Late Weichselian Glaciation of the Russian High Arctic

Quaternary Research 52, 273–285 (1999) Article ID qres.1999.2082, available online at http://www.idealibrary.com on

Late Weichselian Glaciation of the Russian High Arctic Martin J. Siegert and Julian A. Dowdeswell Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, United Kingdom

and Martin Melles Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, Telegrafenberg A43, D-14473 Potsdam, Germany Received December 10, 1998

A numerical ice-sheet model was used to reconstruct the Late Weichselian glaciation of the Eurasian High Arctic, between Franz Josef Land and Severnaya Zemlya. An ice sheet was developed over the entire Eurasian High Arctic so that ice flow from the central Barents and Kara seas toward the northern Russian Arctic could be accounted for. An inverse approach to modeling was utilized, where ice-sheet results were forced to be compatible with geological information indicating ice-free conditions over the Taymyr Peninsula during the Late Weichselian. The model indicates complete glaciation of the Barents and Kara seas and predicts a “maximum-sized” ice sheet for the Late Weichselian Russian High Arctic. In this scenario, full-glacial conditions are characterized by a 1500-m-thick ice mass over the Barents Sea, from which ice flowed to the north and west within several bathymetric troughs as large ice streams. In contrast to this reconstruction, a “minimum” model of glaciation involves restricted glaciation in the Kara Sea, where the ice thickness is only 300 m in the south and which is free of ice in the north across Severnaya Zemlya. Our maximum reconstruction is compatible with geological information that indicates complete glaciation of the Barents Sea. However, geological data from Severnaya Zemlya suggest our minimum model is more relevant further east. This, in turn, implies a strong paleoclimatic gradient to colder and drier conditions eastward across the Eurasian Arctic during the Late Weichselian. © 1999 University of Washington.

INTRODUCTION

Today, glacier ice in the Eurasian High Arctic is restricted to a series of small ice caps, up to about 8000 km 2 in area and 800 m thick, on the archipelagos of Severnaya Zemlya, Franz Josef Land, Novaya Zemlya, and Svalbard (Fig. 1a) (Dowdeswell et al., 1997, in press). For the Late Weichselian, however, the dimensions and timing of ice-sheet growth and decay in the Eurasian High Arctic are critical to the calculation of changing global ice volume, and in accounting for global sea-level depression of about 120 m under full-glacial conditions (Fairbanks, 1989). In addition, general circulation models

of the atmosphere and climate during the last glaciation require the extent and size of the Eurasian ice sheet as an input parameter. Here we present a new reconstruction for the Eurasian High Arctic ice sheet, determined from numerical modeling and compatible with several new geological data sets (Svendsen et al., 1999). Solid-earth and ice-sheet numerical models have predicted a range of Late Weichselian ice masses in the Eurasian Arctic, from a thick and widespread marine ice sheet over the bulk of the region (Peltier, 1994; Grosswald, 1998) to several thinner ice domes, which may or may not have been linked (Lambeck, 1995; Siegert and Dowdeswell, 1995). This variability reflects, in part, the relatively sparse geological information formerly available on ice extent, thickness, and timing, particularly for the Russian sector of the High Arctic (Forman et al., 1997). Recently, however, a number of new geological investigations of terrestrial and marine sequences on and around the Russian High Arctic archipelagos (Fig. 1) have provided new knowledge on the dimensions and chronology of Late Weichselian ice sheet distribution (e.g., Forman et al., 1996, 1997; Polyak et al., 1997; Siegert et al., 1999; Hahne and Melles, 1999; Harwart et al., 1999; Møller et al., 1999). The extent of the Eurasian ice sheet during the Late Weichselian is known relatively well in central Europe, in the North Sea, and in the British Isles (e.g., Mangerud et al., 1996). There is also a consensus that Scandinavia, as well as the Arctic archipelagos of Svalbard, Franz Joseph Land, and Novaya Zemlya, was entirely glaciated during the Late Weichselian (Elverhøi et al., 1995). The northwestern and northern limit of the Eurasian ice sheet was located at the continental shelf break, where the ice margin was prevented from further growth into deeper water by rapid iceberg calving (Andersen et al., 1996). The recent acquisition and interpretation of glacial– geological information has lead to a new reconstruction of the extent of the Late Weichselian ice sheet within the Eurasian High Arctic (Svendsen et al., 1999) (Fig. 1a). Permafrost

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0033-5894/99 $30.00 Copyright © 1999 by the University of Washington. All rights of reproduction in any form reserved.

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FIG. 1. (a) Map of the Eurasian High Arctic. The ice-sheet model was run over a topographic grid of this area. (b) The region around and including Franz Josef Land and Severnaya Zemlya, over which ice-sheet results from the Eurasian High Arctic are extracted and examined in detail. The location of this area is denoted as a shaded region in (a). Bathymetric contours are at 100, 200, 300, 400, and 500 m. (c) Map of Severnaya Zemlya and Taymyr Peninsula indicating modern ice cover and the locations of geological field sites referred to in this paper. The location of mammoth remains are marked 1– 4, from which the following radiocarbon dates have been established: (1) 19,640 6 330 14C yr B.P. (LU-654A), tusk, outer part, and 19,270 6 130 14C yr B.P. (LU-654B), tusk, inner part; (2) 19,970 6 110 14C yr B.P. (LU-688) tooth; (3) 24,910 6 200 14C yr B.P. (LU-749A), bone, and 24,960 6 210 14C yr B.P. (LU-749C), bone; and (4) 11,500 6 60 14C yr B.P. (LU-610), tusk (after Makeyev et al., 1979 and Vasil’chuk et al., 1997).

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profiles and sediment sequences in modern High Arctic lakes indicate that the lowlands of the Taymyr Peninsula, south of Severnaya Zemlya (Fig. 1c), remained unglaciated during the Late Weichselian (Siegert et al., 1999; Hahne and Melles, 1999). This glacial-geological evidence means that the notion of a continuous ice sheet between the Kara and Laptev seas is unlikely. However, we are unable to discriminate between (1) a “maximum” reconstruction in which the Taymyr Peninsula represented the eastern boundary for the Late Weichselian ice sheet and (2) a “minimum” model where glaciation of the Kara Sea was more limited and ice growth on Severnaya Zemlya was restricted to the shorelines of the archipelago. A numerical model of the Eurasian High Arctic ice sheet is used to make predictions about the glaciation of the Kara Sea and Severnaya Zemlya. An inverse approach to modeling is adopted, where ice-sheet margins are forced to match those derived from recent geological investigations. The only parameter allowed to vary within this inverse procedure is the model’s paleoclimate (surface air temperature and accumulation). Thus, model results indicate the thickness and dynamics of the ice sheet over the eastern Eurasian High Arctic, and plausible scenarios for the paleoclimate in this region. NUMERICAL ICE-SHEET MODELING METHODS

In this paper, a simple ice-sheet model is coupled with a model describing the deformation and transport of watersaturated basal sediment. The ice-sheet model is centered around the continuity equation for ice (Mahaffy, 1976), where time-dependent change in the ice thickness of a grid cell is associated with the specific net mass budget of a cell, ­h 5 b s~ x, t! 2 ¹ z F~h, u!, ­t

(1)

where n is the flow-law exponent (equal, in this study, to 3), t b is the basal shear stress (Pa), r i is the density of ice (870 kg m 23), and g is acceleration due to gravity (9.81 m s 22) (Paterson, 1994). The mean ice temperature of a cell is set at 210°C; a temperature often used in isothermal ice-sheet models to determine the ice flow-law parameter A (Payne et al., 1989; Siegert and Dowdeswell, 1995). The velocity due to the deformation of water-saturated basal sediments, u b (m s 21), is determined by u b 5 h bK b

~ t b 2 t *! N2

(3)

(Alley, 1990), where K b is the till deformation softness (0.013 Pa s 21), h b is the deforming till thickness (m), and N is the effective pressure (Pa). Also, t*, the till yield strength, is

t * 5 N tan~ f ! 1 C

(4)

(Alley et al., 1989; Boulton and Hindmarsh, 1987), where u b is the horizontal velocity at the top of the deforming till (m s 21), C is the till cohesion coefficient (4000 Pa), and tan( f ) is a dimensionless glacier bed friction parameter (0.2) (Alley, 1990). The total velocity, u, is then the sum of the internal ice deformation and sediment-deformation velocities. The beddeformation process assumes that there is no change in the effective pressure, N, through the deforming till thickness, h b. Also, neither ploughing nor discrete shearing of the till is accounted for in the bed-deformation model (Alley, 1990). This model has been used previously to reconstruct the western margin of the Barents ice sheet by matching model results of sediment transport to measured volumes of glacigenic material over the Bear Island Trough mouth fan (Dowdeswell and Siegert, 1999). MODEL BOUNDARY CONDITIONS

where ¹ z F(u, h) is the net flux of ice from the grid cell (m yr 21) (a function of the ice thickness, h, and velocity, u, of a cell and its neighboring cells). In this study, the specific mass budget term, b s, is related to the annual surface accumulation and ablation, together with mass loss through iceberg production (Eq. (5)). Equation (1) is solved using an explicit finitedifference technique. The depth-averaged ice-deformation velocity, u i (m s 21), is calculated by (Paterson, 1994) 3

2 A t nb h , n12

(2a)

t b 5 r igh sin~ a !,

(2b)

ui 5 where

Topography of the Ice-Sheet Base The basal topography of the Eurasian High Arctic ice sheet is determined from modern maps and bathymetric information. It is assumed that 30,000 yr ago the bedrock elevation of the Eurasian High Arctic, Scandinavia, and the British Isles region was similar to that of today, such that the present bedrock elevation defines initial conditions in the model. The justification for this assumption is based on sedimentary evidence from central Svalbard and Scandinavia, which indicates interstadial conditions between 50,000 and 30,000 yr ago, when glaciers were not significantly larger than today (e.g., Mangerud and Svendsen, 1992). The bedrock elevation, or topography grid, over which the ice sheet was constructed, is composed of 74,400 (240 north by 310 east) cells, with a width of 20 km per cell. This topographic grid was calculated from a series of topographic and bathymetric maps, together with radio-echo

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sounding data on modern ice thickness where glaciers remain today (Dowdeswell et al., 1986, 1996, 1997). As the ice-sheet model runs, the topographic grid is continually adjusted to account for ice-loading of the crust using the isostasy method developed in Oerlemans and van der Veen (1984). Sea-Level Change and the Loss of Ice through Iceberg Calving During periods of glaciation, global sea level falls due to the displacement of water from the oceans to continental ice sheets, and vice versa during deglaciation. We use the lowlatitude sea-level curve for the last 30,000 yr (after Fairbanks (1989) and Shackleton (1987)) to determine the timedependent change in sea level. We assume that during the Late Weichselian, grounded ice covered the entire Barents Shelf. In order to grow an ice sheet that occupies the entire Eurasian High Arctic, we suppress iceberg calving in all shelf areas, but retain it at the deep-water continental margin. A depth-related calving function is employed to describe the amount of ice removed from the margin of the ice sheet (when it reaches the continental shelf break). The relation used is V c 5 70 1 8.33h w,

(5)

where V c is the calving velocity (m yr 21) and h w is the water depth (m). This has been deduced from a statistical analysis of calving glaciers from several polar locations, including Svalbard (Pelto and Warren, 1991). The credibility of a linear depth-related calving function has been enhanced by Hughes (1992), who detailed a depth-related physical mechanism by which glacier and ice-sheet calving might occur. As the model runs from nonglacial to full-glacial conditions, relative sea level is adjusted through a feedback involving eustatic sea level and the crustal response to ice loading (Siegert and Dowdeswell, 1995). Because the water depth of the calving front determines the rate of iceberg calving (Eq. (5)), the rate of calving will be affected by relative sea-level change through the model run. Paleoenvironmental Forcing The numerical model requires paleoclimatic inputs in the form of air temperature and precipitation, and their timedependent behavior with respect to altitude and location. However, there is a lack of continuous proxy-records from which to reconstruct the climate history of the Eurasian High Arctic region. In order to construct a simple paleoclimate for the period of the last glaciation, a number of assumptions are made. It is first assumed that, at present, the climate of the Svalbard–Barents Sea region is similar to the altitude–precipitation relationship defined as “Polar Mix” (PX) by Pelto et al. (1990) and that the climate over Scandinavia is similar to that defined as “Sub-

Polar Mix” (SX) by Pelto et al. (1990). It is also assumed that, if the present Barents Sea moisture source were curtailed, then a more continental-type precipitation regime would operate, similar to the “Polar Continental” (PC) altitude–precipitation relation given by Pelto et al. (1990). This is supported by palynological investigations on the Taymyr Peninsula, to the south of Severnaya Zemlya, which indicate a cold and dry climate (leading to Arctic desert on the northern part and graminoid-herb-tundra on the southern part of the peninsula) during Late Weichselian time (Hahne and Melles, 1999). Thus, the eastern Eurasian High Arctic is described by a Polar Continental-type relation. In the numerical model, the equilibrium-line altitude (ELA) is related to temperature through an adiabatic lapse rate of 5.1°C km 21 (Fortuin and Oerlemans, 1990). Thus, a temperature depression of 3°C would move the ELA downward by 600 m. Fleming et al. (1997), through surface energy-balance modeling of northwestern Spitsbergen glaciers with a modern ELA of about 400 m, showed that typical depression caused by a 3°C temperature change would shift the ELA below sea level, implying some validation of the simpler approach adopted here. The air-temperature depression over the Eurasian High Arctic at the glacial maximum is set at 10°C (Manabe and Bryan, 1985). An assumption is made that, since glaciers on Svalbard were not significantly larger at 30,000 yr ago than today (Mangerud et al., 1992), the temperature conditions at 30,000 yr ago were similar to those of today. There is a very good correlation between high-latitude paleo-air temperatures (recorded in, for example, the Vostok Ice Core, East Antarctica) and indicators of global ice volume, such as the global sea level, carbon dioxide, or oxygen isotope curves (e.g., Siegert, 1993). Because of this, air-temperature change through time in the Svalbard–Barents Sea region can be calculated empirically by one of the three indicators of global ice volume (since the actual time-dependent function of air temperature is unknown for the Barents Sea region). Our results use the carbon dioxide forcing function, but previous experiments have shown that selecting either of the other available forcing functions has little effect on the ice-sheet size (Siegert and Dowdeswell, 1995). The model begins the glacial simulation under interstadial conditions 30,000 yr ago (Siegert and Dowdeswell, 1995; Dowdeswell and Siegert, 1999; Landvik et al., 1998) and ends after the attainment of full-glacial conditions 15,000 yr ago. In summary, the paleoclimate used in the initial model runs consists of (1) accumulation of ice in Scandinavia, Svalbard, and the eastern Eurasian High Arctic governed by SX, PX, and PC accumulation/elevation algorithms (Pelto et al., 1990), (2) air temperature depression of 10°C, which is forced to adjust the accumulation through an adiabatic lapse rate of 5.1°C km 21, and (3) air temperature change through the last 30,000 yr, which is matched to the CO 2 signal representing a proxy for global ice volume. The paleoclimate (mean annual surface

LATE WEICHSELIAN GLACIATION OF THE RUSSIAN ARCTIC

temperature and rate of ice accumulation) that results at 15,000 yr ago from our environmental boundary conditions is given later. GEOLOGICAL EVIDENCE FOR ICE-SHEET EXTENT

The eastern extent of the Eurasian ice sheet within the Barents and Kara seas is still debated. While many suggest a complete ice cover (Solheim et al., 1990; Forman et al., 1996), some propose a more-restricted glaciation, with the major parts of the Barents Sea being covered instead by perennial sea ice (Pavlidis, 1992; Velichko et al., 1997). Contradicting interpretations also exist for the extent of glaciation in the Kara Sea (Punkari, 1995; Grosswald, 1998), the ice extent on Severnaya Zemlya (Bolshiyanov and Makeyev, 1995; Pavlidis et al., 1997; Svendsen et al., 1999), and the ice advance onto the West Siberian mainland (Tveranger et al., 1995; Astakhov, 1997; Astakhov et al., 1999; Mangerud et al., 1999). Marine geological evidence indicates that the 400-m-deep St. Anna Trough, east of Franz Josef Land, was covered by ice at the maximum phase of glaciation, and may have been the site of a large ice stream that drained ice from the central Barents Sea into the Arctic Ocean (Polyak et al., 1997). In contrast, the Laptev Sea and most parts of the Taymyr Peninsula remained free of ice during the Late Weichselian. Evidence for the Laptev Sea, located east of Severnaya Zemlya, comes from the occurrence of Late Weichselian permafrost and polygonal ice wedge systems in this shelf area, neither of which can be formed beneath grounded glacier ice (Romanovski, 1993; Kleiber and Niessen, 1999). Geological evidence for ice-free conditions on the Taymyr Peninsula during the Late Weichselian, which we use as a boundary condition for our numerical model, comes from investigations of permafrost and lacustrine sequences on land and unconsolidated sediment obtained from modern lakes (e.g., Mo¨ller et al., 1999). For example, Melles et al. (1996) and Siegert et al. (1999) found evidence of continuous, syngenetic permafrost and ice wedge development since Middle Weichselian time at the shore of Labaz Lake, Taymyr Lowland (Fig. 1c), which clearly excludes the presence of Late Weichselian glaciers. Similar findings were made on the northern part of the peninsula, at the shore of Taymyr Lake (Kind and Leonov, 1982; Mo¨ller et al., 1999). Vasil’chuk et al. (1997) compiled the available radiocarbon ages of mammoth remnants in permafrost deposits. Altogether 62 dates, spread relatively evenly between .50,000 and 10,000 14C yr B.P., are widely distributed over the Taymyr Peninsula and indicate the continuous presence of mammoths during the Late Weichselian. Additional evidence for ice-free conditions since Middle Weichselian time comes from continuous limnic sedimentation in Levinson Lessing Lake, to the west of Taymyr Lake (Fig. 1c) (Hahne and Melles, 1999; Niessen et al., 1999; Ebel et al., 1999). Evidence for continuous limnic sedimentation in Lama Lake indicates that the western foreland of the plateau re-

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mained unglaciated (Hahne and Melles, 1997; Harwart et al., 1999). An exception is the Putoran Plateau on the southern Taymyr Peninsula (Fig. 1c), where glaciers in high-altitude areas existed during the Late Weichselian (Hahne and Melles, 1997; Svendsen et al., 1999). Hence, the last large-scale glaciation of the Taymyr Peninsula appears likely to predate the Middle Weichselian (Mo¨ller et al., 1999). Its occurrence during the Early Weichselian is indicated, for example, by glacial deposits and buried glacier ice of this age in permafrost sequences at the shores of the Yennisey River (Astakhov and Isayeva, 1988) and Labaz Lake (Siegert et al., 1999). Geological evidence for the Late Weichselian glaciation of Severnaya Zemlya is rather sparse. However, it indicates that ice extent over the archipelago was not significantly greater during the Late Weichselian than the present value of 18,300 km 2, or 50% of the archipelago (Dowdeswell et al., in press). For example, radiocarbon dates of mammoth remains indicate ice-free conditions at 25,030 6 210 (Lab. no., LU-749B; dated material, tibia), 19,970 6 110 (LU-688; molar), 19,270 6 300 (LU-654B; tusk), and 11,500 6 60 (LU-610; tusk) 14C yr B.P. (Vasil’chuk et al., 1997), all expressed as uncorrected radiocarbon years. The mammoth remains were sampled close to the modern ice margins on Severnaya Zemlya (Fig. 1c). This originally led Makeyev et al. (1979) to conclude that the glaciation was in fact smaller than that of today (in the period 20,000 –18,000 14C yr B.P.). Moreover, Velichko et al. (1984), on the basis of the same dating, indicated that the glacial maximum on Severnaya Zemlya occurred between 18,000 and 14,000 14C yr B.P. (or approximately 22,000 –16,000 cal yr ago). MODEL RESULTS

Our ice sheet was forced to match geological information by making adjustments to the paleoclimate. The ice sheet around Severnaya Zemlya and Franz Josef Land was then examined in more detail. The result represents a “maximum-sized” reconstruction for the Late Weichselian, although even this is very much smaller than the huge Arctic ice sheet of Grosswald (1998). A full series of ice-sheet sensitivity analyses was performed on this section of the Eurasian High Arctic ice sheet. Although we do not claim to be modeling the paleoclimate of the Late Weichselian Eurasian High Arctic explicitly, we regard the environment obtained after the conclusion of our inverse procedure as representing a plausible scenario. Thus, after completing our inverse approach to modeling, we obtain an ice-sheet reconstruction that is compatible with geological information from the Taymyr Peninsula region. A further experiment explored the possibility of a “minimum-sized” ice sheet within the Eurasian High Arctic, where ice growth over the Kara Sea is curtailed by an extreme polar desert environment. The paleoclimate obtained from this experiment is also presented.

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MAXIMUM ICE SHEET

In our initial numerical model runs it was concluded that the full-glacial ice sheet reconstruction over Severnaya Zemlya and the eastern Laptev Sea was too large to be compatible with geological features marking the location of nonglaciated terrain over the Taymyr Peninsula (Hubberten et al., 1996). The present and Late Weichselian climates over Severnaya Zemlya are characterized by low temperatures and low iceaccumulation rates compared with Svalbard to the west (Dowdeswell et al., in press). Consequently, it is assumed that the lack of ice to the east of the archipelago is due to a low rate of accumulation, rather than to an ice-surface ablation mechanism. The lowering of surface temperatures (a model input) by several degrees over this region had the effect of reducing the amount of precipitation, and therefore ice accumulation, over the Laptev Sea. In this way, we were able to restrict the ice margin of the Eurasian High Arctic ice sheet to the eastern Kara Sea and Severnaya Zemlya. Thus, the reconstruction became more compatible with glacial– geological information from the Taymyr Peninsula. The model indicates that, between 25,000 and 22,000 yr ago, several ice domes developed within the Eurasian High Arctic. Specifically, 25,000 yr ago the ice sheet was characterized by ice domes over Svalbard (400 m altitude, 400 m thick), Novaya Zemlya (400 m altitude, 400 m thick) and Scandinavia (1600 m altitude, 1200 m thick) (Figs. 2a and 2b). After 22,000 yr ago, ice continued to build up over Scandinavia and the Barents Sea. Ice flowed northward from Scandinavia into the Barents Sea, causing the marine portion of the ice sheet to thicken. By 20,000 yr ago the Svalbard–Barents ice mass became the northern component of a Scandinavian–Barents ice-sheet divide (Figs. 2c and 2d). The increase in ice thickness was moderated by the development of ice streams within the bathymetric troughs on the western and northern Eurasian continental shelves which drained ice from the Barents Sea (Fig. 1). Ice also flowed eastward across Novaya Zemlya, resulting in a continuous ice sheet over the Barents and Kara seas. The ice thickness in the Barents Sea was about 1000 m, whereas in the southwestern Kara Sea it was 900 m (Figs. 2e and 2f). At 15,000 yr ago the maximum ice thickness over Scandinavia was ;2700 m, while over the Barents Sea it was between 1500 and 1800 m (Fig. 2e). Maximum ice thickness across the Kara Sea varied from 1200 m close to Novaya Zemlya to a grounded ice margin along the eastern coast of the Kara Sea. The volume of the Eurasian High Arctic ice sheet 15,000 yr ago was calculated as almost 8,000,000 km 3 (Fig. 3). The model’s paleoclimate at 15,000 yr ago indicates a strong precipitation gradient across the Eurasian High Arctic, where the accumulation of ice is 40 cm yr 21 over the southwestern Barents sea but only negligible over the eastern Kara Sea (Fig. 4). The mean annual air temperature ranged between 216 and 222°C across the Barents and Kara seas. However, farther

east, the Latpev Sea climate was characterized by mean annual temperatures of ca. 230°C (Fig. 4). Growth of Ice over Severnaya Zemlya and the Northern Kara Sea A closer inspection of the ice sheet modeled between Franz Josef Land and Severnaya Zemlya (Fig. 2) allows the determination of a plausible Late Weichselian glacial history for the region. For 25,000 yr ago, we model the northern Barents Shelf as being covered by a relatively thin (,700 m) ice sheet (Figs. 2a and 2b). The basal ice-sheet topography manifests itself within the ice-surface morphology as small ice domes over Severnaya Zemlya, Franz Josef Land, and Novaya Zemlya that feed ice outward in a pseudo-radial fashion from each archipelago. At that time, the ice thickness over Severnaya Zemlya and the northern Kara Sea was 100 m, and 200 m in the western Kara Sea (Fig. 2a). By 20,000 yr ago, ice thickness over Severnaya Zemlya and the northern Kara Sea had only increased to about 150 m. Ice flow 20,000 yr ago was dominated by the ice divide located over the central Barents Sea and Novaya Zemlya, where the ice is about 1000 m thick (Figs. 2c and 2d). Although the ice sheet over the central Barents Sea grew between 20,000 and 15,000 yr ago, the ice thickness and surface elevation over Severnaya Zemlya and the northern Kara Sea remained largely unchanged during this period (Fig. 2). Thus, at 15,000 yr ago, the ice thickness across Severnaya Zemlya was 200 m in the south and the island was free of ice in the north. In contrast, ice thickness over Franz Josef Land was between 900 m in the south and 300 m in the north. The ice thickness within the central southern Kara Sea was about 900 m (Fig. 2e). The volume of ice across the Kara Sea (within an area denoted in Fig. 1a) was 400,000 km 3 about 15,000 yr ago (Fig. 3b). Ice Stream Development within the St. Anna Trough At 25,000 yr ago the grounded margin of the reconstructed ice sheet had not reached the mouth of the St. Anna Trough (Figs. 2a and 2b). Ice velocities were low and very little ice was calved along the northern ice-sheet margin, aiding the buildup of ice in the Kara Sea. Subsequent to grounded ice expansion to the shelf break along the northern margin of the Barents Sea at about 22,000 yr ago, ice streams were activated within several bathymetric troughs, draining ice from the central Barents Sea to the Arctic Ocean. By 20,000 yr ago, a grounded ice stream flowed over the entire St. Anna Trough. The ice velocity at the grounded margin of the ice stream was about 350 m yr 21 (Fig. 2d). By 15,000 yr ago, the ice velocity within the St. Anna Trough had increased to 650 m yr 21, draining 15 km 3 yr 21 of ice from the Eurasian High Arctic ice sheet (Fig. 2f). However, the mouth of the trough had deglaciated between 16,000 and 15,000 yr ago, causing an embayment of grounded ice east of Franz Josef Land (Figs. 2e and 2f). It should be noted that detail at this

FIG. 2. The “maximum-sized” Eurasian High Arctic ice sheet. (a) Ice thickness (m) 25,000 yr ago. (b) Surface elevation (m) 25,000 yr ago. (c) Ice thickness 20,000 yr ago. (d) Surface elevation (m) 20,000 yr ago. (e) Ice thickness 15,000 yr ago. (f) Surface elevation (m) 15,000 yr ago. Ice thickness contours are given at 50, 150, and 300 m, and thereafter at 300-m intervals. Surface elevation contours are given in 300-m intervals from 0 m. In each case, maximum ice-stream velocity (m yr 21) is indicated in surface elevation maps.

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identified with reference to the distributions of air temperature and ice accumulation (Fig. 4) and our iceberg calving algorithm (Eq. (5)). The ice sheet was found to be most sensitive to the alteration in accumulation, which produced a change in ice-sheet volume to between 360,000 and 430,000 km 3 (Fig. 5). The iceberg calving experiment yielded an ice volume of between 340,000 and 400,000 km 3, whereas the sea-level test showed a variation between 385,000 and 405,000 km 3. One other sensitivity experiment involving large (610%) changes in the surface air temperature (and hence the ELA) produced comparatively smaller changes in ice volume (Fig. 5) and is therefore not considered as important to the ice-sheet sensitivity. Furthermore, significant changes (610%) in the dynamics of the ice-sheet model (i.e., parameters relating to the calculation of the flow of ice and the deformation of basal sediment) produced smaller ice-volume variations than the sea-level depression experiment. In conclusion, although the ice-sheet dimensions are most sensitive to alterations of imposed environmental conditions, relatively large (610%) changes in the inputs of accumulation, iceberg calving, and sea level do not adversely affect the main results and conclusions of this reconstruction. MINIMUM ICE SHEET

FIG. 3. The volume (1000 km 3) of (a) the Eurasian High Arctic ice sheet and (b) the ice-sheet section over the Severnaya Zemlya (as depicted in Fig. 1b) during the last 30,000 yr for both the maximum and the minimum reconstructions.

relatively small scale is at the limit of the resolution of our continental-scale ice-sheet model. Sensitivity of Model Results Several experiments were undertaken to examine the sensitivity of the ice-sheet dimensions around the Kara Sea and Severnaya Zemlya (Fig. 1) to changes in single environmental inputs used to force the model, where all other inputs were held at standard model values. The ice-sheet volume was recorded at 15,000 yr ago, over the area around Franz Josef Land and Severnaya Zemlya illustrated in Fig. 1b. Ice-sheet volumes recorded in these sensitivity tests are compared with the “standard” ice-sheet volume of 400,000 km 3, calculated from “maximum model” environmental forcing inputs. Inputs controlling directly the mass balance of the ice sheet (i.e., rates of accumulation and iceberg calving), and the maximum sea-level depression, were adjusted by a maximum of 610% of the standard value. The entire ice sheet is subject to variations in model inputs. Adjustments of 610% to eustatic sea level represented falls of 132 and 108 m below the modern level, respectively. The value of alterations to other variables can be

Further adjustment to the model’s palaeoclimate resulted in a situation in which accumulation of ice is curtailed across the Kara Sea and Severnaya Zemlya (Fig. 4c). Cold temperatures (222°C mean annual) do not permit surface melting, so ice is allowed to build up slowly via (a) sea-ice thickening over the Kara Sea and (b) ice flow from the Barents Sea. Under these extreme environmental conditions a small ice mass develops over the Kara Sea with a maximum thickness of 300 m at 15,000 yr ago (Fig. 6e). The ice volume across the Kara Sea was 160,000 km 3 (Fig. 3b), and 5,500,000 km 3 over the entire Eurasian Arctic (Fig. 3a); about 40 and 70% of the maximum model size in these respective areas. The ice surface of our minimum reconstruction is characterized by an ice divide over the Barents Sea, with development of major ice streams to the north and west of the continent within bathymetric troughs (Figs. 6b, 6d, and 6f). Ice flows more slowly from the ice divide east over Novaya Zemlya (Fig. 6). Large bathymetric troughs to the north of the Kara Sea remained largely free of grounded ice. These regions were surrounded by grounded ice at a depth of 200 m below modern sea level. We note that instability in the ice margin along the northern Kara Sea could lead to rapid, short-lived glaciation of these trough regions. A maximum ice thickness of only 50 m is modeled to the south of Severnaya Zemlya (Fig. 6), while in the north, similar to the “maximum reconstruction,” ice-free conditions existed. Thus, it appears likely that much of Severnaya Zemlya may have actually remained free of ice (e.g., nunataks and regions where topography inhibits the growth of ice).

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FIG. 5. Sensitivity of the “maximum-sized” ice sheet to variations in paleoenvironmental conditions. The graph shows how the ice-sheet volume 15,000 yr ago, over the Franz Josef Land–Severnaya Zemlya region (Fig. 1b), responds to 610% changes in (1) accumulation of ice (2) mean annual temperature depression, (3) sea level, and (4) iceberg calving.

CONCLUSIONS

A numerical ice-sheet model was used to describe the Late Weichselian glaciation of Severnaya Zemlya and the northern Russian High Arctic. An inverse approach to modeling was adopted, where numerical results were matched with geological information on the extent of the ice sheet. The environmental input to the model was varied in order to develop an ice sheet compatible with available geological information. Thus, plausible Late Weichselian paleoclimatic conditions and former ice extent in the northern Russian High Arctic were determined. ● The model calculates a continuous ice sheet over the Eurasian Arctic and northwestern Europe. The region around Severnaya Zemlya and the St. Anna Trough was then examined in more detail. In this way, the model accounts for the flux of ice from the central Barents and Kara seas northward to the continental margin north of Severnaya Zemlya. ● Two glacial scenarios are developed from the geological literature. The first (a “maximum-sized” ice sheet) is a full Late Weichselian glaciation of the Barents and Kara seas, with an ice margin on the west coast of the Taymyr Peninsula. In the second (a “minimum-sized” ice sheet) ice growth is restricted

FIG. 4. Paleoclimate inputs 15,000 yr ago for (a) mean annual accumulation of ice (mm yr 21) in the “maximum” reconstruction (b) mean annual temperature (°C) in the “maximum” ice sheet reconstruction, and (c) mean annual accumulation of ice (mm yr 21) in the “minimum” ice sheet reconstruction. Mean annual air temperatures for the minimum ice sheet reconstruction are not significantly different from those of the maximum reconstruction in (a).

FIG. 6. The “minimum-sized” Eurasian High Arctic ice sheet. (a) Ice thickness (m) 25,000 yr ago, (b) surface elevation (m) 25,000 yr ago, (c) ice thickness 20,000 yr ago, (d) surface elevation (m) 20,000 yr ago, (e) ice thickness 15,000 yr ago, and (f) surface elevation (m) 15,000 yr ago. Ice-thickness contours are given at 50, 150, and 300 m, and thereafter at 300-m intervals. Surface-elevation contours are given in 300-m intervals from 0 m. In each case, maximum ice-stream velocity (m yr 21) is indicated in surface elevation maps.

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by adjustments to the model’s paleoclimate (curtailing the rate of accumulation of ice across the Kara Sea). ● In our maximum ice-sheet reconstruction, the onset of glaciation occurred about 28,000 yr ago. Ice grew initially from the archipelagos that surround the Barents Shelf, forming a series of relatively small ice domes. Thus, between 28,000 and 25,000 yr ago, ice flowed in a pseudo-radial fashion from the center of these land masses (Fig. 2). After 25,000 yr ago, ice buildup over the central Barents Shelf occurred. The eastern ice margin of the Eurasian High Arctic ice sheet was located west of the Taymyr Peninsula, leaving this landmass free of ice. Between 20,000 and 15,000 yr ago, an ice divide existed over the central Barents Sea, separating the eastward and westward flow of ice. The ice sheet was drained by ice streams within the bathymetric troughs along the western and northern continental margin. By 15,000 yr ago, ice flowed eastward from the Barents Sea across Novaya Zemlya, and northward of this archipelago toward the Arctic Ocean (Fig. 2). ● At 15,000 yr ago, ice thickness over Severnaya Zemlya varied from 200 m in the south to ice-free conditions in the north (Fig. 2). Across Franz Josef Land at this time the ice thickness varied from 900 m in the south to 300 m in the north. An ice stream within the St. Anna Trough (located between these archipelagos) drained about 15 km 3 yr 21 of ice from the ice sheet. The maximum velocity of the ice stream at 15,000 yr ago was 650 m yr 21. ● Our minimum ice-sheet reconstruction indicates that if the precipitation across the Kara Sea were suppressed by a polar desert environment, ice flow from the Barents Sea eastward across Novaya Zemlya would allow a small grounded ice mass to form with a maximum ice thickness of ;300 m (Fig. 6). In this reconstruction, ice thickness over Severnaya Zemlya is not significantly different than that of today. ● Limited geological evidence from Severnaya Zemlya is compatible with our minimum model, while geological information from the Barents Sea is in closer agreement with our maximum-sized ice sheet. ● Examination of the palaeoclimate required for our ice sheet reconstructions shows that, to avoid growth of ice over Severnaya Zemlya, a climatic gradient is required over the Kara Sea limiting the accumulation of ice in a polar desert setting (Figs. 3 and 6). ACKNOWLEDGMENTS Funding for this project was provided by Nuffield Grant SCI/180/95/54/G to M.J.S. and NERC Grant GR3/9958 and EC Grant ENV-CT97-0563 to J.A.D. The paper is a contribution to the European Science Foundation program on Quaternary Environments of the Eurasian North (QUEEN). We thank two anonymous referees for providing constructive reviews.

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