modelling the eurasian ice sheet through a full ... 2006 readings... · topographic and bathymetric...

Ž .Global and Planetary Change 31 2001 367–385www.elsevier.comrlocatergloplacha

ž /Modelling the Eurasian Ice Sheet through a full Weichselianglacial cycle

Martin J. Siegerta,), Julian A. Dowdeswella,1, Morten Haldb,2, John-Inge Svendsenc,3a Bristol Glaciology Centre, School of Geographical Sciences, UniÕersity of Bristol, Bristol BS8 1SS, UK

b Department of Geology, UniÕersity of Tromsø, N-9037, Tromsø, Norwayc Centre for Studies of EnÕironment and Resources, UniÕersity of Bergen, N-5020 Bergen, Norway

Received 7 December 1999; accepted 23 May 2001

Abstract

Recently acquired glacial geological and oceanographic datasets provide information on the Weichselian glaciations ofScandinavia and the Eurasian Arctic. A numerical ice-sheet model, forced by global sea level and solar insolation changes,was run to reconstruct ice sheets compatible with these data. A ‘maximum’ reconstruction assumes that the modern-typetemperature distribution across the Eurasian Arctic is reduced by 108C at three stages during the Weichselian, which arerelated to minimum levels of solar insolation. Conversely, a ‘minimum’ model incorporates a reduction in temperature ofonly 5 8C in Early and Middle Weichselian time. The ‘maximum’ reconstruction employs the relatively larger sea-level fallsuggested by thed18O deep-sea record, while the ‘minimum’ run uses the more conservative sea-level estimate from NewGuinea coral reef terraces. The maximum model predicts three major glacial advances in the Weichselian. These comparewell to geological evidence for ice-sheet growth during the Early, Middle and Late Weichselian. Geological evidence for theLate Weichselian ice sheet is compatible with either reconstruction if ice growth across the Taymyr Peninsula is curtailed.The models show that ice-sheet advance caused by the interaction of sea level and solar insolation changes yields atime-dependent ice volume function similar to that established from the geological record. Periods of seasonally open waterwithin the seas bordering the Eurasian Arctic generally occur prior to glaciation, and may provide a source of precipitationfor ice-sheet growth. In contrast, periods of ice-rafted debris deposition and depletion in surface-oceand18O in sea-floorsediments compare well with the model’s determination of ice-sheet decay and melting.q2001 Elsevier Science B.V. Allrights reserved.

Keywords: Weichselian; ice sheets; Eurasian Arctic; glacial cycle; numerical modelling

) Corresponding author. Tel.:q44-117-928-8902; fax:q44-117-928-7878.

Ž .E-mail addresses: [email protected] M.J. Siegert ,Ž [email protected] J.A. Dowdeswell ,

Ž [email protected] M. Hald , [email protected]Ž .J.-I. Svendsen .

1 Tel.: q44-117-928-9068; fax:q44-117-928-7878.2 Tel.: q47-776-44412; fax:q47-776-45600.3 Tel.: q47-555-84251; fax:q47-555-89687.

1. Introduction

The size and timing of the Late WeichselianŽ .glaciation of the Eurasian Arctic Fig. 1 has recently

been reconstructed by comparing numerical ice-sheetŽmodel results with geological evidence Dowdeswell

and Siegert, 1999; Siegert et al., 1999; Svendsen et.al., 1999 . The Late Weichselian ice sheet was initi-

0921-8181r01r$ - see front matterq 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0921-8181 01 00130-8

( )M.J. Siegert et al.rGlobal and Planetary Change 31 2001 367–385368

Ž . Ž .Fig. 1. a The location of the Eurasian Arctic. The model was run over a topographic grid of this region. b The maximum extent ofŽ .Weichselian ice sheets, adapted from Svendsen et al. 1999 .

( )M.J. Siegert et al.rGlobal and Planetary Change 31 2001 367–385 369

ated at about 28,000 years ago, and grew to its lastŽ .glacial maximum LGM extent of at least 5,000,000

km2 with over 2 km of ice thickness over centralScandinavia and a 1 km thickness of ice over theBarents Sea. Fast-flowing ice streams within bathy-metric troughs on the western and northern marginsof the Barents Sea correspond well with observedconcentrations of glacigenic sediments deposited at

Žthe mouths of major cross-shelf troughs Dowdes-.well and Siegert, 1999 . Ice extent to the east, over

the Kara Sea, was limited by a cold and dry climateŽ .such that most of the Taymyr Peninsula Fig. 1

Žremained free of ice Svendsen et al., 1999; Alexan-.derson et al., 2001-this volume , and the northern

Kara Sea was covered by only a few hundred metresŽ .of ice at most Siegert et al., 1999 .

Despite these recent advances in our knowledgeof the extent and dynamics of the Late WeichselianEurasian Ice Sheet, details about the ice sheet priorto the LGM have yet to be identified fully. In thispaper we employ a numerical ice-sheet model toreconstruct the form of the Eurasian Ice Sheetthroughout the Weichselian period, from the Eemianinterglacial at about 115,000 years ago to the start ofthe Holocene 10,000 years ago. The paper is organ-ised as follows. First, the numerical model is de-scribed. The model inputs and boundary conditions

Žare then detailed e.g. the palaeoclimate used to drive.the model . Following this, palaeoceanographic con-

ditions are outlined, some of which are used to refinemodel inputs while others are reserved for compari-son with model results. After this, geological infor-mation of the former ice sheet is provided. The papercontinues with the procedures employed in the nu-merical modelling experiments, followed by the ice-sheet results themselves. Finally, the model resultsare interpreted and compared with geological andpalaeoceanographic information in order to put for-ward a synthesis of glacial events for the Weich-selian Eurasian Arctic.

2. Model

The ice-sheet model used in our investigation isŽ .the same as that used in Siegert et al. 1999 . The

model is based on the continuity equation for iceŽ .Mahaffy, 1976 , where time-dependent change in

the ice thickness of a grid cell is associated with thespecific net mass budget of a cell:

Ehsb x ,t y=PF u,H 1Ž . Ž . Ž .s

Et

Ž .where F u,H is the net flux of ice from a grid cellŽ 2 y1. Žm year the flux of ice being the product of ice

.velocity, u, and ice thickness,H . In this investiga-tion, the specific mass budget term,b , is related tosthe annual ice-surface accumulationrablation and

Ž .iceberg calving from marine margins. Eq. 1 issolved using an explicit finite difference technique.

Ž y1.The depth-averaged ice velocity,u m year , iscalculated by the sum of depth-averaged internal icedeformation,u , and basal motion,u :i b

2 At nHbusu qu s qu 2aŽ .i b bnq2where

Est sr gH 2bŽ .b i

Ex

Ž .and n is the flow-law exponent equal here to 3 ,t bŽ .is the gravitational driving stress Pa ,r is the icei

Ž y3.density 870 kg m ,s is the ice-sheet surface andŽ y2 . Žg is acceleration due to gravity 9.81 m s Pater-

.son, 1994 . The mean ice temperature of a cell is setat y10 8C, a temperature often used in isothermalice-sheet models to determine the flow-law parame-

Ž y16 y3 y1 .ter A 10 kPa s ; Huybrechts et al., 1996 .As the ice-sheet model runs, the topographic grid iscontinually adjusted to account for ice-loading of thecrust after the isostasy method developed in Oerle-

Ž .mans and van der Veen 1984 .The processes by which large-scale glacial sedi-

mentation occurred over the Eurasian Arctic troughmouths during the Quaternary had the capacity totransport several thousand cubic kilometres of mate-rial within a few thousand years. Dowdeswell and

Ž .Siegert 1999 assumed that the deformation, andsubsequent down-slope transport of water-saturatedbasal sediment, are the major mechanisms by whichglacial sediments were transported to the ice-sheetmargin on the scale required. The model describingsediment deformation beneath an ice sheet is adapted

Ž .from Alley 1990 . The model allows for the rapidŽdeformation of basal till which is coupled to till

.geotechnical properties and basal stresses as a com-


ponent of the total ice velocity. The velocity due tothe deformation of water-saturated basal sediments,

Ž y1. Ž .u m s , is determined by Alley, 1990 :bt yt )Ž .b

u sh K 3Ž .b b b 2NŽwhere K is the till deformation softness 0.013 Pab

y1. Ž .s , h is the deforming till thickness m , andN isbŽ .the effective pressure Pa . The till yield strength,

t ), is derived from known geotechnical propertiesŽ .of the subglacial material Alley, 1990 . Full details

of howt ) is calculated in our model are provided inŽ .Dowdeswell and Siegert 1999 and Siegert et al.

Ž .1999 . This aspect of the model is not discussedhere because sediment transport to continental mar-gins is not examined in detail.

3. Model boundary conditions

3.1. Bedrock eleÕations

It is assumed that, at 120,000 years ago, thebedrock elevation of the Eurasian High Arctic, Scan-dinavia, the northern European mainland and theBritish Isles and their surrounding seas, was similar

Ž .to that of today Fig. 1 . This allows the modernbedrock elevation and bathymetry to be used todefine initial model conditions. The justification forthis assumption is based on oxygen isotope recordsand geological evidence indicating interglacial condi-tions prior to 120,000 years ago when glaciers were

Žprobably smaller than at present e.g. Mangerud,.1991; GRIP Project Members, 1993 . The bedrock-

elevation grid over which the ice sheet was con-Žstructed is composed of 74,400 square cells 310

.north by 240 east , with a width of 20 km per cell.The bed elevations were derived from a series oftopographic and bathymetric maps and radio-echosounding data on modern ice thickness in the

Žglacierised Eurasian Arctic archipelagos e.g..Dowdeswell et al., 1986 .

3.2. Sea-leÕel change and iceberg production

During periods of ice-sheet growth, global sealevel falls due to the transfer of water from theoceans to ice sheets. We use two sea-level curves:

the record from coral-reef terraces in New Guinea,18 Žand thed O record from deep-sea sediments Fig.

. Ž . 182a Chappell and Shackelton, 1986 . Thed Ocurve, which indicates greater sea-level fall duringthe Early and Mid Weichselian than suggested by thecoral reef record, is regarded as a ‘maximum’ valuefor sea-level depression, while the New Guinea ter-race curve is used as our ‘minimum’ sea-level func-tion. In either case, relative sea level is determined inthe model by summing the eustatic sea level with theisostatically adjusted bedrock elevation at any pointin time.

A depth-related iceberg calving function is em-ployed to describe the amount of ice removed fromthe marine margin of the ice sheet. The relation usedis:

w xV sM 70q8.33h 4Ž .c wŽ y1.whereV is the calving velocity m year andhc w

Ž .is the water depth m . This relation has been de-duced from a statistical analysis of calving glaciersfrom several polar locations, including SvalbardŽ .Pelto and Warren, 1991 .

3.3. Palaeoclimate forcing

The western margin of the Eurasian High Arctichas, in relation to its high latitude, an anomalouslymild annual climate. This is due to relatively warmsouth-westerly prevailing winds and ocean currentswhich transfer heat and moisture northward through

Ž .the Norwegian–Greenland Sea Hisdal, 1985 . Thetemperature of the Norwegian–Greenland Sea is in-fluenced by the meridional Norwegian Current, whichtransports relatively warm water from the NorthAtlantic. Eastward, towards the Russian High Arctic,

Žthe climate becomes colder and drier Dowdeswell,.1995 . This climate gradient is illustrated by a mean

annual air–temperature gradient of 0.6–1.08C perŽ100 km across the Svalbard archipelago Simoes,˜

.1990 .The numerical ice-sheet model requires climatic

inputs in the form of air temperature and precipita-tion, and their behaviour with respect to geographicallocation and altitude. However, there is a lack ofcontinuous proxy records from which to infer theclimatic history of the Eurasian High Arctic over thelast glacial cycle. To construct a simple palaeocli-


Ž . 18Fig. 2. Numerical ice sheet model forcing functions: a sea-level curves, the maximum model uses thed O curve, the minimum modelŽ . Ž .uses the New Guinea terraces curve; b solar insolation and related air temperature depression for the ‘maximum’ model; c sea-level and

related temperature depression for the ‘minimum’ model.


mate for the period of the last glaciation, a numberof assumptions are made, following Dowdeswell and

Ž .Siegert 1999 . It is first assumed that, at present, theclimate of the Svalbard–Barents Sea region is simi-lar to the altitude–precipitation relation defined as

Ž .Polar Mix by Pelto et al. 1990 , and that the climateover Scandinavia is defined as Sub-polar Mix, fol-

Ž .lowing Pelto et al. 1990 . Secondly, if the presentBarents Sea moisture source was curtailed, then amore continental-type precipitation regime would ex-ist, similar to the Polar Continental altitude–precipi-

Ž .tation relation in Pelto et al. 1990 . Thus, the east-ern Eurasian High Arctic is described by a PolarContinental-type relation. The equation that definesthe altitude related mass balance is:

b z sA eyx 1 z2qA eyx 2 z

25Ž . Ž .1 2

whereb is the mean annual mass balance at altitudeŽz, A is the ablation at the margin equal toy1.29261

m for the Polar Continental regime,y2.8490 for.Polar Mix and 7.5768 for Subpolar Mix ,A is the2

Žaccumulation at the margin which is 0.22998, 0.8378.and 2.5689 m for the three climate regimes ,x is1Žthe decay exponent of ablation with elevation equal

to 1.5023=10y6, 8.5345=10y6 and 2.7733=y6 .10 , respectively andx is the decay exponent of2

Žaccumulation with elevation with values of 3.6194=10y8, 3.4053=10y8 and 2.9162=10y7, for the

. Ž .respective climate zones Pelto et al., 1990 .In the numerical model, the equilibrium-line alti-

Ž .tude ELA is related to temperature through any1 Žadiabatic lapse rate of 5.18C km Fortuin and

.Oerlemans, 1990 . Thus, a temperature depression of3 8C will move the ELA downward by 600 m.

Ž .Fleming et al. 1997 , through surface energy-bal-ance modelling of north-west Spitsbergen glacierswith a modern ELA of about 400 m, showed that a 38C temperature change would shift the ELA belowsea level, providing support for the simple approachadopted here. Thus, through adjustments to the airtemperature, the surface mass balance of the icesheet can be modified.

In initial experiments, the air temperature depres-sion over the Barents Sea at the glacial maximum is

Ž .set at 108C Manabe and Bryan, 1985 . An assump-tion is made that, since glaciers on Svalbard were not

significantly larger at 120,000 years ago than todayŽ .Mangerud et al., 1998 , the temperature conditions

Žat 120,000 years ago and 10,000 years ago the timeat which Svalbard most recently became largely icefree, and warm waters entered the Svalbard coastal

.region; Salvigsen et al., 1992 were similar to thoseat present.

There is a clear correlation between high-latitudeŽpalaeo-air temperatures recorded in, for example,.Greenland ice-core records and indicators of global

Ž .ice volume e.g. Barrett, 1991 such as the global sealevel and oxygen isotope curves. Because of this, airtemperature change through time in the EurasianArctic region can be calculated empirically by one of

Žthe several indicators of global ice volume since theactual time function of air temperature is unknown

.for the Barents Sea region . In this paper, we presenttwo sets of model results; a ‘maximum’ and a

Ž‘minimum’ Weichselian ice-sheet history see the.later section named ‘Modelling procedure’ . Our

‘maximum’ model uses the time-dependent solarinsolation at 608N. This relationship between airtemperature and solar insolation controls the position

Ž .of the ELA through time Fig. 2b . We note that theinsolation values at higher latitudes are similar to thesignal at 608N and so choosing an alternative insola-tion signal at, say, 708N is unlikely to affect theresults of our model. In our ‘minimum’ reconstruc-tions, we run the model using the New Guineasea-level function to control air temperature. This isto examine the sensitivity of the ice sheet to areduced level of air temperature forcing through the

Ž .Weichselian Fig. 2c .We note that our simple Eurasian palaeoclimate is

comparable in terms of precipitation and mean-an-nual surface temperature to that derived from unpub-lished GCM simulations of the LGM climate in theEurasian Arctic using the Hadley Centre modelŽ .Marsiat, personal communication .

4. Deep-sea records of ocean conditions and ice-sheet behaviour

Recent advances in our understanding of the We-ichselian glaciation of the Eurasian Arctic have re-sulted from our knowledge of the palaeoceanographyof the ocean surrounding this region. Here we outline


several of the most important new findings and detailtheir relevance to ice-sheet behaviour.

4.1. High-productiÕity zones

Analysis of sea-floor sediment cores suggests thatthe rapid growth of ice in the Barents Sea around theLGM was associated with enhanced precipitationover this region, fed by a nearby moisture source in

Ž .the Norwegian–Greenland Sea Hebbeln et al., 1994 .These cores display sections with a high abundanceof planktonic foraminifera, including subpolar specieswhich live in relatively warm waters. Our under-standing of these forams indicates that the sea sur-face was likely to be at least seasonally free of seaice when they were alive. Periods when theseforaminifera are present are referred to as ‘high

Ž . Ž .productivity’ HP zones Fig. 3a . In the periodsbefore and after HP zones, the sea is likely to be

Ž .perennially ice covered Dokken and Hald, 1996 . Inthe Arctic Ocean today, summer sea-surface temper-

Ž .atures SSTs are around 0 toy1 8C in the uppersea-ice covered layer. This sets the minimum SSTfor the HP zones. However, the high concentrationof the polar speciesNeogloboquadrina pachydermaŽ .sinistral during HP zones indicates that the maxi-mum temperature must have been less than 58CŽ .Pflaumann et al., 1996 . The interpretation of HPzones presented here is derived from studies ofsea-floor sediment cores on the western Svalbard andBarents margins, reflecting the northern end-member

Žof the North Atlantic Drift Dokken, 1995; Dokken.and Hald, 1996; Hald et al., 1996 .

HP zones are associated with an increased flux ofthe subpolar speciesGlobigerina quinqueloba, andsuggest an influx of warm Atlantic Water on anumber of occasions during the Weichselian. It isassumed that, during these periods, there was north-wards advection of Atlantic Water. HP zones acrossthe western Norwegian–Greenland Sea are recordedduring periods noted in Fig. 3. In the time betweenthe HP zones it is assumed that oceanographic circu-lation was modified so that a year-round sea-ice

Ž .cover existed, more in line with the CLIMAP 1981reconstruction ofAglacialB circulation. Although theHP zones are likely to be linked with an increase inprecipitation over the western Eurasian Arctic, it isnot yet known which HP zones cause the build-up of

the ice sheet, and which are associated with warmconditions during deglaciation.

This paleoceanographic reconstruction, using in-ferences based on evidence of HP zones, compareswell to several previous studies showing evidence ofthe influx of Atlantic Water to the northern reachesof the Norwegian–Greenland Sea during marine iso-

Ž . Žtope stage MIS 5 and early MIS 4 Streeter et al.,.1982; Henrich et al., 1989; Baumann et al., 1993 .

More recent investigations have shown that AtlanticWater was advected into the Norwegian–Greenland

ŽSea repeatedly during the last 120,000 years Henrichet al., 1995; Sarnthein et al., 1994; Fronval et al.,

.1995; Rasmussen et al., 1996 . Such Atlantic Watermay have been present along the eastern margin ofthe Norwegian Sea for as much as 50% of the

Ž .Weichselian Dokken and Hald, 1996 . In fact, evenduring the coldest stages, such as MIS 2 and 4,Atlantic Water may have reached as far north as the

Žcontinental margin off western Svalbard Hebbeln et.al., 1994; Dokken and Hald, 1996 and the Arctic

ŽOcean Nørgaard Pedersen et al., 1998; Knies and.Stein, 1998 . The zones HP 1 and HP 2 are recorded

synchronously across these regions and show verysimilar high levels of planktonic foraminifera tothose recorded in a high-resolution core from the

Ž . ŽFaroe–Shetland Channel FSC Rasmussen et al.,.1996 . A similar correlation is seen between HP

zones 1–3 and the periods when high carbonatepercentages are observed in cores off the northern

Ž .Barents margin Knies and Stein, 1998 . There arealso indications of a correlation between HP 0 andHP 4 and increased abundance of planktic

Žforaminifera in the central Arctic Ocean Nørgaard.Pedersen et al., 1998 . At present, Atlantic Water

Ž .enters the Norwegian Sea through the FSC at 628N .The correlation of HP zones from the western Bar-ents margin and the Arctic Ocean suggests thatadvection of Atlantic Water during these periodsoccurred extensively across the polar seas between

Ž .628N and to the north of 808N Fig. 3 .There is an increasing amount of evidence for the

formation of deep-water in the Norwegian–Green-land Sea during the Weichselian. Labeyrie et al.Ž . Ž .1987 and Duplessy et al. 1988 interpreted benthicd13C andd18O records as evidence for active forma-tion of cold and well-oxygenated bottom water dur-

Ž .ing MIS 5 and early stage 4. Veum et al. 1992 and

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Ž . Ž . Ž .Fig. 3. Correlation between main periods of a advection of Atlantic Water represented by HP zones 1–12, Dokken, 1995; Dokken and Hald, 1996; Hald et al., 1996 , b HPŽ . Ž . Ž .zones north of Svalbard and Franz Josef Land, c HP zones in the Arctic Ocean Nørgaard Pedersen et al., 1998; Knies and Stein 1998 , d the glacial history ofSvalbard and

Ž . Ž . Ž . Ž .the Barents Sea Mangerud and Svendsen, 1992 , e glacial history of Western Scandinavia Mangerud, 1991 , these glaciation curves indicate the form of ice advance x-axisbetween, in ascending order, no ice, small terrestrial ice caps, ice growth limited to the present shorelines, ice growth over the continental shelf and, finally, ice growth to the

Ž . Ž . Ž .shelf break, f–g main meltwater and IRD events off western Svalbard and the western Barents Sea margin Dokken, 1995; Dokken and Hald, 1996; Hald et al.,1996 , h JulyŽ . Ž . 18 Ž . Ž . Ž .insolation at 788N Berger and Loutre, 1991 , i thed O record of the GISP ice core from the summit of Greenland Grootes et al., 1993 and j marine isotope stages MIS

Ž . Ž .Martinson et al., 1987 . The time scale is given in calibrated years by converting the radiocarbon years-30,000 years according to Bard et al. 1990 .


Ž .Sarnthein et al. 1994 also identified ventilated bot-tom water during MIS 2. Further, the Faeroe–Shet-

Ž .land record of Rasmussen et al. 1996 shows evi-dence of repeated deep-water convection and shutdowns from the beginning of sub-stage 5a to stage 1.Increased abundance ofC. wuellerstorfi, a benthicforaminifera indicative of a modern-type of deep-water, implies active deep-water formation duringMIS 1, 2, 4 and 5. These periods of deep-waterformation correspond to periods with inflow of At-lantic surface water to the area.

4.2. Ice-rafted debris and d18O records

Ž .High amounts of ice-rafted debris IRD in sea-floor sediment cores off western Svalbard reflectperiods when the Svalbard–Barents Sea Ice Sheetextended beyond fjords onto the continental shelfŽ . ŽElverhøi et al., 1995; Dokken and Hald, 1996 Fig..3g . A smaller component of the IRD probably

reflects iceberg rafting from the Western Scandina-vian Ice Sheet, as chalk grains with a source re-stricted to the North Sea area have been identified in

Ž .some of the cores Hebbeln et al., 1994 . Largepeaks in IRD are likely to be associated with en-hanced production of icebergs due to the break up ofthese marine ice masses, while smaller peaks aremore likely to reflect short-lived ice-sheet fluctua-tions that may occur over the cycle of ice growth and

Ž .decay Dowdeswell et al., 1999 .Five major phases of deglaciation are recognised

from IRD during the Weichselian over the followingŽ .time intervals Fig. 3g . The IRD-events at around

15,000 calendar years BP and around 55,000–50,000years BP reflect the deglaciation of two very largeice sheets when they retreated from the continentalshelf edge. The youngest event correlates with thedeglaciation from the LGM over Svalbard and the

ŽBarents Sea Mangerud et al., 1998; Dowdeswell et. Žal., 1999 and western Scandinavia Baumann et al.,

.1995 . The older IRD events correlate to glaciation‘E’ over Svalbard and the Barents Sea and ‘L’ across

Ž .western Scandinavia Fig. 3 . The IRD events arealso associated withd18O depletions, indicative ofincreased supply of isotopically light meltwater to

Ž .the ocean Fig. 3f . The isotope and IRD signalsassociated with the two youngest deglaciations can

be traced over the entire eastern North AtlanticŽ .Duplessy et al., 1988; Stein et al., 1996 and the

Ž .Arctic Ocean Nørgaard Pedersen et al., 1998 . ZonesHP 6 and HP 1, respectively, precede these twodeglaciations. These data are not used to force themodel. Instead, they provide a significant datasetwith which to compare model results.

5. Geological evidence for ice-sheet extent

A recent synthesis of marine and terrestrial geo-logical evidence has revealed the timing and extentof the Eurasian Ice Sheet during the WeichselianŽ . Ž .Svendsen et al., 1999 Fig. 1b . Our model resultsare compared with geological evidence in two forms.The first is the maximum extent of the ice sheet atseveral time slices, when the ice sheet was at the

Ž .height of a ‘maximum’ phase of expansion Fig. 1b .These ice-sheet limits are compatible with the mostrecent geological evidence from several regions

Žacross the Eurasian Arctic Forman et al., 1999;Knies et al., 2000; Gataullin et al., 2001-this volume;

.Polyak et al., 2000 . The second comprises gener-alised glaciation curves for Svalbard and Scandi-

Ž .navia Fig. 3 , based on a large number of glacialsedimentological and geomorphological investiga-

Žtions combined with chronological evidence e.g.,Svendsen et al., 1999; Mangerud and Svendsen,

.1992; Mangerud et al., 1996, 1998 .The generalised glaciation curves show that Sval-

bard was completely covered by ice between 28,000Ž .and 14,000 years ago Late Weichselian , and around

Ž .60,000 years ago Mid-Weichselian , whereas a morelimited ice sheet occurred at 90,000 years ago, and at

Ž . Ž .110,000 years ago Landvik et al., 1998 Fig. 3 .Across the northern Barents Sea margin, ice sheetsflowed across the shallow shelf regions at 20,000

Ž .and 60,000 years ago Knies et al., 2000 . Overwestern Scandinavia, the Late Weichselian glaciation

Ž .is recorded clearly Mangerud et al., 1996 . In addi-tion, glacial advances at 40,000, 60,000, 90,000 and110,000 years ago have also been identifiedŽ .Mangerud et al., 1998 . The exact timing of theseevents is less well known the further back in timethey occur; however, this geological evidence repre-sents an interpretation of geological data with whichice-sheet model reconstructions can be compared.


6. Modelling procedure

Ž .Siegert et al. 1999 modelled the Late Weich-selian Eurasian Ice Sheet by producing maximumand minimum models of glaciation through adjust-ments to the model’s climate forcing function. Theyfound that the geological data from the westernBarents Sea and Scandinavia were consistent withtheir maximum model, while further east the mini-

Žmum model was more relevant Svendsen et al.,.1999 .

A modelling procedure similar to Siegert et al.Ž .1999 is adopted here, where a ‘maximum’ and‘minimum’ model of glaciation is presented. The‘maximum’ model is based on the ‘most likely’

Ž .palaeoclimate of Siegert et al. 1999 . The palaeoen-18 Žvironment is forced by thed O sea-level curve Fig.

.2a and b and air temperature is related linearly toŽ .the value of solar insolation Fig. 2b, Table 1 . Three

periods of major temperature depression are calcu-lated in the maximum model, each of around 108C,corresponding to minimum levels of solar insolationŽ .Fig. 2b . The sea level also experiences minimum

Žlevels at 90,000 years ago when the sea level was.y60 m below present day , 70–60,000 years ago

Ž .when sea level reduction was 80 m and at aroundŽ20,000 years ago when the sea level reduction was

.120 m .The ‘minimum’ model is calculated by using the

Ž .New Guinea sea-level curve Fig. 2c and air temper-Ž .ature changes forced by this curve Table 1 . In this

model, the sea level is not lowered by more than 60m until 30,000 years ago, whereupon a sea-levelreduction of 120 m occurs. Likewise, the air temper-ature depression prior to 30,000 years ago is lessthan 58C. However, at the LGM the air temperature

Ž .depression is 108C. As in Siegert et al. 1999 ,accumulation is curtailed across the Kara Sea in the

Table 1Environmental forcing used in our ‘maximum’ and ‘minimum’ice-sheet reconstructions

Temperature forcing Sea level forcing18Maximum model June Insolation at 608N Marine d O

Minimum model New Guinea Sea Level New GuineaSea Level

minimum model. This produces a very dry polardesert environment in the eastern Eurasian HighArctic, and a more maritime climate in the west. The‘minimum’ model should be regarded as an experi-ment to examine the sensitivity of the ice sheet to alow level of air temperature changes during theWeichselian.

In both ‘maximum’ and ‘minimum’ models, azero level of precipitation is set across the Taymyr

Ž .Peninsula as in Siegert et al., 1999 . These recon-structions are then compared with geological evi-dence, from which the ‘most likely’ glacial scenariois determined. Note that the Late Weichselianpalaeoclimate in both models is very similar to that

Ž .of Siegert et al. 1999 .The accumulation of ice is calculated from an

altitude-dependent precipitation curve, and its varia-tion is in accordance with the change in mean-annualtemperature over time. In addition, the occurrence ofan HP zone may induce an enhanced rate of precipi-tation across the western Barents Sea. We accountfor this possibility by increasing the precipitation

Ž .rate by 20% in each HP zone Fig. 3a in both ourmaximum and minimum model runs. This alterationaffects the amount of precipitation, but not the gen-eral form of the altitude–precipitation relationship

Ž .from Pelto et al. 1990 outlined earlier. A full seriesof sensitivity tests reveals that this procedure addsnoise to the ice-volume signal, and alters the ice-sheetvolume by an amount compatible with the increasein accumulation. This aspect of the work is impor-tant, because it reveals that ice sheet build-up is notsolely caused by the presence of an HP zone.

7. Ice sheet results

7.1. Maximum model

The first point to note about the maximum recon-struction is that there are four prominent phases ofglaciation, which become progressively larger from

Ž .Early to Late Weichselian Fig. 4 . A first, relativelyshort phase of glaciation at around 110,000 years agocaused small ice caps to form on Svalbard andScandinavia. These ice caps then decayed by 105,000years ago. After this time, ice accumulated steadilyacross Scandinavia and the Arctic archipelagos. The


Fig. 4. Ice-sheet volume through the Weichselian for the ‘maxi-mum’ and ‘minimum’ ice sheet reconstructions.

Ž .first major period of glaciation Early Weichselianpredicted in the model was at around 90,000 yearsago, when a 1.75-km-thick ice cap grew over Scan-dinavia, and a 0.75-km-thick ice mass developedfrom Novaya Zemlya, covering most of the Barents

Ž .and Kara seas Fig. 5a . The deepest regions of theseepi-continental seas, such as the Bear Island Troughand the St. Anna Trough, were not glaciated at this

Ž .time Fig. 5a . The maximum volume of this ice3 Ž .sheet was around 4,000,000 km Fig. 4 . The ice

sheet decayed by 80,000 years ago.This was succeeded by 20,000 years of ice growth,

Žto a maximum at 60,000 years ago the Middle.Weichselian , where the entire northern and western

Eurasian continental margins were covered byŽ .grounded ice Fig. 5b . The ice divide was located in

the central Barents Sea, where ice thickness was over1.25 km. Fast-flowing ice streams were activatedwithin topographic troughs, which delivered glacialsediments to the trough–mouth regions on the conti-

Ž .nental shelf edge Dowdeswell and Siegert, 1999 .To the south, Scandinavia experienced less ice thanin the previous glaciation, and the southern Barentsand Kara Seas were not covered by grounded ice.The maximum volume of this ice sheet was justunder 5,500,000 km3. Deglaciation began at around56,000 years ago until 50,000 years, which left asmall ice cap over Scandinavia that existed for around20,000 years.

After 30,000 years ago, the ice sheet grew againŽ .to a maximum, Late Weichselian, position Fig. 5c .

Ice thickness over Scandinavia was around 2.75 km,while over the Barents Sea it was around 1.75 km.The ice divide was located over Novaya Zemlya.The entire Barents and Kara seas were covered bygrounded ice, with fast-flowing sediment-transport-ing ice streams within the bathymetric troughs. Themaximum volume of this ice sheet was around8,000,000 km3. Late Weichselian deglaciation startedat around 16,000 years ago and was completed by10,000 years ago. By a small adjustment to themaximum model, where the accumulation of ice iscurtailed across the eastern Kara Sea during the LateWeichselian, ice thickness across the northern Kara

ŽSea is limited to a few hundred metres due to ice.flow from the Barents Sea , and no ice is developed

Ž .across the Taymyr Peninsula Siegert et al., 1999 .

7.2. Minimum model

In our ‘minimum’ model, the New Guinea Ter-races sea-level function is used to force temperature,and air–temperature depression is limited to 58C

Žthroughout the Early and Middle Weichselian Fig..2c . A very different pattern of glaciation is mod-

Ž .elled Fig. 6 . Although several stages of glacialactivity are reconstructed for similar times to themaximum model, prior to the Late Weichselian onlyrelatively small ice masses were developed. At90,000 years ago, virtually no ice was modelled overScandinavia, and a small 500 m thick ice cap grewover Svalbard and the northern Barents and Kara

Ž .seas Fig. 6a . A slightly larger ice mass existed at60,000 years ago, when a 750-m-thick ice cap waslocated over the southern region of Scandinavia, anda 750-km-thick ice mass existed over the northern

Ž .Barents Sea Fig. 6b . This ice sheet, and that pre-ceding it, were too thin to induce major ice streamswithin the bathymetric troughs. The ice mass overScandinavia did not decay completely prior to theLate Weichselian. However, the ice sheet over theBarents Sea had disintegrated by 50,000 years ago.Ice then built up across the Eurasian Arctic to a

Ž .maximum Late Weichselian position Fig. 6c . Thisice sheet had a volume of 5,000,000 km3, a maxi-mum ice thickness of over 2 km in Scandinavia and

Žover 0.75 km across the Barents Sea Figs. 4 and.6c . The northern Kara Sea was glaciated, but only


Ž . Ž . Ž .Fig. 5. Ice-sheet thickness for the ‘maximum’ model reconstruction at a 90,000, b 60,000, and c 20,000 years ago. Contour interval250 m.


Ž . Ž . Ž .Fig. 6. Ice-sheet thickness for the ‘minimum’ model reconstruction at a 90,000, b 60,000, and c 20,000 years ago. Contour interval250 m.


limited ice cover was modelled over SevernayaZemlya, the southern Barents Sea and the southernKara Sea.

7.3. Limitations of the model results

Two main limitations are apparent in the ice-sheetmodel results. The first is that the location of maxi-mum ice loading is different in the ‘maximum’ and‘minimum’ reconstructions. Present-day isostatic re-bound of Scandinavia indicates that the maximumice load was situated in the northern Gulf of Bothnia.This ties in well with the ‘maximum’ model’s icedistribution but less so with that of the ‘minimum’reconstruction. Model results have not been tunedwith respect to the modern uplift of Scandinavia andthey should be viewed with this in mind. The secondlimitation is that the ‘maximum’ model’s southernice-sheet margin is anomalously straight at the LGMŽ .Fig. 5c . The reason is that the ‘maximum’ recon-struction is too large to be compatible with thegeological evidence in the east of region and so themodel was left without further adjustment. However,

Ž .in the ‘minimum’ model of Siegert et al. 1999 thereconstruction was forced to match the geologicaldata by slight adjustments to the mass balance of thesouthern ice sheet margin. Again, the results of themodelling should be interpreted bearing this in mind.

8. Interpretation of model results

Our model is forced primarily by changes in theposition of the ELA and sea level. The time-depen-dent variation of the ELA is determined by assumingthat there is a linear relationship between air temper-ature and ELA position through a lapse rate of 5.18Ckmy1, and that there is a linear relationship betweenair temperature and solar insolation through time. Itis the interaction of sea-level variations and ELAposition that has the most influence in the growthand decay of the Weichselian ice sheets in ourmodel. Model results should therefore be interpretedwith this forcing relationship in mind.

8.1. Comparison with geological data

The generalised glaciation curves for SvalbardŽ .and western Scandinavia Fig. 3d–e show a clear

similarity with the time-dependent ice-volume curveŽ .in our ‘maximum’ reconstruction Fig. 4 . It is inter-

esting to note that the geological record in the Rus-sian Arctic also agrees with the modelled time-de-pendent development of ice. New evidence from thePechora Basin indicates that the Barents–Kara IceSheet grew to its maximum position as early as100–80,000 years ago when a large ice-dammedlake flooded the lowland areas of the continentŽ .Mangerud et al., 2001-this volume . There is alsosome evidence for a younger major Middle Weich-selian ice sheet advance that terminated on the north-ern rim of the Eurasian continent 60–50,000 years

Žago Mangerud et al., 2001-this volume; Houmark-Nielsen et al., 2001-this volume; Alexanderson et al.,

.2001-this volume .However, it should be noted that there are consid-

erable discrepancies between the model simulationsand the empirical reconstructions concerning the ge-ographical extension of the former ice sheets duringthe various time slices. For example, during theLGM our ‘maximum’ model underestimates the sizeof the Scandinavian Ice Sheet, especially in the

Ž .southeast see Section 7.3 . In contrast, the LateWeichselian ice sheet in our ‘maximum’ reconstruc-tion is noticeably larger in the Barents and Kara Searegion than indicated by the geological observationsŽ .Figs. 1b and 5 , which are more similar to the‘minimum’ reconstruction, particularly in the east.This problem is rectified if the accumulation of ice iscurtailed across the Kara Sea. Our minimum recon-struction is only compatible with geological data forthe Late Weichselian as earlier Weichselian ice sheetsare much too small. We conclude that, prior to theLate Weichselian, geological evidence is more con-sistent with our ‘maximum’ scenario.

Our model results can also be compared withsedimentary evidence from the Bear Island and Stor-fjorden trough–mouth fans, located across the west-

Žern margin of the Barents Sea e.g. Laberg, 1994;.Dowdeswell and Siegert, 1999 . One interpretation

of the geological information acquired from thesetwo large glacial-sediment fans is that they maydiffer because sediments from only the Late Weich-selian are present in the Bear Island Fan, whileseveral episodes of glacial deposition are present in

Ž .the Storfjorden Fan Laberg, 1994 . This agrees wellwith our Early Weichselian ‘maximum’ model, and


our Late Weichselian ‘minimum’ reconstructionsŽ .Figs. 5 and 6 . If Middle Weichselian sedimentswere deposited over the Bear Island Fan, then our‘maximum’ model is also a plausible result. If not,then the ‘maximum’ model over-predicts ice extentacross the Bear Island Trough.

Bearing in mind that there are some significantvariations between the model simulations and thesize of ice sheets interpreted from geological evi-dence, we maintain that our experiments suggestthree major glaciations occurred in Eurasia duringthe Weichselian.

8.2. Comparison with oceanographic data

The build-up of the Weichselian ice sheet corre-sponds with the occurrence of HP zones as followsŽ . ŽFigs. 3 and 4 . The Early Weichselian ice sheet at

.90,000 years is preceded by HP zones across thewestern margin of the Norwegian–Greenland SeaŽ . ŽHP 9 . The Middle Weichselian growth at around

. Ž60,000 years corresponds with HP 6 and possibly.HP 7 . The Late Weichselian glaciation is associated

with HP 1–2 across the Norwegian–Greenland Sea,

and also similar zones across the northern BarentsŽ .Sea margin Fig. 3b . Ice-sheet decay is also associ-

ated with a number of HP zones. For example, HPs6–4 coincide with the deglaciation of the MiddleWeichselian.

Periods of enhanced iceberg calving are predictedŽ . Ž .in our ‘maximum’ reconstruction Fig. 7 at 1

110–105,000 years ago during the decay of the first,Ž .relatively small ice sheet of the Weichselian; 2

100–90,000 years ago during the maximum phase ofŽEarly Weichselian glaciation although the limitedŽ .growth of ice in the Barents Sea Fig. 5a precludes a

. Ž .very large calving event ; 3 around 80,000 yearsago during the initial build-up of the Mid-Weichselian

Ž .ice sheet; 4 60–55,000 years ago due to the break-Ž .up of the Mid-Weichselian ice sheet; and 5 after

15,000 years ago due to decay of the Late Weich-selian ice sheet. All of these iceberg calving eventscorrelate with IRD events interpreted from the ma-

Ž .rine sedimentary record Figs. 3g and 7 .Periods of net ablation of the ice sheet also corre-

late reasonably well with meltwater events recordedin the isotopic ratios of sea-floor sediments. Ice-sheet

Ž .decay is predicted between 1 110,000 and 105,000

Fig. 7. Rates of ice accumulation and iceberg calving through the Weichselian for the ‘maximum’ reconstruction. Note that negative iceaccumulation represents melting. The thick curve is iceberg production and the thin curve is accumulationrmeltwater production. The areas

Ž .between the two curves relate to periods of accumulation of the ice-sheet when accumulation is greater than calving or periods of ice-sheetŽ .decay when calving is greater than accumulation . IRD events, as shown in Fig. 3, are displayed in grey shade.


Ž . Ž .year ago; 2 90,000 and 85,000 years ago; 3Ž .65,000 and 55,000 years ago; and 4 18,000 and

Ž .10,000 years ago Fig. 7 . Meltwater events arerecorded in sea-floor sediments at 105,000, 82,000,

Ž57,000–55,000 and 18,000–10,000 years ago Fig..3f .

8.3. Synthesis of Weichselian glacial history

The comparison between our numerical recon-struction of the Weichselian Eurasian ice sheet, andgeological and oceanographic evidence allows us topropose a glacial history for Eurasian Arctic over thewhole Weichselian glacial cycle as follows:

v Large-scale glaciation of the WeichselianEurasian Arctic is controlled largely by sea level,and equilibrium-line altitude changes forced by vari-ations in solar insolation. The enhanced precipitationthat may occur as a result of high productivity zonesallows the rapid build-up of ice sheets, but is notuniquely responsible for ice-sheet growth.

v Prior to 100,000 years ago, the ice-sheet modelproduces only a very small ice mass. Geologicalevidence on Svalbard and Scandinavia indicates thatthis ice sheet extended to the shelf break. However,in the model, relatively high sea level at this timecauses an enhanced level of iceberg calving at themarine margin of the ice sheet which precludes thebuild up of ice to the continental margin.

v The first large-scale glaciation of the EurasianArctic was at 90,000 years ago according to ourmodel. Ice-sheet growth was related to a period ofHP in the nearby Norwegian–Greenland Sea whichacted as a moisture source for precipitation. The icesheet covered most of the Barents and Kara seas, but

Ž .left their deeper regions free of ice Fig. 5a . There-fore, glacigenic sediments were not transported tothe mouths of either the Bear Island or St. Annatroughs at this time. However, the mouths of shal-lower bathymetric troughs such as the StorfjordenTrough may have experienced a build up of materialover the continental margin fan system.

v The large amounts of icebergs and melt watersŽ .released during deglaciation Fig. 7 caused an ob-

servable increase in IRD and depletion of18O in theŽ .nearby Norwegian–Greenland Sea Fig. 3f,g .

v The second large-scale glaciation began ataround 80,000 years ago, caused by a reduction in

Žsea level and a decrease in the ELA forced by an.decrease in solar insolation , and aided by a nearby

moisture source due to the influx of Atlantic Waterinto the eastern Norwegian Sea. The ice sheet reacheda maximum at 60,000 years ago, when the westernand northern margin of the Barents and Kara seas

Ž .were covered by a 1.25-km-thick ice sheet Fig. 5b .Glaciation in the south of the Barents and Kara seaswas less extensive than in the Early Weichselianglaciation. Similarly, the ice sheet over Scandinaviawas smaller than that at 90,000 years ago.

v Deglaciation of this ice sheet was rapid, andassociated with an increase in sea level. This causedenhanced iceberg calving over the Barents Sea andthe decay of the marine portion of the ice sheet.These icebergs produced an IRD signal in Norwe-gian Sea sediments.

v By 50,000 years ago only a small ice cap wasleft over Scandinavia. However, this small ice massreflects the strong influence of precession-led forcingŽ .41 ka periodicity of glaciation over Scandinavia,compared with further north where axial-tilt cyclesŽ .20 ka periodicity dominate the glacial record. TheScandinavia ice cap existed in a relatively steadyform between 50,000 and 28,000 years after which aresurgence in glacial activity occurred. After thistime, open-ocean conditions across the eastern Nor-wegian Sea and the Barents margins of the ArcticOcean provided a source of moisture for rapid ice-sheet growth.

v By the LGM, the largest ice sheet of the Weich-Ž .selian existed over Scandinavia)2 km thick and

Ž .the western Barents Sea)0.75 km thick . This isreflected in the curves for total ice volume of the

Ž .Eurasian Ice Sheet Fig. 4 because the Fennoscan-dian sector is its largest contributor. The westernmargin of the ice sheet was characterised by fast-flowing ice streams in bathymetric troughs, trans-porting sediment to trough mouth fans. To the east,ice-sheet growth was limited to at most a thin,250-m-thick ice cap over the northern Kara Sea, andvery little ice to the south of this. The TaymyrPeninsula remained free of ice at the LGM.

v Deglaciation occurred primarily through icebergcalving, such that the marine portions of the ice sheetdecayed first. These icebergs left a noticeable IRDand meltwater sequence in ocean sediments. By10,000 years ago, deglaciation was complete.


The episodic build-up of continental-scale icesheets across the Eurasian Arctic during the Weich-selian has implications for global sea level. Thiscontribution is simply the volume of ice that existsabove the level of ice-sheet buoyancy. Assuming thesurface area of the world’s oceans remains un-changed through the Weichselian, each 1 km3 of icewhich is melted and added to the oceans is equiva-lent to 2.76mm of sea-level rise. Under this assump-tion, the Eurasian Ice Sheet contributes around 14 mto global sea-level reduction at the LGM, and about11 m of sea level during the Mid-Weichselian. Forearlier Weichselian glaciations, and the period be-tween the Mid and Late Weichselian, the EurasianIce Sheet contributed less than 5 m to global sea-levelfall.

9. Conclusions

A numerical ice-sheet model was used to recon-struct the growth and decay of the Eurasian Ice Sheetduring the full Weichselian glacial–interglacial cy-cle. Model results are compared with a variety ofterrestrial and marine geological data from which arecord of past ice-sheet extent and activity has been

Ž .interpreted e.g. Svendsen et al., 1999 . The growthand decay of the ice sheet is forced by the time-de-pendent variations in sea level and ELA. Two recon-structions, representing ‘maximum’ and ‘minimum’scenarios, are determined. The ‘maximum’ model,which is forced by thed18O sea-level function andmean air temperature related linearly to solar insola-tion, shows four distinct glacial episodes in theWeichselian, three of which result in major expan-sions of the ice sheet at 90,000, 60,000 and 20,000

Ž .years ago Fig. 5 . The ‘minimum’ model which isforced by the New Guinea sea-level function, and airtemperature related linearly to this sea level, predicts

Ž .very little ice prior to the Late Weichselian Fig. 6 .However, the ice sheet at the Late Weichselian issimilar to the maximum model. Geological evidencefor former ice-sheet size is more compatible with our‘maximum’ model prior to the Late Weichselianthan the ‘minimum’ reconstruction. However, bothmodels produce a Late Weichselian ice sheet compli-ant with geological data if the accumulation of ice iscurtailed across the Kara Sea.

Marine geological and oceanographic data, de-Žtailing periods of IRD input, meltwater from the

18 16 . ŽOr O ratio and sea-ice free conditions HP.zones , are available throughout the Weichselian for

the western and northern margins of the BarentsŽ .Shelf Fig. 3 . By comparing our model results with

these data, we are able to propose a glacial historyfor the Eurasian Arctic during the Weichselian.

Twelve HP zones are identified across the easternNorwegian Sea. These periods correspond with sea-

Ž .ice free conditions at least seasonally . It is there-fore possible that these zones reflect periods when amoisture source is available for precipitation over thenearby ice sheet. Alternatively, these zones couldreflect the warmer conditions experienced duringdeglaciation. Our maximum model indicates thatseveral of these zones are related to the build-up ofthe ice sheet, where an enhanced rate of precipitationis conducive to rapid ice growth. However, adjust-ments to the rates of precipitation at these times donot adversely affect the results of the ‘maximum’model. We conclude that the development of Weich-selian ice sheets is strongly dependent on the posi-tions of sea level and ELA, and that enhanced pre-cipitation has a lesser but nonetheless importanteffect. The ‘maximum’ model’s calculation of ice-sheet mass balance also agrees well with IRD andmeltwater events recorded in the Norwegian Sea. Allmeltwater signals are related to the decay of the icesheets in the Weichselian.

Acknowledgements

We acknowledge financial support from EU GrantENV4-CT97-0563 to the project Ice Sheets and Cli-mate in the Eurasian Arctic at the Last Glacial

Ž .Maximum Eurasian Ice Sheets . The European Sci-Žence Foundation QUEEN Quaternary Environment

.of the Eurasian North programme provided fundingfor workshops where a number of the ideas pre-sented in this paper were discussed. MH and JISacknowledge funding from the Research Council ofNorway for the projects entitledAThe Climate andOzone ProgrammeB and APaleo Environment and

Ž .Climate History of the Russian ArcticB PECHORA ,respectively. We thank Peter Clark and Tony Paynefor providing constructive reviews of our work.


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