees with the relative abundanceof littoral and aerophilous species of diatoms. Here,

the oligohalobous taxa of Diatom Group 1 are almostabsent with relative abundance less than 0.2%.

Increases in the relative abundance of meso andpolyhalobous diatoms result from different groups ofdominant species: for instanceN. hustedtiana(Fig-ure 4c) andN. constrictadominate in SB-09, whileH. lentiginosusand M. braunii (Figure 4d) domi-nate in SB-03. Only one of these increases (SB-

363

Figure 7. Salinity tolerances of diatom taxa.

10) can be related to higher relative frequencies ofC.choctawhatcheeana(Figure 4e).

The increase in the relative frequency of meso-halobous species that prefer sulfate waters such asM. braunii and R. gibberula towards the top of thesedimentary sequence (SB-03 and SB-08), indicateschanges in the geochemical behaviour of sulfate,whichthemselves can be related to changes in variables suchas wind and temperature that affect evaporation (Mai-dana, 1994).

Discussion

To interpret by analogy the significance of the pale-olimnological evidences found in Salinas del Bebederoand other localities of South American mid-latitudeswith well developed proxy records, it is important toanalyze the current atmospheric behaviour.

Current atmospheric behaviour in South Americanmid latitudes

In a very simplified generalization (after Gonzalez1990; Gonzalez & Minetti, 1990), the atmosphericcirculation of South American mid-latitudes, includ-

ing Argentina, is dominated by two air masses comingfrom:

a) South Atlantic Anticyclone: it advects on thecontinent wet air masses from the Atlantic Ocean andproduces wet winds from the east and north-east (East-erlies).

b) South Pacific Anticyclone: it advects on the con-tinent wet air masses from the Pacific Ocean and orig-inates wet winds from the west and south-west (West-erlies). They are also called West Zonal Circulation(Minetti et al., 1982), Sub-Tropical Anticyclone (Har-rison et al., 1984) and Semi-Permanent Pacific Anticy-clone (SPPA; Minetti & Vargas, 1983). The SPPA hasbeen continuously advecting the Westerlies toward itseast and north-east continental areas. Thus, indirectly,its strength have been conditioning the access of thewet Easterlies on that areas.

The effects of those winds on mid latitudes of thecontinent are important and very different. Westerliesthat arrive in Argentina, first rose over the Andes (meanheight of about 5000 m asl, with several peaks higherthan 6000 m asl at the latitude of Salinas del Bebederobasin; that is the catching area of the Desaguadero Riv-er). On these mountains, by the ‘Topographic Factor’(Minetti et al., 1982) the winds are cooled and precipi-tate their moisture as ice and snow. Thus, they fall very

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Figure 8. 1a,b:Achnanthes biasolettiana(a: raphe valve, b: rapheless valve); 2:Amphora coffeaeformis; 3: Cocconeis placentulavar. lineata(rapheless valve); 4:C. choctawhatcheeana; 5: Cymbella cistula; 6: C. pusilla; 7:Denticula elegans; 8a,b:Diatomella balfouriana(a: valve view,b.- girdle view); 9:Epithemia argus; 10: Fragilaria brevistriata; 11: F. construens; 12: Melosira moniliformis; 13: Hyalodiscus lentiginosus;14:Hantzschia amphioxys; 15: Navicula perminuta; 16: N. cincta; 17: N. mutica; 18: N. peregrina; 19:Nitzschia constricta; 20: N. elegantula;21: N. hustedtiana; 22: N. tergestina; 23: Mastogloia lanceolata; 24: M. elliptica; 25: M. braunii; 26: Rhopalodia gibberula; 27: Tabulariafasciculata.

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dry on eastward plain areas, leading to current arid andsemi-arid environments.

On the contrary, Easterlies are not intercepted bymountainous barriers and arrive to the continent withtheir original moisture. For this reason, the currentrainfall in Argentina has been mainly oscillating at thesame way that the competence between Easterlies andWesterlies oscillate.

In addition, Minetti & Vargas (1983) showed theimportant seasonal shift of the SPPA and its influenceon the continent. In winter (July) the southern bound-ary of the SPPA is around 28�/29� S and in summer(January) it is at 33�/34� S. For this reason, in win-ter Westerlies are advected to northern latitudes andeastern longitudes with regard to summer. Thus, inthis season they generate a large north-eastward retreatof Easterlies and their moisture. On contrast, in sum-mer SPPA shifts to its southernmost location. In thatcondition Westerlies retreat to higher latitudes and lon-gitudes, smoothing the access of Easterlies to south-western Argentine areas.

This seasonal shift of the SPPA drives changes inthe dominance of the mid-latitudinal winds along theyear and the consequent changes of the rainfall. Fig-ure 5 (after Gonzalez & Minetti, 1990) shows the meanvalue of Easterlies vs. Westerlies in winter and in sum-mer, since 1941 up to 1960. Data are related to thedominance of Easterlies; thus, positive data indicatedominance of Easterlies and vice-versa. Note that the‘zero-line’ (line of equilibrium between opposite airmasses) alternatively advanced 350/400 km toward thenortheast in winter (cold season) and retreated towardthe southwest in summer (warm season).

Past (late Pleistocene) atmospheric behaviour inSouth American mid latitudes

Several works concerning late Tertiary and Quaternarypaleoclimates in South American mid-latitudes, indi-cated evidence of aridity correlated with cold episodes.Gonzalez & Trombotto (1990) reported a widespreadMiocene sand desert in the northern part of Patago-nia (Argentina). They presented evidence of its devel-opment under cold climate and tentatively correlatedit to the evidence of the Miocene glacial stages (tilland related deposits) reported by Mercer (1976, 1982,1984, 1985) on the South Patagonian Andes.

Late Pleistocene dry paleoenvironments are repre-sented in mid-latitudes of Argentina by transgressionaldune-fields (Gonzalez, 1981, 1983, 1989, 1990, 1994),wide sand-blankets (Gonzalez & Weiler, 1984), clay-

dunes, or ‘lunettes’ (Weiler & Gonzalez, 1988), andloess and loess-like deposits (Ferpozzi, 1992; Ferpozzi& Suriano, in press).

In eastern Uruguayan areas, the rocks of the SanMiguel Hills, currently covered by a dense forest, havetypical paleo-desertic features, such as ‘honey-comb’and ‘under-cutting’ (Gonzalez, 1990).

In Brazilian areas, Bigarella & Mousinho (1965)described late Pleistocene arid paleolandscapes in thesoutheastern region.Baker (1978) pointed out evidenceof arid phases during late Pleistocene times in theAmazonian Basin. Tricart (1969, 1974) and Thomas& Goudie (1984) indicated development of true sanddeserts, like ergs, in eastern Brazilian areas during latePleistocene.

In addition, in the Brazilian Atlantic shelf there areaditional evidence of past arid environments. Coresfrom this shelf showed dominance of feldspars relatedto quartz grains, suggesting less chemical weatheringin the Amazonian basin, perhaps due to less rainfall,during late Pleistocene (Damuth & Fairbridge, 1970).

Kowsman & Costa (1979) also indicated evidenceof aridity in the late Pleistocene Brazilian Atlanticshelf, at that time exposed by the glacioeustatic fall.Muehe (1983) also pointed out deposition of loess onthe subaerial exposed Brazilian Atlantic shelf duringLate Pleistocene times, indicating arid environments.

By analogy with the current atmospheric behav-iour, these evidences of past dry environments in SouthAmerican mid-latitudes during glacioeustatic fall, sug-gest that during global cold episodes, dried Westerliesreached much lower latitudes and longitudes than theircurrent influence in winter. That would be explainedby some climatic mechanisms that could have occurredsimultaneously, increasing their respective dry effect:

1) Flohn (1984) indicated an enhancement of theextratropical Westerlies during late Pleistocene globalcold episodes. That also was pointed out by Harrisonet al. (1984) for mid-latitudes of both hemispheres.

2) The lower oceanic temperature during globalcooling, would involve less oceanic evaporation and,thus, it would encourage arid environments in the SouthAmerican continent (Lauer & Frankenberger, 1984).

3) A northward shift of the SPPA, larger than itscurrent shift of winter (Figure 6) would have led toextremely arid environments in the continental areas.In this regard, researchers in the Bolivian Andes andAltiplano found an important increase of moisture,chronologically related to the main stages of the Wis-consinian glaciation (Kessler, 1963, 1984; Servant &Fontes, 1978; Lauer & Frankenberger, 1984). These

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researchers related such moisture to wet winds fromthe west (Westerlies) and to a probable shift of theSPPA to lower latitudes.

Possible past (Late Pleistocene and Holocene)behaviour of Salinas del Bebedero Basin

The glacial growth in the Andes under global coldconditions led to a large amount of seasonal meltwaterthrough the Desaguadero River (Gonzalez, 1988), par-tially flowing into the Salinas del Bebedero basin bythe Bebedero stream channel. This inflow generatedthe above commented high Late Pleistocene lacustrinestages (LS), whose ages agree with other evidence ofglacial advances on the Andes (Table 1). They will beanalyzed from younger to older samples.

Two posthumous inflows of meltwater from theAndes to Salinas del Bebedero (youngest LS), builttwo berms at roughly 10 m on the current floor of thebasin. The younger of them dated in the studied out-crops, has 325�95 yr. BP (mean of two dates; Tables2 and 3). The older one was dated in the oppositeside of the basin and has 630�110 BP. These radio-carbon dates corrected to sidereal ages (according tothe method of Stuiver & Becker, 1986; Table 2 and 3)range between AD 1280/1420 and AD 1443/1656 (onesigma), respectively.

Dendrological data from the Andes of Patagonia,indicate that two cold episodes occurred between AD1270/1380 and AD 1520/1680 (Villalba, 1990), whichwere correlated to the Little Ice Age. These dendrolog-ical ages agree with the sidereal ages of these youngerLS of Salinas del Bebedero and reinforce the correla-tion of the posthumous inflow of water to this lake withthe Little Ice Age (Gonzalez, 1993, 1994).

As shown by Maidana (1994), in the deposits relat-ed to these last inflow of water,C. choctawhatcheeanashows one of its highest peaks of relative abundance(peak I, Figure 4e). Since this planktonic diatom ofbrackish water appears in all the samples, we assumethat its peaks of relative abundance would indicatehigher salinity and high water level resulting frombrackish water supplied by the Desaguadero Riverthrough the Bebedero stream channel. Then, it is pos-sible to use this species as indirect indicator of coldstages at the Andes.

Thus, the following two high peaks ofC.choctawhatcheeana(II and III in Figure 4e) recordedfor the older samples, are interpreted as representingepisodes of large inflow of meltwater from the Andesduring cold conditions. Although these two peaks still

remain undated, it is possible to attempt some approx-imation to their chronology.

In this basin, Gonzalez & Weiler (1984) indicatedan episode of drying occurred at around of 5240� 100yr BP. They tentatively correlated it to global evidenceof glacial advances at ca. 5000 yr BP (Rotlisberger etal., 1981). In a similar way, the above mentioned peaksII and III of C. choctawhatcheeana, would be related tothe Neoglacial Advances indicated by Mercer (1976,1982, 1984, 1985).

Between 9070�180 and 8570� 130 yr BP (Sam-ple Group III), the lack of diatom frustules and theabundance of archaeological material indicate subaer-ial conditions at the sampled site allowing seasonalhuman settlements. After 8570� 130 yr BP, there arenew lacustrine deposits.

In basal samples (Sample Group I; between11 600� 140 and 9070�180 yr BP) peaks IV andV of C. choctawhatcheeana(Figure 4e) are interpret-ed as two important episodes of water inflow from theDesaguadero fluvial system, attributed to two glacial‘pulses’ in the highest Andes. In this group of samples,C. choctawhatcheeanafluctuates inversely to littoral,oligohalobous diatoms (i.e.Epithemia argus, Fragi-laria brevistriata, F. construens). The latter show twopeaks (relative abundance> 30%; Figures 4b and 7),that could be correlated with important local rainfallby dominance of Easterlies (warm conditions), as wasalso inferred by geomorphological (Gonzalez, 1993)and diatomological features (Maidana, 1994).

Conclusions

Peaks ofC. choctawhatcheeanaare interpreted asepisodes of meltwater inflow through the Desaguaderofluvial system, correlated to glacial ‘pulses’ in the high-est Andes, during episodes of strong Westerlies. Theabundance of this diatom fluctuates inversely to littoral,oligohalobousdiatoms (i.e.Epithemia argus, Fragilar-ia brevistriata, F. construens). That, in turn, would becorrelated with important local rainfall during warmepisodes and thus, during episodes of strong Easter-lies.

Thus, we found evidence of at least five episodes ofhigh water level in Salinas del Bebedero basin, relatedto water inflow from the Andes that could be due tofive cold episodes that occurred during the Holocene.The youngest of them were correlated with the LittleIce Age and the older ones to the Neoglacial Advances.

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In spite of their indirect origin, these are the moreprecise chronologies of the Little Ice Age in the higestAndes of Mendoza and San Juan provinces (Argenti-na). This youngest cold episode seems to produce twopeaks of water inflow to Salinas del Bebedero: theoldest of them occurred around of AD 1280/1420 andthe youngest one occurred around of AD 1443/1656.These ages highly agree with dendrochronologicalevi-dence of the Little Ice Age in the Patagonian Andes,that indicate two cold peaks at AD 1270/1380 and AD1520/1680, respectively.

The lack of diatoms in the sediments related tothe older archaeological site (Sample Group III), indi-cates a lower level of water in Salinas del Bebederobasin. It would be attributed to a lack of water inflowfrom the Andes that, in turn, could be attibuted to awarm episode. According to their chronologies (basal:9070�180 BP; top: 8570�130 BP), it would indi-cate an early Holocene warmth in South Americanmid-latudes.

Acknowledgements

We would like to thank Dr Jorge E. Wright for cor-recting the English language; to the Carl C:zon Calde-nius and the Buenos Aires University by several help.Special acknowledge to the kind suggestions from theeditors of JOPL and from the anonymous reviewers.

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