DETERMINING THE GLACIAL EQUILIBRIUM ... -...

Analele Universităţii de Vest din Timişoara, GEOGRAFIE, vol. XIV, 2004, pp. 9-32

DETERMINING THE GLACIAL EQUILIBRIUM LINE ALTITUDE (ELA) FOR THE NORTHERN RETEZAT MOUNTAINS,

SOUTHERN CARPATHIANS AND RESULTING PALEOCLIMATIC IMPLICATIONS FOR THE LAST GLACIAL CYCLE

A.U. REUTHER†*, C. GEIGER†, P. URDEA‡, H.-P. NILLER†, K. HEINE†

†Department of Physical Geography, University of Regensburg, Germany ‡Department of Geography, West University of Timisoara, Romania *Department of Physical Geography, Universitaetsstr. 31, D-93040 Regensburg, Germany, [email protected]

Abstract: The Carpathians are one of the dominating mountain ranges in Europe. Their transitional climatic situation between maritime (west) and continental (east) air masses arouses special interest with respect to the timing of the glaciations. In reconstructing the past history of the Pietrele-Nucşoara glacier complex on the northern slope of the Retezat mountains, Southern Carpathians, a combined geomorphological, pedological and geochronological (surface exposure dating) study was carried out. A detailed GPS survey assisted in reconstructing the paleoequilibrium line altitude (pELA) in a Geographical Information System (GIS). Two glacial stages were identified. Using exposure ages, the younger (Capra-Judele, M2) advance dates back to about 16.5 kyr, delayed in comparison to the onset of deglaciation in other mountain ranges like the Alps. The respective pELA-depression was determined to about 1050 m (present-day snowline, approx. 2900 m asl as reference level). Due to the absence of datable boulders, it was not possible to date the older (Lolaia, M1) advance. Hence, pedological investigations assisted in estimating the age of the Lolaia advance. Our results indicate that the maximum advance of the Pietrele-Nucşoara glacier complex corresponds to the early Wuermian (MIS 4?). The pELA-depression of the Lolaia advance during the early Wuermian was estimated to be approx. 1100 m (present-day snowline as reference level). Further investigations in other mountain ranges in the Southern Carpathians are required to confirm our preliminary interpretations.

Keywords: Retezat, Carpathians, wuermian, ELA, weathering indices, chronostratigraphy. Cuvinte cheie: Retezat, Carpaţi, Würmian, ELA, indici de meteorizaţie, cronostratigrafie.

A.U. REUTHER, C. GEIGER, P. URDEA, H.-P. NILLER, K. HEINE

1. INTRODUCTION

The Carpathians (Fig. 1) are one of the dominating mountain ranges in Europe and are an important mountain range between the Alpine, the Balcanic and the Himalayan orogens. The Carpathians are located in the transitional climate zone between the maritime air masses in the West and the more continental air masses in the East. This transitional climatic situation awakes special interest in respect of the the timing of the glaciations in this region. During the Pleistocene, only the higher mountain ranges (above ca. 1500 m) of the Carpathians were glaciated. Due to their low elevation and the continental climate, the glaciation of the Carpathians was patchy in nature (PAWLOWSKI 1936, MIHAILESCU 1966); the glaciation consisted primarily of cirque glaciation with small valley glaciers restricted to the mountain ranges and not extending into the forelands. No present-day glaciers or perennial snow patches can be identified in the highest mountains

Fig. 1. Overview map of the Romanian Carpathians with sketch of Retezat Mountains.

(HOREDT 1988). An absolute glacial chronology in the Carpathians has controversially been discussed (e.g. LEHMANN 1903, PAWLOWSKI 1936, MIHAILESCU 1966, NICULESCU et al. 1983 and 1987, URDEA 2004). Two major phases of glaciation and several smaller readvance or recessional moraines can be reconstructed from field observations. In the absence of absolute ages, this controversy is still a matter of discussion. URDEA (2004) has published a tentative chronology based on the correlation of the glacial advances in the Carpathians with the well-constrained glacial chronologies in the Alps and the Tatra mountains. He assigns the maximum glacial advance (local terminology: Lolaia phase) to the Rissian glaciation (MIS 6) and the second major glacial advance (local terminology: Capra-Judele phase) to the Wuermian (MIS 2) and correlates the different smaller glacial advances to the traditional Late-glacial Alpine phases (GLÜCKERT 1987). Our recent field work provides an absolute age constraint on

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Determining the glacial equilibrium line altitude (ELA) for The Northern Retezat Mountains…

the glacial advances in the northern Retezat mountains utilising surface exposure ages supported by pedological weathering indices. Our results determine the timing of the glaciation and underline the exceptional climatic position in this transitional mountain range. Based on detailed mapping and subsequent glacier modelling, a reconstruction of different paleo-glaciers and their respective equilibrium line altitudes (ELAs) was achieved. The ELA was calculated, thereby acting as an important indicator for climatic fluctuations.

2. STUDY AREA

The Carpathians are part of the Alpine-Himalaya orogen chain and are the dominating relief element in Romania (Fig. 1). The study area is located in the southern part of the Carpathian arc, the W-E running Southern Carpathians or Transsylvanian Alps. The Southern Carpathians form the highest mountain chain in the Carpathians and extend between the valleys of Timiş and Cerna in the West and the Prahova-Valley in the East. The Southern Carpathians consist of crystalline massifs separated by transversal valleys (e.g. Olt, Jiu, Sebeşul) and intramontane basins (e.g. Haţeg, Petroşani, Loviştea basin) (BERZA et al. 1994b, ZWEIGEL 1997). They are the main segment of the Carpathian fold and thrust belt. The Retezat mountains are part of the Danubian nappes that are of metamorphic character and intruded by an igneous body, the Retezat granitoid pluton (BERZA et al. 1994a, WILLINGSHOFER et al. 2001). The loess-covered Transylvanian basin extends to the north of the mountain chain. The Pietrele Valley is located in the central Retezat mountains with a homogeneous lithology of massive granitoide. The relief in the Retezat mountains is glacially overprinted, jagged mountain summits with elevations up to 2509 m asl (Peleaga peak) surmount the glacially smoothed relief as former nunataks (Fig. 2. The well developed U-shaped valleys are covered by thick glacial deposits. Periglacial and gravitational processes dominate the Holocene landscape (URDEA 1992, 1993). The Pietrele valley (ca. 45.4°N, 22.8°E) is a north-facing valley that is drained by the Pietrele river, with the Mureş and Theiß river as local erosion basis.

The climate in Romania is in general moderately continental but due to the topography locally variable (GĂSTESCU et al. 1975, COLDEA 2003). The climatic stations in the Southern Carpathians record mean annual temperatures of -2,5°C at 2502 m asl (Vf. Omu), 0.5°C at 2180 m asl (Ţarcu), 3.3°C at 1585 m asl (Parîng) and 4.4°C at 1450 m asl (Cuntu). The mountain stations record negative mean temperatures for over six months of the year. The mean annual precipitation of the stations is 1277 mm at 2502 m (Vf. Omu), 1178 mm at 2180 m (Ţarcu), 1400 mm at 1585 m (Parîng) and 1301 mm at 1450 m (Cuntu). Over 40% of the precipitation above 1500 m is snow or snowrain (GǍSTESCU et al. 1975, VELCEA and BADEA 1983, URDEA 2000). Continuous snow cover exists at 2502 m asl (Vf. Omu) for approx. 215 d/yr, at 2180 m asl (Ţarcu) for approx. 190 d/yr and at 1585 m asl

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(Parîng) for approx. 148 d/yr (URDEA and SARBOVAN 1995). In the Retezat mountains, the treeline (pinus cembra) is located at approx. 1800 m (STARMÜLLER and STARMÜLLER 1995, COLDEA 2003).

Fig. 2. Total glaciated area of the Retezat mountains at (1) the Lolaia-, (2) the Capra-Judele- and (3) the Beagu-advance (youngest glacial advance from Early Holocene).

3. METHODOLOGY

Fieldwork Detailed geomorphological mapping of glacial features was carried out in the

field to reconstruct different moraine sequences. The field work study areas focused on the Pietrele valley and three adjacent tributary valleys (Stânişoara,

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Galeşu, Valea Rea). Particular attention was paid to the two moraine deposits representing the two most extensive glaciations. We surveyed all moraine ridges with a hand-held Global Positioning System (GPS) Garmin eTrex summit© and an altimeter with an accuracy of 3-7 m; the data points were placed every 5-10 m. The different moraine deposits were correlated across the valley according to their geographical and altitudinal positions. In the cirques, the trimlines, transfluence steps and nunatak positions were surveyed in order to reconstruct peripherical glacial borderlines of the former glacier for different glacial advances (Fig. 3).

Fig. 3. Glacial features in the Pietrele valley as mapped in the field.

Soil pits (KP1, KP2, KP5, KP6) were excavated on two moraine systems in association with the most extensive glaciations (Fig. 2). The pits KP5 and KP6 are located at road cuts through moraine ridges. KP1 and KP2 are located in till sediments. KP1 is located on a plateau at the side of the valley and KP2 is located at the spur where the tributary valley Stânişoara feeds into the Pietrele valley (see Fig. 4 for location). Both are located in NE-facing positions on the moraine ridges. All soil pits reached the initial till sediment (Cv-horizon). The profiles were described and profiled in the field and samples were taken from each horizon. For surface exposure dating we sampled erratic boulders on moraines and glacially polished bedrock with hammer and chisel. No suitable erratic boulder could be found on the oldest moraines.

Digital elevation model A topographic map (1:25,000) with an equidistance of 10m contour lines

was used as a data basis for the Digital Elevation Model (DEM). Unfortunately, no high-resolution map was available. The map was georeferenced using geographical coordinates plotted on the map sheet and verified at obvious landmarks (e.g. summits). The contour lines were digitized on-screen and transformed into elevation points. A TIN (triangulated network) dataset was calculated from the triangulation of the elevation points. For better visualization, a three-dimensional view was plotted. The GPS data points were imported into a Geographical Information System (GIS) with the software package Fugawi©, transformed into

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the map projection (Gauss Krüger with the Krasovski ellipsoid) using the software Transdat© and the results were verified with test coordinates (e.g. cabin, summit).

Parameterization of the glacier The former glacier´s peripherical borderlines were reconstructed from the

DEM by correlating the location of moraine deposits, the trimline altitude and transfluence elevation. Careful interpretation of field data facilitated the construction of polygons serving as best estimation of the former glacier´s peripherical borderlines (respectively for both maximum glacial advances). A more detailed description of the reconstruction of the glacier´s surface is described in REUTHER (2004). A rough estimation of the glacier´s equilibrium line altitude (ELA) is calculated by means of the Höfer method (see section 4), dividing the glacier in an accumulation zone above the ELA and an ablation zone below the ELA. An estimation is hereby possible by applying well known glacier surface morphology principles (e.g. HESS 1904) and verified by comparison with ice-contour maps of present-day glaciers. The elevation of the lateral and terminal moraine crests were recognised as the elevation of the ice margin in the glacier´s ablation zone. The elevation of the trimlines and the cirque headwall respectively mark the ice margin in the accumulation zone. Interpolation between moraine crests on either side of the glacier determines the inclination of the glacier tongue. The maximum thickness of the glacier is located along the depth contour of the catchment area, represented by the flow path of rivers. In general, the longitudinal profile of a glacier shows a concave shaped surface in the accumulation zone (above the ELA) and a convex shape in the ablation zone (below the ELA). Contours run normal to the valley walls in the vicinity of the ELA (Fig. 5). This causes a shallow depression in the topography of the accumulation zone and an arching of the glacier surface at the glacier tongue (ablation zone). Contours in the ablation zone usually show a rapid change in curvature from the valley walls to the glacier surface due to high ablation at the ice margin. Contours in the accumulation zone however, show a smooth change in the direction from the bedrock to the ice-surface contour (Fig. 5b). Along the latitudinal axis, the contours are bulging in nature with the maximum bulging along the line of maximum thickness of the glacier. The curvature of the ice-surface contours are characterised by fairly straight lines in the transition zone, progressively more convexly curved in the ablation zone and more concavely curved in the accumulation zone of the glacier. The trimlines at nunataks pose as reliable control mechanisms for the bulging of the ice surface.

The ice-surface contours were reconstructed for two paleo-glacier advances. A grid of the ice surface was interpolated for each glacial advance and then visualized by means of a three-dimensional view (Fig. 4). This was achieved and carried out using the ArcGIS© 8.2 software with the extension packages Spatial Analyst and 3D Analyst.

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Fig. 4. Reconstructed glacier surfaces for the two glacial advances (M1 and M2) as indicated and the location of the four soil pits (KP) (scale varies with perspective).

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Pedological investigations and exposure ages The soil samples were analysed in the laboratory using standard analytical

methods and a number of samples were analysed to determine the soil profile´s condition of weathering. Bulk samples were initially dried at 40°C and then passed through a 2 mm sieve to separate the <2 mm fraction to be used for the analysis. To determine the grain size distribution of the <2 mm fraction, a combined sedimentation (pipette method) and wet-sieving method (DIN 19683, SCHLICHTING and BLUME 1995) was initialised. The pHCaCl2-value was measured with an electronic pH-meter after 30 min and 24 h. The soil colour (wet) was determined using the MUNSELL (1975) chart. Carbonate pre-tests were carried out with 10% HCl.

Fig. 5. Sketch of a) longitudinal profile of glacier with flow lines, b) contour lines of ice surface and curvature of bedrock contour lines in the contact to the glacier (modified from Schreiner 1997 and Hess 1904).

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Clay mineral analysis was carried out by using smear glass slides with different preparations (Mg, K, ethylene glycol solvation and heating, c.f. VÖLKEL 1995) and measured with the XRD (Siemens D 5000, Co-kα-beam). The diagrams were plotted using MACDIFF 4.2.5 (PETSCHICK 2001). The X-ray diagrams were interpreted following the clay mineral identification methods from MOORE and REYNOLDS (1997) and TRIBUTH and LAGALY (1991). Each sample was analysed for pedological sesquioxides (Fe, Al, Mn) on a flame-AAS (Solaar 939, ATI UNICAM). Pedogenic iron was extracted using dithionite citrate-bicarbonate to determine Fed (MEHRA AND JACKSON 1960), with acid ammonium oxalate to determine Feo (SCHWERTMANN 1964) and with Na-pyrophosphate to determine Fep (MAHR 1989). Sequential extractions were undertaken following the procedure described in VEERHOFF (1992) and VÖLKEL (1995).

The rock surface samples for exposure dating were prepared and analysed in accordance with Kohl and NISHIIZUMI (1992), IVY-OCHS (1996) and LICCIARDI (2000). Ages were calculated according to formulations published in LAL (1991) (scaling), STONE (2000) (scaling), HEISINGER et al. (2002a) (muon contribution), DUNNE et al. (1999) (shielding), GOSSE and PHILLIPS (2001) (shielding), using the production rate from STONE (2000).

4. THEORY ELA DETERMINATION

The definition of the ELA and other terms describing a similar mass budget index (e.g. snowline, firnline) has been controversially discussed (e.g. GROSS et al. 1977, Hoinkes 1980, Anonymous 1969, HAWKINS 1985). In this study, the ELA is defined as the altitude on the glacier where net accumulation is balanced by the net ablation of the glacier´s ice, integrated over the period of a budget year in accordance with UNESCO/IAHS (1970). The ELA is a commonly used and recognised parameter applied to characterize Pleistocene and Holocene climate conditions in mountainous areas and used to correlate former glacial advances. Vertical ELA fluctuations indicate changes in the mass budget of a glacier and consequently climatic changes. This mass budget determination approach is however simplified and the climatic interpretation of glacial fluctuations is time scale dependent. The time scale resolution that can be inferred from geomorphological evidence refers to a larger time scale and thus only produces time-integrated statements on glacial advances. In order to indicate that the ELA has been reconstructed for a certain glacial advance and hence producing an integrated value for a period of glacial equilibrium, the term paleoELA (pELA) has been used. The ELA depression indicates the vertical fluctuation between two different glacial advances. An ELA depression is always calculated relative to a defined reference level (e.g. present-day ELA).

As the ELA can only be determined for paleo-glaciers by interpreting geomorphological features or by extrapolations from studies on present-day glacier

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dynamics, different approaches have been suggested. The well-established methods have been applied in this study and are summarized below.

(1) The accumulation-area ratio (AAR) method is usually expressed as the area of accumulation of a glacier in relation to its total extent (ANONYMOUS 1969). Different authors have derived different ratios. A common used value is the AAR of 0.67 (GROSS et al. 1977, KERSCHNER 1990) for glaciers in the European Alps.

(2) The Höfer-method has historically been most frequently applied for calculating the pELA. The pELA is defined by dividing the vertical extension of the glaciated area in different fractions. The original Höfer method (HÖFER 1879, and as modified by MÜLLER 1978, HAWKINS 1985, KUHLE 1986) takes the vertical distance between the median elevation of the ridge that exceeds the glacier accumulation zone to the toe of the glacier. Other well known modifications to the Höfer method exist: (2a) The THAR method (toe-to-headwall-altitude ratio) assumes the location of the pELA at a constant proportion of the vertical distance between the highest (base of headwall or bergschrund) and the lower most glacial limit (e.g. HAWKINS 1985, MÜLLER 1978) and (2b) the TSAR method (toe-to-summit-altitude ratio) the vertical distance between the highest surrounding ridges belonging to the catchment and the toe of glacier to be divided (Louis 1955).

(3) The pELA can also be estimated by the maximum elevation of the lateral moraines (MELM) of a glacier (LICHTENECKER 1938). This method is based on the fact that lateral moraines can only form along the margin of the ablation zones. With respect to glacier dynamics, the deposition of lateral moraines is only possible where the flow lines of the glacier´s ice-body point outwards, conveying the englacially transported till to the surface (Fig. 5a) (ANDREWS 1975, CHARLESWORTH 1957, FINSTERWALDER 1952).

(4) The elevation of the cirque-floor (defined as the base of cirque-floor headwall) is also regarded as an indicator for the pELA, with the base of the cirque-floor headwall being an approximation of the pELA. Cirque glaciers have the highest velocity and thus the greatest erosional potential and mass turnover at the elevation of the pELA (Fig. 5a) (FLINT 1970, MÜLLER 1978, HASTENRATH 1971, ANDREWS et al. 1970, LOCKE 1990, CHARLESWORTH 1957, FINSTERWALDER 1952). The cirque floor represents the upper limit of the pELA.

The methods of pELA determination can be divided in methods that (1) account for mass balance dependency and are based on glaciological principles as do the different AAR approaches and the MELM approach, and (2) the methods that are simply based on topographic indicators such as the different Höfer approaches and the cirque-floor elevation method. The methods that incorporate mass budget relations are generally more reliable than the simplified geometric determination.

In this study, the aforementioned summarized methods were applied to the glacial setting of the study area. The results are listed in Table 1. The best approximation of the pELA will be discussed in Section 5.

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Table 1. Results for pELA calculation.

Capra-Judele Phase (16.5 kyr)

Lolaia-Phase (Early Wuermian)

AAR (0.67) 1848 m asl 1795 m asl Modified Höfer (0.5) 1735 m asl 1595 m asl TSAR (0.5) 1900 m asl 1775 m asl THAR (0.5) 1800 m asl 1675 m asl MELM (min) 1500 m asl 1450 m asl Cirque-floor (max) 2060 m asl 1930 m asl

5. RESULTS

Glacier extent In respect of glacial landscape mappings in the study area, P. Urdea´s in-

depth field knowledge from over a decade of research in the Retezat mountains was of tremendous benefit for the study. For the study we mapped erosional and depositional landforms representing therein the peripherical glacial borderlines of former glaciers (Fig. 6). A more detailed compilation of landforms can be found in URDEA (2000). The GPS-based survey and altimeter measurements assisted in precisely locating landform locations. This was of particular importance as high-resolution maps of the area are not accessible for civil use. With the combination of mappings, GPS-surveying and altimeter measurements, the moraine ridges were assigned to different glacial advances. A distinguishment was made between (1) a maximum advance reaching 1050 m asl (Lolaia) and (2) another major moraine system reaching 1300 m asl (Capra-Judele advance) and several recessional or small readvance moraines (Fig. 7). The more extensive Lolaia-advance will be termed M1 whilst the Capra-Judele glacial advance is termed M2. The reconstruction of the two paleo-glacier´s ice surfaces facilitates an visualization of the areal extent of the glaciation (Fig. 4). During the two maximum glaciations, the Pietrele-Nucşoara glacier complex was composed of the Pietrele valley and the tributary valleys to the West (Stânişoara valley) and the East (Galeşu and Valea Rea valley). The maximum advance (M1) and terminal moraine at 1050 m asl was joined by a small glacier (4.2 km²) from the Beagu valley.

Pedological investigations and exposure ages The combination of pedological investigations and absolute age dating

yields an age control for the glacial advances. The soil profiles KP1 and KP5 were excavated in moraines of the M1-advance (Lolaia) whilst KP2 and KP6 are located in moraines of the M2-advance (Capra-Judele). Strong periglacial sloping is unlikely to have occurred in the profiles KP5 and KP6 due to the well preserved

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prominent moraine ridge. KP1 and KP2 are more than likely influenced by solifluction processes due to their positioning (cf. section 3).

Fig. 6. Morphologically visible glacial and periglacial landforms in the study area (compiled from field survey, Urdea 2000 and Urdea 1988).

The parent material is identical for all profiles - weathered till from homogeneous granodiorit. Boulders in a finer sediment matrix have been identified throughout the profiles. The presumable loess influence in upper horizons (Ah) of each profile is reflected by a considerable increase in the silt fraction (Fig. 8). All samples are strongly acidic with a pHCaCl2 ranging from 3.3 to 4.7.

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Fig. 7. Sketch of location of all glacial advances in the northern Retezat mountains as an altitudinal profile with local names (based on data from URDEA 2000), pELA location for the Lolaia (M1) and the Capra-Judele (M2) advance are marked.

Significant field observations were made in relation to the weathering depth in the different profiles. The soil pits of one particular glacial advance are characteristically similar. For M1-advance (Lolaia) moraines, the soil development depth is 60 to >70 cm compared with 45-50 cm for the M2-advance moraines (Capra-Judele) (Fig. 8). The parameters for the weathering intensity in till suggest a fairly long period of soil development between the deposition of the two different moraine complexes. Clay mineralology analysis identifies the presence of chlorites (14Å mineral) in all horizons. Chlorite has been transformed into vermiculite and smectite1 only in the upper layers of the M1 till. In the M2 till, chlorites are however found throughout the profile. We interpret this as an indicator for significantly longer exposure time of the M1 till to weathering agents in contrast to the M2 till. This interpretation is supported by pedological sesquioxides. The soils that developed in the till of the M1 moraines have a total pedogenetic Fe content that is significantly higher than values in the B-horizons from the M2 moraines (average total content for M1 moraines: 0.85%; for M2 moraine: 0.1%). The Feo/Fed ratio, as indicator for the weathering activity, is significantly increased in the B-horizons of the M1 moraines (Feo/Fed ratio for M1 moraine: 1.8, for M2 moraine: 0.4) (c.f. Fig. 8). We interpret this as a more extended time period to surficial weathering of the M1 moraines. The lower Feo/Fed ratios in the upper

1 This transformation process was experimentally reproduced by Proust et al. (1987).

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horizons of all profiles is explained by the accumulation of allochtonous material (influence of loess) and is in accordance with MAHR (1998). It should however be noted, that the total sesquioxide concentration is fairly low and hence small changes in the total amount result in an amplified change in the ratio. All soil characteristical indicators suggest a significant older age for the M1 moraine (Lolaia) in contrast to the M2 moraine (Capra-Judele).

Fig. 8. Selected profiles from the moraines of the Lolaia (M1) and the Capra-Judele (M2) advance (see text), with a picture of the section, the laboratory results and a plot of the absolute values of the pedogenetic Fe-content (bars in the pictures are 10 cm).

Absolute ages could only be determined for the M2 moraine. The amount of boulders located on the M1 moraine were very few and small in size (height <30 cm) which we regard as not suitable for exposure dating in a glacial landscape. The

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lack of suitable boulders is another indicator for a longer moraine exposure to weathering. We dated 16 samples from erratic boulders on the M2 moraine as well as glacially polished bedrock surfaces in the ice transfluence step up in the cirque that was geometrically correlated to this glacial advance. We calculated an error-weighted mean of the 16 ages of approx. 16.5 kyr, assuming 5 mm erosion/kyr. Details on the exposure ages are discussed in REUTHER et al. (in preparation). The late-glacial age for the M2 advance (Capra-Judele) and the aforementioned strong indicators of an extensive weathering period on the older profiles (KP5, KP1) derived from soil analysis suggest that the maximum glacial extent in the Carpathians (M1, Lolaia advance) may have occurred during the early Wuermian (MIS4 or 5b) rather than during the Late Wuermian glacial stage (MIS2).

Determination of pELA and ELA depression pELA modelling is based on reconstructed glacier surfaces for both glacial

advances. The well preserved and precisely mapped erosional and depositional glacial features facilitate a fairly reliable modelling of the peripherical glacial borderlines in the Pietrele Valley, therein reducing the uncertainty associated with geometric data that the pELA modelling is based on. The simple geometry and the restricted extent of the Pietrele-Nucşoara glacier encourages the application of the aforementioned summarized pELA-determination approaches without having to implement adjustments to the methods. The glacier is N-E exposed with a mean slope of 9.2°. The reconstructed glacier surface and bed can be characterised by a gentle sloping subglacial topography and the absence of steep steps.

The different pELA determination approaches are based on algorithms that are presented in detail in REUTHER (2004). The MELM-method yields an absolute pELA minimum as the lateral moraines are more than often not fully preserved. The lateral moraines in the vicinity of the pELA in the narrow Pietrele valley, are not preserved or covered with Holocene block streams, thereby producing a very low MELM. The MELM value is hence interpreted as a minimum portraying the realistic altitude of the EL. The cirque-floor elevation defines the upper limit of the pELA. The cirque-floor elevation is determined by lithological constraints (cf. MEIERDING 1982) and hence does not produce a meaningful pELA approximation in our study area. Both of these methods have been intensively used in the literature but the approaches are not sensitive enough to produce high resolution results as feasible in a GIS-based study combined with exact field survey of glacial landforms (see Tab. 1 for values). The pELAAAR

and the pELAHöfer yield values that are located well within this defined altitudinal range. An empirically determined value of 0.67 was applied to the AA-ratio (GROSS et al. 1977, SLUPETZKY 1974), often applied to mid-latitude mountain ranges (e.g. MAISCH 1987, AMMANN et al. 2001, BRAITHWAITE AND MÜLLER 1978). In order to avoid the circular argumentation associated with the HÖFER approach (1879), we determine the mean elevation of the surrounding ridges in the GIS. An elevation (z-value) mean for

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each pixel that is part of the catchment area was taken and the resulting elevation of approx. 2150 m asl was applied in the calculations. As suggested in the original literature, the vertical distance was divided in half (0.5). This value is considerably lower than the pELA that was determined by the AAR-method (Tab. 1). For the modified Höfer methods, we also applied vertical distance (0.5) bisectioning to determine the pELAs (Fig. 9).

Fig. 9. Calculated ELAs on the glaciers for a) the Lolaia advance (M1) and b) the Capra-Judele advance (M2) (explanation see text).

The AAR-method is the most reliable method as it takes the glacier´s areal extent as well as the catchment geometry into account and is based on glaciological principles. The low values derived from the Höfer (0.5)-method can be eliminated when considering the effect of possible avalanche nourishment in the accumulation zone and debris cover of the glacier in the ablation zone. The effect of avalanche nourishment (Benn and Lehmkuhl 2002, SISSONS AND SUTHERLAND 1976) and debris cover (CLARK et al. 1994, BENN AND LEHMKUHL 2002, KULKARNI 1992) have been discussed in the literature. The steep cliffs surrounding the cirques and the long unglaciated ridges along the valleys possessing an average slope of 35° (max. 79°) and a total 3D-area of 7,4 km² enforce a significant increase in the total amount of snow delivered into the catchment area. If this area is included in the accumulation area of the glacier and a pELAAAR determined from an increased surface area, the pELAAAR is located approx. 100 m higher. Numerous avalanche tracks along the valley sides are evidence for avalanche activity. The Romanian

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“Valle Pietrele” means “valley of large boulders” originating from the frequent occurrence of block streams, blocky talus slopes and rock glaciers in the valley today (URDEA 1988) (c.f. Fig. 6). Supposedly, glacial erosion leads to an increased undercutting of the steep valley sides and results in the large subsequent debris delivery onto the glacier. Debris cover on a glacier reduces the ablation and thus increases the altitude of glacial mass budget equilibrium in the study area. The AAR (0.67) is a realistic pELA approximation for the study area. This study therefore suggests that the Höfer method ratio need be modified as has been recommended and carried out in a number of other studies (e.g. MEIERDING 1982, PORTER 1975, PÉWÉ AND REGER 1972). The altitude determined by the AAR-method resembles a Höfer ratio of approx. 0.65. Furthermore we point out that both avalanche nourishment and debris cover have more than likely occurred in the study area and thus the calculated pELAs are probably minimum values.

The ELA depression is a number that is relatively independent of local topographic and climatic influences and can hence be used when comparing glaciated areas.

In order to calculate the ELA depression, a reference level has to be determined. A modelled theoretical present-day snowline (HOREDT 1988) was applied in this study. The calculated values are listed in Table 2.

Table 2. ELA-depression relative to the modern snowline (HOREDT 1988) using the AAR (0.67)-value.

6. CONCLUSION AND SUMMARY

The late Pleistocene Pietrele-Nucşoara glacier represents a former dominant glacial ice surface in the northern Retezat mountains (Fig. 2). The glacier´s former catchment area has been sufficiently studied and mapped. The former glacier was a north-east facing cirque glacier with four glacier tongues. The glacial landforms including moraines, trimlines, transfluence steps, a U-shaped valley, roche motounées and cirque-floor lakes have been well preserved.

Detailed mapping, GPS surveying and pedological investigations supported by surface exposure dating led to the unequivocal distinction of two very extensive glacial advances. These moraines had been long identified by local geomorphologists but a glacial chronostratigraphy was only applied by correlating with glacial advances in other mountain ranges in the absence of absolute ages. An important parameter for glacier mass balance, the pELA, was modelled for these

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two glacial advances applying a GIS-software. The GIS approach yields reliable high-resolution information on the catchment and the reconstructed glacier surface. Spatial analysis, in particular planimetric and hypsographic curves, are very reliable and accurate and are thus important for the calculation of the pELA. CARRIVICK AND BREWER (2004) convincingly showed the inaccuracies that evolve in glacier parameterization when relying on traditional map interpretation. The ice-surface integrative AAR method approach yields the most reliable results for a ELA calculation. Applying the cirque-floor method and the MELM method in determining the maximum and minimum pELA thresholds proved very beneficial. In respect of palaeoclimatic conclusions and correlations regarding glacial advances, the ELA depression has been applied as a proxy value independent of local climatic and topographic conditions. We determined a pELA depression of 1052 m for the M2 advance and 1105 m for the M1 advance.

The deposition of the boulders on the moraines of the M2 advance (Capra-Judele) dates to approx. 16.5 kyrs. The early late-glacial age for the moraine deposition suggests that the glacier might have been restrained and hindered in its melting process in comparison with the onset of the deglaciation in other mountain ranges e.g. the Swiss Alps (e.g. IVY-OCHS et al. 2004). This may be due to local climatological influences or a possible thick debris glacier surface cover resulting in a retarded deglaciation in sheltered positions. Before this issue can be conclusively solved, further absolute age dating is required for the northern slopes of the Southern Carpathians.

The M1 advance (Lolaia) has up to now been referred to and associated with the Rissian glaciation (e.g. URDEA 2004) located at approx. 250 m lower than the terminal moraines of the M2 advance (Capra-Judele). We showed that these moraines (M1) were exposed to weathering for a considerable longer period than the younger moraines (M2) were. No evidence could be found for a glacial advance that occurred during the pleniglacial around 21 cal kyr BP synchronic with the maximum global ice extent. The freshness of the landforms, such as the well-preserved moraine ridges in this fairly steep terrain and the degree of weathering of the till suggest that the moraines were deposited within the last glacial cycle. Our results and the interpretation indicate a major glacial advance during the early Wuermian (MIS4 or cold stage of MIS5) on the northern slope of the Retezat mountains. During the MIS4, a maximum extensional glacial phase has already been reported in Europe for the Pyrenees (GARCIA-RUIZ et al. 2003), the French Alps (GUITER et al. 2005), the Massif Centrale (MONJUVENT and NICOUD 1988) and the Vosges (SERET et al. 1990). A paleoclimatic interpretation will be presented at a later date. Additional absolute age dating is required to resolve this question. Further work will be carried out in the other large north-facing mountain ranges in the Southern Carpathians, the Parîng and the Făgăraş mountains to confirm interpretations presented in this study.

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Acknowledgements

This study was funded by the German Science Foundation (DFG) under grant number HE 722/32-1. The University of Regensburg (AAA) provided travel grants for C. Geiger, S. Lell and F. Unger. We greatly appreciate help in the field from M. Ardelean and F. Vuia (Western University, Timişoara) and S. Lell and F. Unger (University of Regensburg). Thanks to. P. Navratil, J. Daferner and E. Ardelean (University of Regensburg), who assisted with Figures 1 and 2 and 6. S. Kunz (University of Regensburg) assisted with the soil analysis. A special thanks to J. Völkel (University Regensburg) for providing the XRD, AAS measurements and for discussion. We thank L. Duffy for checking the English language on the manuscript. Permission to enter the Retezat Nationalpark was granted by the National Park´s Director of Administration.

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