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Quaternary Science Reviews 92 (2014) 409e428

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Quaternary Science Reviews

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Glacial and vegetation history of the Polar Ural Mountains in northernRussia during the Last Ice Age, Marine Isotope Stages 5e2

John Inge Svendsen a,b,*, Linn Cecilie Krüger c, Jan Mangerud a,b, Valery I. Astakhov d,Aage Paus c, Dmitry Nazarov d, Andrew Murray e

aDepartment of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, NorwaybBjerknes Centre for Climate Research, P.O. Box 7810, University of Bergen, 5020 Bergen, NorwaycDepartment of Biology, University of Bergen, P.O. Box 7803, 5020 Bergen, Norwayd St.Petersburg State University, Geological Faculty, Universitetskaya 7/9, 199034 St. Petersburg, Russiae The Nordic Laboratory for Luminescence Dating, Department of Geoscience, Aarhus University, DTU Risø Campus, DK-4000 Roskilde, Denmark

a r t i c l e i n f o

Article history:Received 13 February 2013Received in revised form20 September 2013Accepted 5 October 2013Available online 31 October 2013

Keywords:Ural MountainsArctic RussiaGlaciationIce capLake coresBarentseKara Ice SheetPollen stratigraphyIce-dammed lakesPaleoclimateOSL chronology

* Corresponding author. Department of Earth ScienBox 7803, N-5020 Bergen, Norway. Tel.: þ47 5558351

E-mail address: John.Svendsen@geo.uib.no (J.I. Sve

0277-3791/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.quascirev.2013.10.008

a b s t r a c t

We describe and discuss the glacial and climate variations in the Polar Ural Mountains in northernRussia over the last 100 000 years. A series of optically stimulated luminescence (OSL) and radio-carbon ages from sediment cores demonstrate that there has been continuous deposition of lacustrinesediments throughout the last 65 ka in Lake Gerdizty, a lake basin situated on the eastern foothills ofthe Urals. Below the lacustrine sequence are two till beds; the uppermost (unit B) was probablydeposited by a large mountain-centred glacier complex that covered the lake basin during MarineIsotope Stage (MIS) 4. This till bed might be a counterpart of the Usa Moraine in the western foothillsof the Polar Urals, which is dated with OSL- and 10Be cosmogenic nuclide methods to between 68 and58 ka, consistent with the OSL ages from the lowermost lacustrine sediments in Lake Gerdizty. Duringthis glaciation some of the northern Uralian outlet glaciers probably merged with the adjacentBarentseKara Ice Sheet that reached onto the adjacent lowlands forming large ice-dammed lakes onboth sides of the Ural Mountains. The equilibrium line altitude (ELA) on the local ice cap was then atleast 1200 m lower than at present. The pollen stratigraphy in Lake Gerditzy reflects an open tundraand steppe-like vegetation throughout MIS 3e2 (60e11,7 ka). The first significant change in vege-tation, shown by an increase of dwarf-shrub communities of Salix and Betula, took place during theLateglacial at around 15 ka or shortly after. Birch and spruce trees approached the area for the firsttime soon after the Holocene transition w11.7 ka, and became dominant elements in the vegetationafter a few hundred years. Our reconstruction suggests a very cold summer climate during MIS 4 andpresumably also during the preceding MIS 5b (95e85 ka) glaciation, probably amplified by the largeice-dammed lakes that existed during these two glaciations. A somewhat milder climate prevailedduring MIS 3 (60e25 ka), but a treeless vegetation nevertheless suggests that mean summer tem-peratures did not exceed 10e12 �C before the earliest Holocene.

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1. Introduction

In this paper we present results from field studies in the PolarUralMountainswith the aim of improving our understanding of theenvironmental changes during the Last Glacial cycle. We attachconsiderable importance to lithological and pollen stratigraphicdata from a well dated lake record that are combined with results

ce, University of Bergen, P.O.0.ndsen).

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from exposed sediment sections, geomorphological observationsand various geochronological data from a wider area (Fig. 1). Theresults are used to constrain the timing and extent of the mostrecent glaciation that embraced the northern part of the mountainrange and adjacent lowland areas during Marine Isotope Stage(MIS) 4. From the pollen data we reconstruct the vegetation historysince this last extensive ice cap retreated from the foothills.

The Polar Urals have been affected by both the Barents-Kara IceSheet and locally sourced glaciers. At least once during an early stage(>60 ka) of the Last Ice Age (Weichselian), the BarentseKara Ice Sheetencroached the northern tip of the Ural Mountains (Fig. 1) (Astakhovet al., 1999; Svendsen et al., 2004). In contrast, during the Last Glacial

Fig. 1. A. Digital elevation model of the Polar Urals. Some of the most distinct end moraines (white lines) and all described sites (blue stars) are marked. B. Map of Lake Gerdizty. Allcores used in this study were taken close to the mark in the centre of the lake. Note the small catchment area. C. Key map showing the location (square) of Fig. 1A. The ice sheetlimits during the Late Weichselian (white line) and during the Early Weichselian glacial maximum (black line) are marked (From Svendsen et al., 2004).

J.I. Svendsen et al. / Quaternary Science Reviews 92 (2014) 409e428410

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Maximum (LGM, 20e25 ka), the ice sheet did not reach the northernrimof themainland.At this timemostly small cirqueglaciers appear tohave existed in the Polar Urals (Mangerud et al., 2008a). According tothese studies, most of the areawithin and around this mountain areahas remained ice-free over the last 50e60 ka. However, the configu-ration and timing of the preceding glaciations has remained unclear.Likewise, our knowledge of the vegetation and landscape develop-ment is fragmentary and poor. We cored several lakes in the antici-pation that they would provide continuous sediment records thatcould be utilized to reconstruct the environmental changes since themainvalleysbecamepermanently ice free.Herewepresent the resultsfrom sediment cores retrieved from Lake Gerdizty on the easternfoothills of the Polar Urals, as well as other relevant data used toreconstruct anddate theglacial history (Fig.1). Basedona compilationof all available data from the whole region, we outline the environ-mental and climatic variations over the greater part of the Last IceAgeand well into the early Holocene.

1.1. Regional setting

The Polar Urals, the segment of the Ural Mountains north of theArctic Circle, are characterized by glacial landforms such as cirques,U-shaped valleys and lakes (Fig. 1). The highest peaks reach 900e1400 m above sea level (m a.s.l.) whereas the trunk valley floors areonly 200e300 m a.s.l. Today there are a number of active cirqueglaciers and also a few thin hanging glaciers on north-facingmountain slopes, but none is >1 km2 (Troitsky et al., 1966;Grosswald and Kotlyakov, 1969; Solomina et al., 2010). The climateis cold and continental with permafrost and mean summer tem-peratures range between 10 and 13 �C in the adjacent lowlands. Theannual precipitation is w500 mm in the foothills, but may locallyexceed 800 mm in the central part of the mountain range. Thetheoretical climatic snow line is estimated to occur at 1600 m a.s.l.(Troitsky et al., 1966), i.e. well above the highest mountain peaks.The current glaciers exist due to local conditions that favour accu-mulation over ablation such as shadow effects, wind drift and snowavalanches (Troitsky et al., 1966; Mangerud et al., 2008a).

Lake Gerdizty (66�50’N, 66�00’E, 213 m a.s.l.) is a shallow lakesituated w9 km to the north-east of the village of Kharp on theeastern foothills of the Polar Urals (Fig. 1A). It is w2 km long, 400e900 m wide and has a water depth of only 2e3 m. The catchmentarea is limited to a narrow zone around the lake (Fig. 1B). Thus,surface water supply is restricted, but the lake may also receivesome additional inflow of groundwater from the mountains. Thegently undulating landscape in the surrounding area is charac-terised by solifluction and other slope processes and is absent ofglacial landforms. Mean July and January temperatures, recorded atthe Salekhard weather station, w40 km to the SE (Fig. 1), are 13 �Cand�23 �C, respectively (http://meteo.infospace.ru/climate/html/).Vegetation is of tundra type, but the lake is located close to thenorthern limits of birch, larch and spruce (cf. Kremenetski et al.,1998; MacDonald et al., 2000) and scattered stands of these treesare found in the vicinity. According to the ice sheet reconstructionof Svendsen et al. (2004) during the Last Ice Age (Weichselian) theinvestigated site was outside the range of any BarentseKara IceSheet. However, the glacier limits around the Polar Urals are notwell constrained, and here we provide new interpretations for theWeichselian ice margin positions in this area.

2. Material and methods

2.1. Lake coring and documentation of the sediments

Coring of Lake Gerdizty was carried out from lake-ice in March2006, using heavy motorized geotechnical drilling equipment.

Metre-long core segments with a diameter of 10 cmwere recoveredin PVC tubes mounted inside a steel coring tube. In the field, thecores were stored in a heated shed to prevent them from freezing.After arrival in Bergen, the cores were split length-ways in adarkroom to prevent exposure to light. The reference sections werecarefully wrapped in black plastic bags and stored in a dark cool-room. The split core sections were photographed before descrip-tion of the lithology. Magnetic susceptibility (MS) was measuredusing a GEOTEKMultisensor Core Logger, and major elements wereanalysed using an ITRAX X-Ray Fluorescence (XRF) Core Scanner(Croudace et al., 2006), at a spatial resolution of 2 and 0.2 mm,respectively. Organic and carbonate contents were estimated bypercentage loss-on-ignition (LOI, % of dry weight, e.g. Heiri et al.,2001) by incineration at 550 �C and 950 �C, respectively.

2.2. Geomorphic and glacial geological investigations

Ice marginal features were identified from stereoscopicallystudied aerial photographs at scales of 1:25 000. We also consultedRussian high-altitude aerial photographs and satellite images,including Google Earth. Moraines showing former glacier lobesalong the flanks of the Polar Urals have previously been mappedand described by Astakhov et al. (1999) and Astakhov (2011). Someof these features are also shown on a recent overview map byFredin et al. (2012). Most of the mapped geomorphic features thatare reproduced on Fig. 1A have been checked and verified byground observations during our field expeditions. Sediment expo-sures along rivers were described in the field and sampled fordating.

2.3. Pollen analysis

Pollen samples (1 cm3) at intervals of 0.5e10 cm, were spikedwith Lycopodium tablets for estimation of concentration(Stockmarr, 1971), and were treated following standard techniques(Fægri and Iversen, 1989). Taxonomic identifications follow Moe(1974), Fægri and Iversen (1989), Moore et al. (1991) and modernreference material at the Department of Biology, University ofBergen. The pollen diagrams were drawn using the computerprogram CORE 2.0 (Kaland and Natvig, 1993). The pollen sum (SP)includes all terrestrial pollen taxa, thus excluding spores andaquatics. The diagrams presented here are divided into pollenassemblage zones (PAZs) by optimal sum of square partitioning(Birks and Gordon, 1985) using the program ZONE version 1.2(Juggins, 1991). The number of statistically significant zones wasdetermined by comparison with a broken-stick model (Bennett,1996) using the program BSTICK version 1.0 (Birks, H.J.B. andLine, J.M., unpublished).

2.4. Numerical analyses of the pollen composition

Patterns in the palynological data from Lake Gerdizty weredetected using the software CANOCO for Windows 4.5 (ter Braakand Smilauer, 2002). Due to the high amount of reworked pollenin unit A, the samples of this unit were not included in the analysis.The Lake Gerdizty data was also merged with data from LakeYamozero (65�01’N, 50�14’E) on the Timan Ridge, approximately750 km to the south-west (Fig. 1C) (Henriksen et al., 2008). All datawere square-root transformed to stabilize the variances. DetrendedCorrespondence Analysis (DCA) resulted in first axis gradientlengths of 1.233 for the Lake Gerdizty data set and 1.390 for themerged data set, suggesting linear response curves. Thus, PrincipalComponent Analysis (PCA) was the chosen ordination technique.Palynological terrestrial richness at Lake Gerdizty was estimated byrarefaction analysis (program RAREPOLL, Birks and Line, 1992). The

Fig. 2. A composite sediment log from Lake Gerdizty. The lithostratigraphic units are labelled by letters (AeD) and the pollen zones by roman numbers (IeIV). Loss-on-ignition(LOI), carbonate (CaCO3) and water content are shown to the right. OSL (�1s) and calibrated 14C ages are also plotted.

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Table 2Optically stimulated luminescence (OSL) dates on quartz from Lake Gerdizty.

Sample name Risø no.a Depth below sediment surface (cm) Unit Age (ka � 1s) Dose (Gy) n Dose rate, (Gy/ka) w.c. (%)a

506-33-01-50 08 25 01 96.5e104.5 D/C 17 � 1 40.7 � 1.2 18 2.39 � 0.10 25506-33-01-110 11 25 11 106e114 C1 35 � 10 82 � 22 8 2.32 � 0.09 32506-33-01-90 08 25 02 136.5e143.5 C3 37 � 2 85 � 3 18 2.28 � 0.10 21506-33-02-20 08 25 03 166e173 C2 41 � 2 80 � 2 21 1.94 � 0.08 24506-33-02-180 11 25 10 175.5e183.5 C2 40 � 3 70 � 3 24 1.74 � 0.07 28506-33-02-45 08 25 04 192e199 C2 45 � 2 81.5 � 1.9 19 1.81 � 0.07 25506-31-02-10 08 25 07 206e213b C2 52 � 3 102 � 3 17 1.98 � 0.08 18506-33-02-70 08 25 05 216e223 C1 57 � 3 112 � 4 20 1.94 � 0.09 21506-31-02-30 08 25 08 228e233c C1 63 � 3 131 � 3 23 2.08 � 0.08 29506-33-02-225 11 25 12 227e230 C1 58 � 5 118 � 9 18 2.02 � 0.08 17506-33-02-85 08 25 06 232e238 C1 65 � 3 138 � 4 24 2.13 � 0.09 23506-31-05-70 08 25 09 519e524 A 166 � 10 335 � 13 17 2.01 � 0.08 24506-31-06-30 08 25 10 626e633 A 128 � 8 281 � 9 18 2.19 � 0.10 40506-31-06-80 08 25 11 677e683 A 123 � 7 335 � 12 18 2.72 � 0.11 26

a Assumed water content.b The depth interval is from a parallel core (506-31-02), and the dated level has been correlated with the main core based on lithostratigraphy.

Table 1Radiocarbon dates of bulk samples from Lake Gerdizty.

Lab. no.a Sample ID Depth below sedimentsurface (cm)

Unit Sampleweight (g)

Age � 1s(14C a BP)

Cal age, 2s (cal a BP) Remarkb

Poz-27638 506-33-01 31 80.5e81.5 D 16.2 7660 � 40 8390e8540 AAAPoz-38293 506-33-01 95e97 cm 95e97 D 35.6 10 160 � 60 11 410e12 080 AAAPoz-38177 506-33-01 107e109 cm 107e109 C3 41.5 15 790 � 80 18 710e19 330 AAAPoz-27640 506-33-01 61 110.5e111.5 C3 35.9 18 290 � 170 21 430e22 300 AAA, 0.38 mg CPoz-27641 506-33-01 85 134.5e135.5 C3 29.5 23 160 � 120 27 680e28 470 TOCPoz-27679 506-33-02 31 180.5e181.5 C2 40.4 24 840 � 310 28 980e30 420 TOCPoz-38178 506-33-02 183e184.4 cm 183e184.4 C2 39.6 20 790 � 170 24 320e25 260 AAA, 0.6 mg CPoz-38948 506-33-02 183e184.4 cm 183e184.4 C2 39.6 26 070 � 300 30 330e31 180 TOCPoz-27680 506-33-02 71 220.5e221.5 C2 28.9 35 600 � 1500 37 440e43 610 TOC, 0.67 mg CPoz-38179 506-33-02 224e226 cm 224e226 C1 36.5 25 730 � 210 30 210e31 020 AAAPoz-38949 506-33-02 224e226 cm 224e226 C1 36.5 36 300 � 1000 39 230e42 990 TOCPoz-27628 506-33-02 85 234.5e235.5 C1 29.1 35 100 � 700 38 760e41 570 TOCPoz-41144 506-34-03 50 cm (A) 600 A 40 33 730 � 490 37 080e40 060 AAAPoz-41145 506-34-03 50 cm (B) 600 A 70 30 780 � 320 34 740e36 290 TOCPoz-23699 506-30-02 20 cm 120 D ? 7010 � 40 7820e7871 AAA

a POZ, Pozna�n Radiocarbon Laboratory, Poland.b TOC ¼ total organic carbon; AAA ¼ humin fraction, extracted by the acidealkalieacid method.

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basis for the analysis was 358, i.e. the smallest total number ofidentified pollen and spores per sample.

2.5. Radiocarbon dating

In all, 15 radiocarbon ages were obtained from the cored sedi-ment sequence from Lake Gerdizty (Fig. 2, Table 1). We intended tosample plant macrofossils for AMS radiocarbon dating, but onlysome very small fragments were found. As a result, bulk sedimentsamples were 14C AMS-dated using either the humin (alkali insol-uble, AAA) fraction and/or the total organic content (TOC). All ageswere determined at the Pozna�n Radiocarbon Laboratory, Poland.The ages were calibrated using OxCal v4.1.7 (Bronk Ramsey, 2009)with atmospheric data from Reimer et al. (2009). Calibratedradiocarbon ages are reported as ka BP (thousand years before AD,1950).

2.6. Optically stimulated luminescence (OSL) dating

A series of 14 optically stimulated luminescence (OSL) ages wereobtained from the cored sequence in Lake Gerdizty and a further 22from exposed strata on the western side of the mountains (Tables 2and 3). The samples from the lake basin are 6e8 cm thick sedimentslices cut out from the split core segments in a darkroom. Theothers were collected using opaque PVC tubes hammered into

freshly exposed sediment layers. The outermost end pieces of thesediment tubes were later removed in the dating laboratory toavoid sand grains that may have been exposed to sunlight duringthe field sampling. The equivalent dose measurements follow theSingle Aliquot Regenerated (SAR) dose protocol applied to quartz(Murray and Wintle, 2000; Mangerud et al., 2001) with a 260 �C(10 s) and 220 �C (no storage) thermal pre-treatment before thenatural/regenerated signals and the test dose signals, respectively;stimulations were at 125 �C with blue light, and detection wasthrough 7 mm of U-340 filter. High temperature stimulation wasused at the end of each SAR cycle to minimise thermal transfer tothe next cycle (Murray and Wintle, 2003). All samples weredominated by the quartz fast component; the mean dose recoveryratio for the 14 Lake Gerdizty samples was 0.98� 0.02 (n¼ 49). Weonly have dose recovery results for 4 of the samples (062516-19,Table 3) from the Usa River valley exposed sections, but with anaverage of 1.000 � 0.007 (n ¼ 12) this ratio is again indistin-guishable from unity. We conclude that, for these samples, ourprotocol is able to measure accurately a known dose given in thelaboratory before any thermal treatment.

Finally we caution against placing too much reliability on thequartz OSL dates that yielded ages older thanw120e130 ka (basedon doses larger thanw200 Gy); these include the bottom 3 samplesfrom the Lake Gerdizty core (Table 2) and also sample 062516 fromthe site Meskashor (Table 3). In this dose range, the quartz growth

Table 3OSL ages from investigated sediment exposures.

Risø no.a Fieldsample no.

Site Lat. N(degree,min. sec)

Long. E(degree,min. sec)

Description Age(ka � 1s)

Depth(cm)

Dose,(Gy)

n Dose rate(Gy/ka)

w.c.(%)b

022504 00-2462 Sandur at the UsaMoraine(covered by till)

67.28.48 65.01.12 Proximal to the ridge, covered by till. A 20 cm thicklayer of coarse sand, 4 m above river level (a.r.l.)

158 � 22 700 114 � 14 23 0.72 � 0.04 27

012577 00-2463 " " " 40 cm upstream of 00-2462 104 � 10 690 89 � 7 33 0.85 � 0.04 33012578 00-2464 " " " About 30 m upstream of the moraine ridge.

Fine-coarse sand with some gravel, 9 m a.r.l.101 � 7 200 87 � 3 30 0.85 � 0.05 33

022507 00-2465 " " " About 100 m distal to the ridge, 6 m a.r.l.Weakly laminated medium-coarse sand, 80 cm thick

66 � 4 400 57.7 � 1.1 24 0.87 � 0.05 23

022508 00-2466 " " " Same position and context as 00-2465. 64 � 4 400 67 � 2 24 1.03 � 0.05 22022505 00-2474 " " " Pocket of fine sand in till/moraine, 3 m below surface 60 � 5 160 66 � 5 24 1.11 � 0.05 22002558 00-2475 " " " Same location as 00-2474, but 30 cm higher up 82 � 8 130 93 � 3 19 1.14 � 0.10 23002559 00-2480 " " " Sand bed under till/moraine, 11 m a.r.l. 67 � 5 600 60 � 2 21 0.89 � 0.05 27002560 00-2481 " " " Sand bed, 1 m downstream of sample 00-2480. 63 � 4 550 55 � 2 21 0.86 � 0.04 26012537 00-2469 Sandur outside

Usa Moraine67.28.36 65.00.50 Sand-pocket within gravel, 2.5 m a.r.l. 87 � 7 750 68 � 4 27 0.79 � 0.04 24

012538 00-2470 " 67.28.37 65.00.51 Coarse to medium sand 3.7 m a.r.l.,close to 00-2469

70 � 4 630 65 � 2 18 0.93 � 0.05 23

022503 00-2471 " 67.28.38 65.00.52 Coarse sand with gravel, 7 m a.r.l.100 m upstream of 00-2469

78 � 5 400 70 � 2 24 0.89 � 0.05 13

022501 00-2408 Bol. Usa Valley,8 m terrace

67.28.18 65.43.22 Sand bed (8e10 cm) within beachgravel, 5 m a.r.l.

48 � 4 250 71 � 4 24 1.48 � 0.06 22

012531 00-2409 " 67.28.18 65.43.23 0.5 m thick bed of sand-sandy gravel(fining upwards),w300 m upstreamof 00-2408, 3 m a.r.l.

48 � 3 400 81 � 3 21 1.68 � 0.07 25

022502 00-2410 " 67.28.18 65.43.24 Sand bed (20 cm) within gravel, 3 m a.r.l.,and 3 m downstream of 00-2409.

16.4 � 1.0 400 26 � 0.9 24 1.59 � 0.08 23

62516 05-468 Meskashor,Usa River

66.37.00 62.37.10 Laminated, sandy silt (floodplain?) 171 � 10 1400 203 � 3 30 1.18 � 0.06 ?

82530 05-469 “ “ “ Coarse, cross-bedded sand (fluvial) 74 � 7 1300 55 � 4 24 0.74 � 0.04 3262517 05-470 “ “ “ Coarse, cross-bedded sand (fluvial) 84 � 5 1170 62 � 1.2 27 0.74 � 0.05 ?62518 05-471 “ “ “ Medium, rippled sand (fluvial) 78 � 4 1050 90 � 3 29 1.16 � 0.05 ?82531 05-472 “ “ “ Laminated, medium sand 67 � 4 930 112 � 5 21 1.66 � 0.07 2382532 05-473 “ “ “ Laminated, medium sand, cryoturbated 66 � 4 800 114 � 4 21 1.72 � 0.08 2062519 05-474 “ “ “ Laminated, medium sand with silt seams 64 � 4 730 89 � 3 21 1.38 � 0.06 ?012545 00-0460 Longotyeyan

Valley (sandur)67.15.00 67.06.00 Poorly sorted sand, with gravel, w210 m a.s.l. 48 � 4 450 101 � 6 36 2.11 � 0.09 23

012546 00-0461 “ “ “ Poorly sorted sand with gravel, w2010 m a.s.l. 36 � 2 450 52 � 2 18 1.45 � 0.06 21

a The Nordic Laboratory for Luminescence dating, Aarhus University, Denmark.b Assumed water content.

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Fig. 3. Photos of core segments from Lake Gerdizty. The three core segments illustratethe transition between the main lithostratigraphic units (marked with letters). Thesections are from three parallel boreholes in the central part of the basin: Section I isfrom core 34, II is from 31 and III is from core 33.

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curve is at or close to saturation, and all such ages should beconsidered minimum values.

3. Results

3.1. Coring results from Lake Gerdizty

Five sediment cores (nos. 30e34) were retrieved from the lake;the four studied here were taken only a few metres apart from thecentral part of the basin (Fig. 1B). Three reached the deepest strataand the longest core (no. 31) even sampled a thin slice of the un-derlying bedrock at a depth of 7.6 m. None of the cores has a fullrecovery, but in the laboratory we could correlate the overlappingcore segments. Most of the analyses were conducted on the twomost complete cores (nos. 31 and 33).

Fig. 4. Core log from Lake Gerdzty. The log shows the measured magnetic susce

3.1.1. LithostratigraphyThe sequence is divided into 4 main lithostratigraphic units

labelled A to D from the base (Figs. 1B and 2). Unit A, which isw2.7 m thick, is a compact clayey diamicton with a brownish greycolour. It contains scattered pebble and gravel particles and is forthe most part massive without any recognizable bedding. Nomacroremains of organic matter were seen, but loss-on-ignition valueswere 4e5%. We interpret the clayey diamicton as a till, but cannotexclude the possibility that it consists of lacustrine sediments thatsubsequently have been moulded and homogenized by a glacieradvance.

Unit B is a stiff, clearly over-consolidated gravelly diamictonwith a greyish colour. It differs from unit A by having amuch highercontent of sub-angular pebbles as well as sand. The lower boundaryis sharp, whereas the upper is a little more gradational. In contrastto unit A, the sediments contain very few pollen grains. We alsointerpret this unit as a till bed.

Unit C is divided into 3 subunits. The lowermost (C1), which isw50 cm thick, is composed of sandy silt with a thin, gravelly ho-rizon in the very top. When freshly exposed, unit C1 was charac-terized by dark grey colour due to sulphide staining, but followingoxidation it turned lighter grey. The loss-on-ignition values werew2.5%. The next sub-unit (C2) consists of faintly laminated siltysand with a light grey colour. Above follows a well-definedsequence (C3), w60 cm thick, consisting of a nearly massive greysilt. Supported by the pollen results, we interpret the entire unit Cas a lacustrine deposit.

There is a gradual transition from unit C to the uppermost unitD, which is also evident from the XRF data (Fig. 4). Unit D consists ofbrownish gyttja that gradually becomes increasingly organic to-wards the top (Figs. 2 and 3). The surface sediments in the centralpart of the lake were very loose, and we did not manage to retrievethe uppermost part (50 cm) with our coring device.

3.1.2. ChronostratigraphyIn total, 15 radiocarbon and 14 OSL ages were obtained from the

cores (Fig. 2, Tables 1 and 2). When assessing the results (Fig. 5), wehave also taken into consideration the pollen and lithostratigraphy.Two radiocarbon dates from the lower part of unit D, supported by

ptibility (MS) and XRF data (selected elements) from the investigated cores.

Fig. 5. Age model for the cored sequence in Lake Gerdizty. The figure illustrates theageedepth relationships for the lacustrine sequence (units CeD) based on the datingresults. The constructed age model (solid line) relies on the OSL-dates for the segmentbefore 30 ka and calibrated 14C ages and pollen data from the upper part. 14C datedsamples marked AAA or TOC are from the humin fraction and total organic carbon,respectively. Errors for 14C ages are given as �2s (sometimes smaller than the symbol)and for OSL ages as �1s. YD/H and Bö indicate the Younger Dryas/Holocene boundaryat w11.7 ka and the start of the Bølling interstadial at w14.6 ka respectively.

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the pollen stratigraphy, indicate that this unit started to accumulateat the very beginning of the Holocene. Accordingly, we place thePleistoceneeHolocene transition (w11.7 ka) at the boundary be-tween units C and D.

The radiocarbon (14C) and OSL ages from unit C provided ageestimates in the range 19e41 ka BP and 17e65 ka, respectively. Wenote that OSL yields significantly higher ages than 14C from corre-sponding levels. An OSL date of a sediment slice (96.5e194.5 cm)from near the upper contact of unit C3 gives an age of w17 ka.Provided that there was a continuous sedimentation across theboundary between C and D this suggests an offset in OSL ages of afew thousand years, perhaps arising from a residual dose of w5 Gyat deposition. This deviation might be explained by incompletebleaching of the OSL signal prior to deposition. The second samplefrom the top gives an even higher OSL age of 35�10 ka, but becauseof the unduly large uncertainty connected to age calculation littleimportance can be attached to this particular sample. It should benoted that the age discrepancy between 14C and the consistentseries of OSL dates increases down core, reachingw20e30 ka at thebottom of unit C1, where the 14C dates from this subunit yield agesofw40 ka. If real, such an overestimate in OSL age implies a residualof >50 Gy at deposition; this is extremely unlikely in this envi-ronment (see Wallinga, 2002; Jain et al., 2004; Murray et al., 2012for summaries of residual doses in fluvial sediments at deposi-tion). Incomplete bleaching is a heterogeneous process in both timeand space (Murray et al., 2012) and it is difficult to imagine adeposition process that would lead to a systematic decrease in re-sidual dose from the oldest to the youngest layers. We also call

attention to the fact that the radiocarbon ages were all obtainedfrom bulk sediments with very low carbon content; only minorcontamination by “young” carbon would give rise to age un-derestimates. The divergent ages obtained by radiocarbon datingboth the total organic content and also the humin fractions fromthe same sample indeed suggest that such contamination is aproblem. We conclude that the OSL ages from unit C are morereliable than the radiocarbon ages. Three OSL ages from thelowermost stratigraphic unit (A) provided much older ages (166e123 ka) compared to the series above. However, the natural OSLsignals from these samples were close to saturation on the quartzgrowth curves, and so we consider these samples to give minimumages. Radiocarbon dating of the bulk sediments from unit A pro-vided ages of w39 and w36 ka for the humin and total organiccontent, respectively (Table 1). We consider both of these as non-finite ages and that the real age of the dated organic material ismuch older, in line with our favoured OSL chronology for theoverlying strata.

3.1.3. Pollen stratigraphyThe pollen records from units A, C and D are statistically divided

into four pollen assemblage zones (PAZ), labelled G-I to G-IV (Figs. 6and 7; the few pollen grains found in unit B are not shown).

PAZ G-I. This zone includes all the pollen spectra of unit A, whichis interpreted as a till (Fig. 6). No distinct changes in pollencomposition can be detected upwards. Arboreal pollen taxa (Alnus,Betula, Pinus, and Picea) dominate (60e70% SP). The relativefrequent occurrences of “exotic” pollen of various deciduous treetaxa including Engelhardia, Ilex, Liquidambar, Nyssa, Platycarya,Pterocarya, Carpinus, Carya, Castanea, and Juglans, are noteworthyas they do not fit in with the known late Quaternary glacial norinterglacial vegetation cover this far north.

PAZ G-II. This zone is split into two sub-zones, G-IIa and G-IIb(Fig. 7), separated by the 20 cm core gap due to incomplete re-covery. G-IIa is characterized by high values of Cyperaceae pollen(40% SP) in the lower part, whereas Poaceae (40% SP) becomesdominant higher up. Other important pollen taxa are Artemisia,Chenopodiaceae, Salix, and Thalictrum. In PAZ G-IIb, the Poaceaecurve remains relatively high. The pollen of Cerastium-type, Sina-pis-type and Solidago-type are continuously present throughoutthe zone, but with low values.

PAZ G-III. The Betula curve shows a marked rise at the boundaryto G-III, whereas Pinus, Artemisia, Chenopodiaceae, and Poaceaedecrease. The green algae Pediastrum increases. At the upperboundary, maxima of Ranunculus acris-type and Ranunculus flam-mula-type appear, whereas the occurrence of Polypodiaceae isreduced.

PAZ G-IV. This zone comprises unit D (Fig. 7). The total con-centration of pollen rises markedly at the lower boundary. TheBetula curve increases and reaches maximum values of 60% SP inthe upper part. The content of Alnus and Juniperus pollen isincreasing. Picea reaches maximum values (16e17% SP) in the up-per half of the zone. The values of Artemisia, Chenopodiaceae, Poa-ceae, Sinapis-type and Solidago-type all decrease.

3.1.4. Numerical analyses of palaeoecological dataThe PCA of the pollen data from Lake Gerdizty (Fig. 8A) con-

centrates light-demanding, mineral-soil pioneers (e.g. Artemisia,Chenopodiaceae) to the left. Tall herbs, shrubs and trees, whichreflect denser vegetation on humus soils, are grouped to the right.Hence, the first PCA axis, explaining 60% of the species variance,captures two underlying correlating gradients of vegetation densityand soil conditions. Each PAZ is clearly separated, with the oldestsamples found to the left together with pioneer taxa and youngersamples to the right (Fig. 8B).

166 ± 10

128 ± 8

123 ± 7

LAKE GERDIZTY, POLAR URALS, RUSSIAUnit A TREES & SHRUBS HERBS ALGAE AQP SPORES SECONDARY

Analysis: Linn C. Krüger

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Fig. 6. Pollen percentage diagram of unit A, Lake Gerdizty (Fig. 2). The calculation basis comprises the terrestrial pollen taxa (sum pollen, SP). For taxa shown to the right of SP thecalculation basis is SP þ X, where X is the relevant taxon. Total terrestrial pollen concentration (grains cm�3) is also shown. The light grey shading represents 10� exaggeration ofthe scale. Most, if not all, pollen grains from this unit (A) are considered to be reworked from pre-Quaternary strata on the Siberian Lowland.

J.I. Svendsen et al. / Quaternary Science Reviews 92 (2014) 409e428 417

The merged data set shows a similar distribution of taxa(Fig. 9A) to that of Lake Gerdizty. Two corresponding gradientsalong the first axis are seen also here, showing open mineral-soilvegetation to the left and denser humus-soil vegetation to theright. Samples of MIS 3e2 age are concentrated to the left, whereasHolocene samples are found close to axis I to the right (Fig. 9B).

The palynological richness remains more or less constantthroughout unit C (Fig. 7) with a slight decrease in unit D.

3.2. Glacial geological investigations

The Polar Urals have repeatedly been affected by the BarentseKara Ice Sheet as well as by local glaciers (Fig. 1). Below we give abrief review of some glacial features and dating results that directlyrelate to the findings from Lake Gerdizty.

3.2.1. The Usa Moraines e traces of a former piedmont glacier onthe western foothills

The distinct Usa Moraine outlines a former horseshoe-formedpiedmont glacier on the western foothills of the Ural Mountains(Fig. 1A) (Astakhov et al., 1999; Mangerud et al., 2008a; Astakhov,2011). The northern flank of the moraine merges with the south-ern end of the Harbei Moraine. The glacier lobe that deposited thelatter moraine was probably connected to the BarentseKara IceSheet that flowed southward along both flanks of the Polar Urals(Astakhov et al., 1999; Svendsen et al., 2004; Astakhov, 2011). Wehave mapped lateral moraines of the Usa lobe up to an altitude ofw400 m a.s.l., which we consider approximates the former ELA ofthis glacier. This implies that most, if not all, trunk valleys within

the central part of the Polar Urals were filled by ice during thisglaciation (Mangerud et al., 2008a).

The river cuts through the Usa Moraine ridge to expose thecorresponding till and related sediments (Figs. 1A and 10). Thesection consists of sub-horizontally bedded boulderly gravel,clast-supported boulder beds (20e30 cm in diameter), and crossbedded gravels. Our interpretation is that these are outwashsediments that accumulated near the front of a glacier occupyingthe upstream part of the Usa Valley. The outwash is overlain by adistinct diamicton with frequent occurrence of glacially-striatedboulders, which is confidently interpreted as a till. In thedownstream direction the till gradually thickens to form aprominent, 10e15 m high, moraine ridge. The till containspockets of the underlying gravel, and in places we also observedglaciotectonic folding of the underlying strata. In the westernpart of the section, the sandur deposit is covered by a sequence ofloess-like silt several metres thick, evidently a younger aeoliandeposit.

Suitable sand beds within the sandur deposits or sand pocketswithin the overlying till for OSL dating were rare, especially sincethese had to be of sufficient thickness (>50 cm) to avoid significantdosimetric discontinuities. Samples were collected along a stretchof w200 m (Fig. 10). Two were retrieved from an isolated sand raftwithin the till bed, whereas the remaining 7 samples are from thesub-till sandur deposit (Table 3). The three samples collected fromthe easternmost end of the section yield significantly older ages(158e101 ka). Because of poorer exposures in this part of the sec-tion we cannot rule out the possibility that the strata sampled inthis area belong to an older formation predating the last glacier

ΣP=100% ΣP+X

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LAKE GERDIZTY, POLAR URALS, RUSSIATREES & SHRUBS HERBS SPORES AQP ALGAE

Analysis: Linn Cecilie Krüger

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Fig. 7. Pollen percentage diagram of units CeD, Lake Gerdizty. The basis for pollen calculations are the same as in Fig. 6. Loss-on-ignition (LOI) and palynological richness curves are given to the left. The dot in the Pinus curve (at 220 cm)indicates a single find of a stoma. Total terrestrial pollen concentration (grains cm�3) is also shown. The light grey shading represents 10� exaggeration of the scale.

J.I.Svendsenet

al./Quaternary

ScienceReview

s92

(2014)409

e428

418

Fig. 8. A. PCA of pollen data from Lake Gerdizty. Eigenvalues of the four first axes are 0.604, 0.131, 0.066, and 0.032, respectively. Pollen types close to the origin are not shown. B.PCA of the pollen samples. Samples in each pollen zone are connected with lines according to their stratigraphic position. The numbers correspond to sample depth in centimetresbelow sediment surface (Fig. 7).

J.I. Svendsen et al. / Quaternary Science Reviews 92 (2014) 409e428 419

advance. The calculated weighted mean age of the other 6 samplesis 65� 4 kawhich, in principle, should represent the maximum ageof the till. However, we find it more likely that the sandur sedi-ments were deposited by the same glacial advance as deposited theoverlying till. At the western end of the exposure the river runsacross a bedrock threshold and distal to this there is an exposure of

-1.0 1.0-1.0

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Urtica

Equisetum

Osmunda

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Lyc. annotinum

Gymnocarpium dryopteris

Astragalus

alpinus-type

Pedicularis

Caryophyllaceae

Ast CichRumex acetosa

Lycopodium sp.

Axis

II

Axis I

A

Fig. 9. A. PCA of palynological data from Lake Gerdizty (this paper) and lacustrine sedimen0.420, 0.130, 0.097, and 0.046, respectively. Pollen types close to the origin are not shown. B.pollen assemblage zone.

similar gravelly sediments as those described above. The sectionhere is 10 m high and consists of sub-horizontally bedded coarsegravel with a high concentration of 10e20 cm diameter cobbles.Three OSL samples from these strata (Table 3) give a calculatedweighted mean age of 75 � 5 ka, and the mean for all 9 samplesfrom both sections is 68 � 4 ka (Table 3).

-1.0 1.0-1.0

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G-III

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G-Ia

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Y-17

Y-16Y-15

Y-14

Y-13Y-12

Y-11

Y-10

Y-9

Y-8 Y-7 Gerdizty

Yamozero

Axis I

Axis

II B

ts from Lake Yamozero (Henriksen et al., 2008). Eigenvalues of the four first axes arePCA of the pollen samples. The symbols indicate the mean score of the samples in each

Fig. 10. A simplified profile illustrating the investigated sediment section across theUsa Moraine. The location is marked with a star in Fig. 1A. The OSL ages are indicatedwithout error, compare Table 3. The error bars associated with the age calculations are<10 ka for all samples except our sample no. 00-2462 158 � 22 ka (Table 3). 10Beexposure dates of glacially transported boulders from the area inside the Usa Moraine(Mangerud et al. 2008a) are shown in the box above the till.

J.I. Svendsen et al. / Quaternary Science Reviews 92 (2014) 409e428420

In order to better constrain the age of the Usa Moraine we alsoinvestigated an 8 m high river terrace located further upstream,well into the mountain valleys (Table 3). This section included twoseparate exposuresw300 m apart, consisting mainly of either puregravel or sandy gravel; material not optimal for OSL dating. Threesamples were collected from the terrace 3e5 m above the presentriver level. Two of these gave OSL ages of w48 ka, whereas a thirdsample yielded a significantly younger age of w16 ka (Table 3). Wehave no good explanation for this unexpectedly young age, whichwe consider to be an outlier and therefore unreliable.

The most reliable minimum age for the Usa Moraine is providedby 10Be Cosmogenic Nuclide (CN) exposure ages obtained fromboulders inside the moraine ridge (Mangerud et al., 2008a).Recalculated with the production rates recently obtained for Nor-way (Goehring et al., 2012), the CN ages of these samples are55.5� 3.5, 54.7� 3.3, 56.7� 2.8 and 64.7� 3.2 ka. Thus, we bracketthe age of the Usa Moraine between a maximum age of 68 � 4 kaobtained from OSL dating of pro-glacial sandur sediments and a

Fig. 11. The sediment sections at the sites Meskahor and Mamontovaya Kurya (Fig. 1). The Mthat the alluvial sequence at Mamontovaya Kurya is younger than almost the entire sequencMeskahor site are listed in Table 3.

minimum age of 58 � 3 ka, the mean value of the 10Be exposureages of glacially transported boulders located inside the moraine.On the basis of the available data we therefore conclude that theglacier advance happened some time during MIS 4 and that theglacier melted away during an early stage of MIS 3.

3.2.2. Glaciolacustrine sediments downstream from the UsaMoraine

The melt-water stream from the Usa Moraine drained south-wards along the Usa River valley (Fig. 1). This valley, incised into aneroded, lake-free landscape of pre-Weichselian moraines(Astakhov et al., 1999; Nikolskaya et al., 2002), contains severalterraces that are partly exposed along the river bluffs. Of particularimportance are two sites that are located near the Arctic Circle,w160 and 170 km downstream from the Usa Moraine.

The northernmost is a 600 m long and 15 m high section in thehighest (third) river terrace, w5 km upstream of the villageMeskashor (66�37.00N, 62�37.10E) (Fig. 1A). The basal unit (A) in thissection consists of crudely laminated silt with thin sand lenses,probably deposited on a former floodplain (Fig. 11). The sedimentsare pierced by ice wedge casts that are w0.3 mwide near their toplevels at the boundary between unit A and B. A single OSL samplefrom unit A gave an age of 171�10 ka (probably non-finite; Table 3)suggesting that this part of the sequence predates the Last Glacialcycle. The next unit (B) is composed of a series of long cross-beddedsand dunes with some fine gravel, interpreted as channel alluvium.Three OSL samples yield ages in the range 84e74 ka. This unit isoverlain by laminated sand (unit C) with irregular 1e2 cm thick siltbands, reaching a total thickness of 3.4 m. The uppermost part(2.5 m) of this unit C contains four horizons with mushroom-shaped involutions, interpreted as products of overpressure inconsecutive active layers atop of a rising permafrost table. Unit Cwas probably deposited by spring floods on top of a point-bar, andthree consistent OSL dates yield a mean age of 66 � 4 ka. The sand

amontovaya Kurya section is slightly modified from Svendsen and Pavlov (2003). Noticee (units AeD) at Meskashor, as indicated with the dashed line. The OSL-dates from the

J.I. Svendsen et al. / Quaternary Science Reviews 92 (2014) 409e428 421

in C is pierced by ice wedge casts that are filled with dark lacustrineclay from the overlying unit D. The latter unit is up to 6 m thick andcontains a 3 m thick layer of varved clay, consisting of w1600annual couplets (Danilov, 1966), that is sandwiched between twolayers of more massive silt-clay sediments. We consider that unit Daccumulated in an ice-dammed lake that inundated the Usa Rivervalley and initiated thawing of the underlying ice wedge polygons.The glaciolacustrine sediments are in turn capped by a metre ofloess-like silt (unit E) from where a third generation of ice wedgecasts pierces the underlying strata.

The Mamontovaya Kurya site, situated another 13 km down-stream along the Usa River (Fig. 1), is well-known for findings ofPaleolithic artifacts and numerous animal remains (Pavlov et al.,2001) uncovered from strata just above the present river level,radiocarbon dated tow40 ka (mean of 11 dates). This bed is coveredby 15 m of alluvial and aeolian sediments (Fig. 11) that according todating results were deposited between 40 and 15 ka (Svendsenet al., 2010). Apparently normal fluvial drainage prevailed be-tween 40 and 24 ka followed by an episode of aeolian depositionduring the late Weichselian (20e15 ka). We conclude that the riverhere has eroded away the older deposits described at Meskashorand thereby created accommodation space for these younger strata.This means that the age of the ice-dammed lake sediments atMeskashor is between w66 ka (Unit C at the Meskashor) andw40 ka (Paleolithic layer at Mamontavaya Kurya) (Fig. 11).

3.2.3. Relics of former glaciers on the eastern side of the Polar UralsWe have not been able to identify end moraines in the vicinity

Lake Gerdizty, possibly due to the fact that this area is heavilydegraded by solifluction and other slope processes. Further to theeast, the Ob River may have removed possible pre-existing mo-raines. In contrast to the “old-looking” landscape between the riverand the mountain range there are some distinct moraine lobesaround the Rai-Iz Mountainmassivew14 km to thewest of the lake(Dolvik, 2004; Astakhov, 2011) (Fig. 1). The small ice cap thatformed these moraines did not reach Lake Gerdizty. However, aseries of large moraine arches fringe the eastern piedmont of theUrals w50 km outside the Rai-Iz Moraines (Gesse et al., 1963;Khlebnikov, 1974; Astakhov, 1998, 2011). These moraines show amore extensive glaciation that certainly covered Lake Gerdizty, butthey are degraded and leave the impression of being considerablyolder. Unfortunately there are no dates available to constrain theirage.

Some 50e60 km to the northeast of Lake Gerdizty there is asoutheast-facing moraine lobe in the Longotyegan River valley(Astakhov, 1979) (Fig. 1A). This moraine (here termed the LongotMoraine) may represent an eastern counterpart of the UsaMoraine.In order to determine its age we investigated an exposure in anescarpment interpreted as a former ice-contact in the northernflank of the moraine (Fig. 12). The sediments consist of poorlysorted, stratified gravel that also contain some silt, sand and rareboulders; probably glaciofluvial deposits emplaced at the marginand distal to a glacier lobe. Two OSL ages obtained from sand beds

Fig. 12. Investigated profile across the Longot. Moraine on the eastern flank of thePolar Urals. The location is marked with a star in Fig. 1A. The OSL ages are listed inTable 3.

near the base of the exposure were 48 � 4 and 36 � 2 ka, slightlyyounger than the expected Usa Moraine age.

The largest and most distinct glacial feature on the eastern sideof the mountain is the Sopkay Moraine, a long moraine ridge up to12 km wide that has been traced from the Urals eastwards acrossthe southern part of the Yamal Peninsula (Fig. 1C) (Astakhov, 1979;Astakhov et al., 1999). We have previously suggested that thismoraine marks the maximum extent of the BarentseKara Ice Sheetduring the Weichselian (Svendsen et al., 2004; Astakhov, 2006,2011). The moraine zone is characterized by an apparently-freshglaciokarst landscape with numerous small thaw lakes. With theexception of the Longot Moraine, described above, there is hardlyany recognizable glacial feature between the Sopkay Moraine andLake Gerdizty. Glaciofluvial gravel on the distal side of the Sopkaymoraine gave an OSL age of 96� 13 ka. This age is supported by fourOSL ages with a mean age of 81 ka from fluvial sand below pro-glacial varved rhytmites at the nearby Sangompan site (Fig. 1),showing that an ice dammed lake existed in the Ob river valley(Astakhov, 2006).

3.2.4. Traces of local glacier in the interior of the Polar UralsMoraines are commonly found in the fore-field of existing cir-

que glaciers. There are also traces of older and much larger glaciersystems that reached well beyond the mouth of the valleys andonto adjacent lowlands. Mangerud et al. (2008a) obtained a seriesof 10Be cosmogenic nuclide (CN) dates from moraines less than1 km outside the present Chernov Glacier, yielding ages in therange 14e28 ka. In contrast, glacially transported boulders furtherdown-valley yielded much older ages in the range 50e60 ka. Ouron-going investigations support the conclusion of Mangerud et al.(2008a) that large ice free areas have existed within the moun-tain chain since w60 ka, although some glaciers may have beensignificantly larger than the Chernov Glacier during LGM. Asmentioned above, we have mapped distinct and fresh lookingmoraine lobes (marked in Fig. 1) from outlet glaciers that outline asmall ice cap terminating on both sides of the Rai-Iz Mountainmassive (Astakhov, 2011), whichwe assume are of LGM age (Dolvik,2004), although we are also open for the possibility that they areolder and of the same age as the Usa Moraine.

4. Discussion

4.1. Reliability and accuracy of the OSL-based chronology

The glacial and climatic events discussed in this paper arebeyond the limit of 14C-dating and rely on OSL dating, supple-mented with a few 10Be exposure dates. Our current understandingof the Late Quaternary ice sheet chronology in the wider area of theRussian Arctic is also based on extensive application of OSL dating(Svendsen et al., 2004; Larsen et al., 2006; Astakhov, 2012). Ourexperience from previous studies in northern Russia is that OSLdates provide consistent results and that ages younger than 40 kaare generally in accordance with available radiocarbon ages. QuartzOSL ages from an Eemian site gave ages w10% younger than ex-pected of w130 ka (Murray et al., 2007). However, excellentagreement was obtained when comparing OSL and U/Th dates fromorganic strata from the penultimate interglacial (Murray et al.,2008). The spread of ages at individual sites show that real un-certainties for OSL ages are significantly larger than the reportedanalytic error bars and the errors may also vary from site to sitedepending on sedimentary environment and stratigraphic setting.Errors may be caused by uncertainties in long-term variations inthe water content of the dated sediments, variation in sedimentcover, incomplete bleaching of the quartz grains, open systemradionuclide behaviour as well as other factors that are yet poorly

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understood. Based on previous experience we consider that thedeviation from the true ages (calendar years) in some cases may beas much as 10 ka.

We consider the lacustrine sediments from Lake Gerdizty asvery well-suited for OSL dating because the lake was only 5e6 mmetres deep when it came into existence and the water depthdecreased as sediments accumulated on the lake floor. The sedi-ments were probably well exposed to sunlight during fluvial andaeolian transport into the lake and also during deposition in theshallow lake. This assumption is substantiated by the slow andgradual sedimentation that seems to have taken place in the lakebasin. In the unlikely event that the dated sand grains were notsufficiently exposed to daylight to fully reset the OSL signal atdeposition, there is every reason to believe that any incompletebleaching resulted in an age offset of less than w5 ka.

We discount potential errors caused by variations in dose ratesdue to water content as the lake sediments have probably alwaysbeen water saturated. Furthermore, the OSL dates decline upwardsin a very consistent manner. We conclude that lacustrine sedi-mentation started at w65 ka (Fig. 5). In accepting this age we es-timate a very low sedimentation rate with a mean value of0.03 mm a�1, which also means that each OSL sample, most ofwhich were 6e8 cm thick, represents a time period of up to 2000e3000 years. We are open to the possibility that sedimentation wasinterrupted periodically during this long time span and we notethat the portion of the age model spanning the period betweenw30 andw12 ka (MIS 2) is not well constrained by dates. However,the pollen stratigraphy does not indicate any long-lasting in-terruptions in sedimentation.

The OSL dates from the Usa Moraine were conducted on sandthat accumulated on a sandur in front of an advancing ice front(Fig. 10). Such sediments are not optimal for OSL dating as there is arisk that they are not sufficiently exposed to sunlight because ofshort transport times and turbid water. We reject the three outliersin the age range 158e101 ka. However, the remaining six dates gavesimilar ages between 82 and 63 ka, in excellent agreement with themean age of 58 � 3 ka from four 10Be exposure dates for erratics onthe proximal side of the moraine. We therefore confidentlyconclude that the Usa Moraine was deposited during (or close to)MIS 4.

The OSL dates fromMeskashor were collected from alluvial sandthat should bewell suited for OSL dating. The sediments are locatedclose to the river level and have therefore always been watersaturated. The obtained ages also give a consistent picture togetherwith the well dated Paleolithic site Mamontovaya Kurya, indicatingthat the glaciolacustrine sediments accumulated during MIS 4 andcan be correlated with the Usa Moraine further upstream.

4.2. Glacial history

Lake Gerdizty was expected to be located beyond the limit of allWeichselian glaciations (cf. Svendsen et al., 2004). We thereforehoped to obtain a continuous sediment record back to the LastInterglacial (Eemian). However, the coring results reveal that theoldest lacustrine sediments started to accumulate at w65 ka, i.e.w50 ka into the Weichselian (Fig. 5). Here we address the impli-cations of this observation for our understanding of the glacialhistory of the area and we also discuss the wider significance in thelight of mapping results and other glacial geological investigationsin the wider region of the Polar Urals.

4.2.1. Unit A e relic of a pre-Weichselian ice sheet advance?The clayey diamicton (unit A) in Lake Gerdizty is the most

difficult to interpret (Fig. 2). Due to the high pollen content, we atfirst considered that it might be a lacustrine deposit. However, it

was not possible to detect any laminations in the sediments orstratigraphic development in the pollen composition. In fact, thediamicton resembles tills known to be deposited by the BarentseKara Ice Sheet in the adjacent lowland areas. Such tills consist forthe most part of silt and clay, reflecting the fine-grained composi-tion of the underlying sediments. Accordingly, we conclude thatunit A is a till deposited by a shelf-centred ice sheet flowingsouthwards and inundating the adjacent foothills of the UralMountains. The many exotic pollen grains in the till were mostlikely picked up by the ice sheet from pre-Quaternary strata on theWest Siberian lowland.

The OSL ages fromunit A (166,128 and 123 ka) appear to suggestthat the till may post-date the Last Interglacial. However, as dis-cussed in Section 3.1.2 (chronostratigraphy) we consider these to beminimum ages, in which case the till may be considerably older. Asimilar dark-coloured clayey till was identified in a road pitw20 kmto the NE of Gerdizty. At this latter site the till is covered by gla-ciofluvial sand with an OSL age of w200 ka (Astakhov, 2006). Itshould be noted that some Late Quaternary ice sheets reachedmuch further to the south than during the Last Glacial cycle, and inthis area the ice thickness during MIS 6 may have been more than akilometre (Astakhov, 2011).

4.2.2. Units BeC e a Middle Weichselian (75e60 ka) glaciation anddeglaciation?

Unit B, confidently interpreted as a till bed, is much morecoarse-grained than the underlying till (Unit A) and dominated bylocal bedrock material, suggesting that it was deposited by aglacier centred in the adjacent mountains (Fig. 2). However, twoco-authors (Astakhov and Nazarov) consider that it represents ayounger till facies from the same Kara Sea centred glaciation as tillA. Regardless of the type of glaciation that left behind these sed-iments, the series of consistent OSL ages from the overlyinglacustrine strata provide a minimum age of w65 ka for thedeposition of till B. Theoretically, the lake basin could have formedlong time after the area became ice free, for example due todelayed thawing of permafrost and/or buried glacier ice(Henriksen et al., 2008). However, if the area has remained ice freesince MIS 6, which was our earlier working hypothesis, we shouldhave found lake sediments from the warm MIS 5e (Eemian)interglacial. In contrast, the pollen spectra from the lower part ofUnit C indicate cold climate (Fig. 7). We therefore conclude thatthe lake was formed soon after the deglaciation related to till Band thus provide an approximate, although in principle minimum,age ofw65 ka for this glaciation. This “young” age of the lacustrinesediments is supported by the fact that there is no analogue to thedistinct Early Weichselian forest phase (MIS 5a, 85e75 ka) thatwas recorded in Lake Yamozero further to the west (Henriksenet al., 2008).

The dating results indicate that till B in Lake Gerdizty (w65 ka)was deposited during the same glaciation that formed the UsaMoraine (dated to between 68 and 58 ka). The Longot Moraine(Fig. 1) may also be of the same age, although the two OSL agesrelated to this moraine provided ages between 36 and 48 ka(Fig. 12). Nevertheless, provided that Lake Gerdizty was ice coveredduring MIS 4 then this glacier complex must have covered a sig-nificant portion of the adjacentmountain area. The formation of theUsa Moraine on the western flank demonstrates that a large glacierexisted in or across the Ural Mountains. Accordingly, we (exceptAstakhov and Nazarov) conclude that this ice cap, or a system ofconnected valley glaciers, filled most of the central valleys withinthe Polar Urals during MIS 4. North of the Usa lobe the outlet gla-ciers merged with the KaraeBarents Ice Sheet (Fig. 13). A weaknessof our reconstruction is that we have not identified endmoraines tothe east of Lake Gerdizty that are attributed to the last glaciation

Fig. 13. A reconstruction of the ice sheet extent and locally formed glaciers in the Polar Urals during the MIS 4 glaciation. We consider the white semi-transparent area to representthe minimum extent of ice during this glaciation. Notice that the dimension of the local glaciers in the mountains in many areas remains uncertain. It is assumed that the glacierswere mainly confined to the valleys and cirques, leaving most of the mountain peaks ice free. The age of the series of east facing moraine lobes in the areas to the south of the LongotMoraine is unknown. The extent of the contemporaneous ice-dammed lakes that flooded the wide river valleys on both sides of the Ural Mountains during MIS 4 are shown in bluecolour.

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covering the lake. One may speculate whether the large, undatedmoraine lobes SW of Lake Gerdizty may stem from the same MIS 4glaciation although they geomorphologically give the impression ofbeing older than both the Usa and Sopkay Moraines. Perhaps moreimportantly, we have not identified counterparts to these largelobes on the western side of the mountain chain to the south of theUsa Moraine, where local moraines are traced only a few kmoutside the mountain front (Gesse et al., 1963). Thus, based on theavailable evidences we believe that the mentioned series of largemoraine arches along the Ob River belongs to an older glaciation,although we are not fully convinced about this assumption.

Although we have not been able to reconstruct the glacier limitsalong the entire northern stretch of the Urals, we conclude thatmost areas were covered by glaciers during the peak of the MIS 4glaciation and that at least some of the northernmost valley glaciersreached the flanks of both sides of the mountain chain (Figs. 1 and13). This picture is consistent with findings further to the east thatsuggest considerable ice growth during MIS 4 on the PutoranaMountain plateau (Astakhov andMangerud, 2007; Astakhov, 2011),the Severnaya Zemlya Islands (Möller et al., 2006, 2011) and theVerkhoyansk Mountains along the Lena River (Stauch andLehmkuhl, 2010).

4.2.3. Large ice-dammed lakes on both sides of the Polar Uralsduring MIS 4

On the basis of a long series of OSL ages, Mangerud et al. (2004)concluded that a large ice-dammed lake, Lake Komi, flooded thevalleys of the Pechora basin at around 90e80 ka (MIS 5b). Thisimplies that a BarentseKara Ice sheet blocked the Pechora Riverthat today empties into the Barents Sea (Fig. 1). These authors alsopostulated another large lake duringMIS 4. The sediment records atMeskashor and Mamontovaya Kurya indeed support that the val-leys downstream from the Usa Moraine were occupied by an ice-dammed lake during MIS 4 (Figs. 11 and 13) whereas a normaldrainage prevailed during MIS 3e2. However, it should be notedthat we have not been able to demonstrate stratigraphically (i.e. inone section) that large ice dammed lakes formed twice in thePechora Basin, i.e. both during MIS 5b and MIS 4, and this

hypothesis relies exclusively on the OSL results. It is noteworthythat our reconstruction of large ice-dammed lakes during MIS 4 isin good agreement with the inferred glacial history for a wider areaof the Russian Arctic. Perhaps the best dated glacial record associ-ated with ice-dammed lake sediments is the site Yuribei-2 locatedon the Gydan Peninsula to the east of the Ob Estuary (Astakhov andNazarov, 2010). A series of 6 OSL samples from this site, collectedfrom a sequence of glaciolacustrine sand above buried glacier ice,gave consistent ages with a mean value of w63 ka. Thus, strati-graphic evidence for large ice-dammed lakes duringMIS 4 has beenfound inWest Siberia (Astakhov and Mangerud, 2007), as well as inthe White Sea basin further to the west (Larsen et al., 2006; Lysået al., 2011).

4.2.4. Mountain glaciers and their interaction with the BarentseKara Ice Sheet during MIS 4

We have earlier considered that the Harbei Moraine along thewestern flank of the Polar Urals was formed by an ice lobe from theBarentseKara Ice Sheet that flowed southwards along the moun-tains during the Early Weichselian (w90 ka) (Fig. 1A) (Astakhovet al., 1999; Svendsen et al., 2004; Mangerud et al., 2008a). Wenow consider that the Harbei Moraine is younger and belongs totheMIS 4 glaciation. Based on the configuration of themoraines, weinfer that outlet glaciers from the Polar Urals merged with theHarbei lobe (Figs. 1 and 13).

We draw attention to the fact that some horseshoe-shaped endmoraines in the Kara Valley in the north-eastern part of the PolarUrals were deposited by an ice tongue flowing uphill and reachingwell into the mountain valley (Fig. 1A) (Astakhov et al., 1999;Astakhov, 2011). This indicates that the BarentseKara Ice Sheetstill reached the northern tip of the Polar Urals at a time when themountain glaciers had retreated to leave space for the ice sheet toadvance into the northernmost valleys.

4.2.5. Smaller glaciers and unhindered fluvial drainage during MIS3e2 (60e11.6 ka)

The sediment record from Lake Gerdizty reveals that glacial icedid not reach the lake after MIS 4 (Fig. 2). The 10Be ages of glacially

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transported boulders in the Usa Valley (see Section 3.2.2) indicatethat at least some of the main valleys in the Polar Urals have beenice free for the last w50 ka. The last advance of the BarentseKaraIce Sheet onto the northern rim of the continent has been OSL datedto an early stage of late MIS 4 or early MIS 3 at around 60e50 ka(Svendsen et al., 2004; Larsen et al., 2006; Astakhov, 2012). There isevidence to suggest that a full deglaciation of the Barents and KaraSea region took place during MIS 3, soon after this re-advance(Möller et al., 2006, 2011; Mangerud et al., 2008b).

Mangerud et al. (2008a) concluded that the small ChernovGlacier (Fig.1) was only about a kilometre larger during the peak ofthe MIS 2 glaciation than today. Even though the glaciers at thistime presumably were more numerous and some also significantlylarger than at present, most were probably confined to shady cir-ques and narrow valleys that acted as traps for drifting snow.However, we are open to the possibility that some larger glaciersformed in mountain areas where the topography was especiallyfavourable for ice growth, for example in the Rai-Iz Mountainmentioned above (Fig.1) and perhaps alsowithin the central area ofthe mountain chain.

4.3. Vegetation history

4.3.1. Tundra and steppe-like vegetation during MIS 3e2 (60e11.7 ka)

The slow sedimentation rate of the lacustrine sediments in LakeGerdizty leads to poor temporal resolution (Fig. 5). The mean res-olution isw300 years between each pollen sample (1 cm intervals)in the upper part of unit C3, but>1000 years (2e10 cm intervals) forthe lower part of the sequence (Fig. 7). Thus, short-lived vegetationchanges may not be resolved, and as mentioned above, we cannotexclude the possibility that there are hiatuses within the sequencealthough no unconformities have been identified. We thereforeassume the pollen record captures the main trends in the vegeta-tion development, but that finer details are missing. One wouldexpect that changes in lake level have occurred during this longperiod and made an imprint on the pollen and lithostratigraphy.However, we have not been able to trace former lake fluctuations inthe sediment record and this possibility remains hypothetical.

Pollen zones G-IIa and b are dominated by herbs (>70% SP),indicating that the landscape was covered by open and light-demanding vegetation without trees during MIS 3e2 (Figs. 7 and8). In the lower part of G-IIa sedges and grasses must have beenimportant components of the vegetation, which probably alsoincluded dwarf-shrubs of Salix and Betula nana. Mesophytic herbslike Thalictrum and Oxyriawere growing in moister areas, whereasArtemisia and Chenopodiaceae represent steppe-like vegetation ondrier ground. Species of Papaver, thrivingonunstable soils,mayhaveestablished on wind-blown ridges and slopes. The relatively lowpercentages (<25% SP) of arboreal pollen, as well as the single pinestoma at 220 cm depth, are considered as results of long-distancetransport and/or redeposition. From w50 ka the increase in Poa-ceae and the decreasing Cyperaceae could reflect less favourablegrowth conditions for sedge vegetation around the lake. A reductionin shrubs (Betula, Salix) suggests that the environment graduallybecame harsher, which is also indicated by a decrease in the pollenconcentrations (Fig. 7). In the pollen assemblage zone G-IIb thesteppe-elements slightly increase in importance whereas plantsrequiring moister substrate (e.g. Thalictrum, Oxyria) decreased.

The main vegetation development at Lake Gerdizty during theMIS 3e2 was similar to that inferred from cores from Lake Yamo-zero (Henriksen et al., 2008), located approximately 750 kmWeSWof Lake Gerdizty (Fig. 1C). However, it should be mentioned that theYamozero data suggest slightly more heterogeneous vegetation(Fig. 9). More importantly, the pollen diagram from Lake Yamozero

also reaches into an older warm interval (possibly MIS 5a) with anopen forest environment (e.g. PAZ Y-11 in Fig. 9). Further to thewest, at Sokli in NE Finland (Fig. 1C), there seems to have been atreeless vegetation during MIS 3 (Bos et al., 2009). In the latter areathe vegetation was described as low arctic tundra with dwarfshrubs and a relatively high proportion of grass and sedges. East ofthe Ural Mountains, a vegetation consisting mainly of grassesedgeassociations mixed with steppe elements appears to have extendedfrom areas surrounding Lake Gerdizty and all the way to the YamalPeninsula further to the north (Andreev et al., 2006). From the sameperiod (MIS 3), a mixture of tundra and steppe-like vegetationwithout trees has also been recorded further east in Siberia; on theTaymyr Peninsula (Andreev et al., 2003), at the Laptev Sea Coast(Andreev et al., 2002a, 2002b, 2009; Sher et al., 2005) and on thefoothills of the Verkhoyansk Mountains (Müller et al., 2010).

The boundary between Gerditzy pollen zones G-II and G-III isrepresented by a pronounced rise in the curves for Betula and Salixas well as the onset of a continuous representation of Juniperuspollen suggesting that dwarf shrub communities re-establishedaround the lake. The higher values of the pollen types R. acris andR. flammulamay indicate increased soil moisture at this time. Thesechanges, as well as the increasing total pollen concentrations, areassumed to reflect responses to a climatic warming; probably theonset of an interstadial. However, there is nothing to suggest thattrees reached the lake area during this period and the typicaltundra-steppe elements (Artemisia, Chenopodiaceae, Poaceae, Tha-lictrum) were still important components of the vegetation. Minordepressions in the Betula and Salix curves and increased Poaceaevalues close to the upper boundary of PAZ G-III may possibly reflectthe Younger Dryas (YD) cooling; however, the signals of this eventare not very clear. The low temporal resolution (300 years betweeneach pollen sample in PAZ G-III) makes it difficult to detect vege-tation changes in periods as short-lived as the YD. Cool and aridconditions during the YD are reflected by increased values ofsteppe-elements in several Russian pollen records (Andreev et al.,1997; Khotinsky and Klimanov, 1997; Andreev et al., 2002a, 2003;Paus et al., 2003; Henriksen et al., 2008).

4.3.2. Forest development during the early HoloceneThe increases in Betula and total pollen concentration at the

onset of pollen zone G-IV reflect the onset of the Holocenewarming. The disappearance of Papaver, Cerastium-, Sinapis-, andSolidago-types, combined with the reduction in Chenopodiaceaeand Artemisia (Figs. 7 and 9) signal stabilized soil conditions andless light due to vegetation closure. The increased loss-on-ignitionvalues probably reflect lowered input of minerogenic material aswell as increased autochthonous production shown by theincreased concentration of green algae colonies. Juniperus, Salixshrubs and Alnus expanded in the surroundings, reflecting presenceof both dry and moister substrates.

The strongly increasing Betula values (pollen percentages andconcentrations) suggests local establishment of birch trees aroundLake Gerdizty during the Holocene. However, due to the lack ofspecies determination and the poor chronological control, it is notpossible to determine exactly when birch trees first arrived at thesite. Just 10e15 kmwest of Lake Gerdizty, and approximately 40 kmto the north within the Polar Urals, birch trees were establishedduring the earliest part of the Holocene interval (Koshkarova et al.,1999; Panova et al., 2003; Jankovská et al., 2006). Birch trees alsoestablished in the Arctic part of European Russia, at this time (e.g.Paus et al., 2003; Kultti et al., 2003, 2004; Henriksen et al., 2008;Jones et al., 2011, see also; Kremenetski et al., 1998). Välirantaet al. (2006) suggest that around Lake Llet-Ti in the Pechora Basin(w300 km to the west of Lake Gerdizty), scattered birch and sprucetreesmay have been present as early as the YD-Holocene transition,

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and that a boreal spruce-birch forest was established between 10.5and 10.2 ka.

At Lake Gerdizty, both percentage and concentration values ofPicea increase from the early Holocene. According to Surova et al.(1975, cited in e.g. Kultti et al., 2004), spruce was already growingin the Polar Urals at the very beginning of the Holocene. From a siteclose to Lake Gerdizty, Picea obovatamacrofossil remains have beendated to w10.6 ka (9.4 14C ka BP, Koshkarova et al., 1999). Inaccordance with this, we suggest that birch and spruce weregrowing in the vicinity of Lake Gerdizty not later than a few hun-dred years into the Holocene. There are no indications from thisrecord that pine trees formed part of the vegetation in this area.This is in accordance with the findings of MacDonald et al. (2000)that Pinus was never important north of the modern treeline inareas east of the Kola Peninsula.

4.4. Climate history

We have reconstructed the environmental and climatic changesover the last w100 ka (Fig. 14). The reconstructed temperaturecurve is based on the inferred vegetation and glacial history.Considering that this area was probably located to the north of thetreeline throughout MIS 3e2 we assume that the mean summertemperatures remained below 10e12 �C. Modelling experimentssuggest that the large ice-dammed lakes that existed south of theBarents-Kara Ice Sheet during periods when it expanded onto thecontinent (MIS 5b and 4), caused a dramatic cooling of the summertemperatures of as much as 8e10 �C across vast areas of Russia(Krinner et al., 2004). Accordingly, we postulate that the summertemperatures during these cold spells dropped below 5 �C. Theclimate curve outlines the approximate trends based on our currentunderstanding of the environmental changes and is accompaniedwith considerable uncertainty.

Fig. 14. Environmental changes in the Polar Urals and the adjacent lowland areas. Thediagram illustrates the reconstructed glacial, environment and climate history over thelast 100 ka. The ice-dammed lakes are from Mangerud et al. (2004) and this study(Meskashor). The inferred environment during MIS 5a (Odderade Interstadial) is basedon the record from Lake Yamozero further to the west (Henriksen et al., 2008).

Unfortunately, we have no observations that can be used todetermine the timing and extent of the mountain glaciers prior toMIS 4, but it seems likely that sizeable mountain glaciers formed inthe Polar Urals during MIS 5b (90 ka) and perhaps also during MIS5d.

The BarentseKara Ice Sheet glaciation during MIS 5b (Svendsenet al., 2004), that corresponds with the ice-dammed Lake Komi inthe Pechora basin (Mangerud et al., 2004), appears to have beensucceeded by an exceptionally warm period during MIS 5a (85e75 ka) when the taiga approached the northernmost parts of thecontinent (Houmark-Nielsen et al., 2001; Henriksen et al., 2008).We infer that the BarentseKara Ice Sheet melted away during thiswarm episode when parts of the Arctic Ocean may have becomeseasonally ice-free (Polyak et al., 2010). The strong warming wasprobably driven by a high summer insolation; June insolation at65 �N increased by as much as 12% between 95 and 85 ka (Bergerand Loutre, 1991; Berger, 1992).

The climate turned cold again during MIS 4 causing a re-growthof the Eurasian Ice Sheet (Svendsen et al., 2004) and once againlarge ice dammed lakes flooded the major river valleys in thelowland areas in northern Russia (Mangerud et al., 2004; Larsenet al., 2006; Lyså et al., 2011). During this period large glaciersformed in the Ural Mountains and deposited the Usa Moraine andthe upper till at Lake Gerdizty. As discussed abovewe now considerthat the BarentseKara Ice Sheet was somewhat larger during MIS 4than assumed previously. The timing of the MIS 3 deglaciation isnot well constrained but observations from Severnaya Zemlya(Möller et al., 2006, 2011), Novaya Zemlya (Mangerud et al., 2008b)and Svalbard (Mangerud et al., 1998) indicate that these Arcticislands were essentially ice free fromw50 ka until the regrowth ofthe glaciers that took place during early MIS 2. Nevertheless, thepollen stratigraphy from Lake Gerdizty reveals that few if any treesexisted in this part of the Polar Urals during MIS 3, suggesting thatthe summer temperatures remained below 10e12 �C (Fig. 13). Thus,MIS 3 was significantly colder than during the partly forested EarlyWeichselian interstadials (MIS 5a and MIS 5c) (Helmens et al.,2007; Henriksen et al., 2008). In comparison, the pollen-basedclimate reconstruction for the Sokli site in the Finnish Laplandsuggests a slightly warmer (10e13 �C) summer climate in that re-gion (Helmens et al., 2007; Bos et al., 2009). Further east, in theLaptev Sea region, fossil insect assemblages as well as palae-obotanical records suggest that the summer temperatures at theground may have been close to the present values and even higherduring the entire Weichselian stage (Hubberten et al., 2004). Initialhuman colonization of the Urals took place during the last half ofMIS 3 at which time the soils must have been sufficiently fertile andmoist to sustain a numerousmammothmegafauna (Svendsen et al.,2010; Slimak et al., 2011). The sediment sequence at MamontovayaKurya (Fig. 11) suggests that the River Usa was at least as deep astoday during theMIS 3/2 transition, suggesting that the climatewasnot unusually dry w35e25 ka (Svendsen and Pavlov, 2003). Sincew25 ka the climate became significantly more dry and frosty, butopen tundra-steppe vegetation seems to have prevailed in at least apart of this northern region (Astakhov and Svendsen, 2008, 2011).

The growth of ice sheets over northern Eurasia during thetransition between MIS 3 and 2 must have been driven by asignificantly colder climate. Inverse modelling indicates that verylow precipitation prevented eastward growth of the Barents IceSheet (Siegert et al., 1999, 2001). The south-eastern margin of theice sheet was probably influenced by cold and dry winds coming offthe ice and causing the winter temperatures to drop below �50 �C(Siegert and Marsiat, 2001; Hubberten et al., 2004). Measured d18Ovalues of syngenetic ice wedges on the Yamal Peninsula indicatethat the winter temperatures were 6e9 �C colder than today(Vasil’chuk et al., 2000). The extremely low winter precipitation

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may explain why large ice caps did not form in the Polar Urals atthis time, in spite of a colder climate. We also call attention to thefact that the BarentseKara Ice Sheet did not block northboundfluvial drainage during MIS 2 (25e11,6 ka), in contrast to the situ-ation during the earlier, more extensive glaciations when large ice-dammed lakes flooded the lowlands. Thus, it is not unlikely that thesummer temperatures during MIS 2 were higher than during thepreceding MIS 4 glaciation. The widespread deposition of aeoliansand that appears to have taken place during the period 20e14 kasuggests that the climate on both sides of the Polar Urals wassignificantly drier (Astakhov et al., 2004; Astakhov and Svendsen,2008; Heggen et al., 2012).

According to the pollen data from Lake Gerditzy, the first sig-nificantwarminghappened at around14e15ka (Fig. 7). As a result ofthis warming, the permafrost started to melt, forming some of thenumerous lakes that characterize the present-day landscapes in theArctic (Henriksen et al., 2008). The Younger Dryas cooling is hardlyrecognizable in the Gerdizty data, probably because of the poortemporal resolution, but the abrupt Holocene warming is clearlyreflected in the pollen diagram. No later than a thousand years intothe Holocene, when spruce trees established around the lake, thesummer temperatures must have become higher than today.

5. Conclusions

The Lake Gerdizty cores, from the eastern Polar Urals, contain acontinuous lacustrine sequence that accumulated over the last65 ka, i.e. during MIS 3e1.

The lacustrine sediments rest on a gravelly till (Unit B) that isassumed to reflect the former existence of a large glacier complex inthe Polar Urals during MIS 4 (�65 ka) that we correlate with thepronouncedUsaMoraineon thewesternflankof themountain chain.

During the MIS 4 glacial maximum some of the mountain gla-ciers merged with the BarentseKara Ice Sheet that at this timeinundated the northern rim of the continent, causing large ice-dammed lakes on the lowland areas in West Siberia as well as inthe European sector of Northern Russia (Fig. 13).

Much smaller glaciers existed during MIS 2; The Barents-KaraIce Sheet did not reach onto the adjacent lowland and there wereonly small local glaciers in the Polar Urals.

The pollen stratigraphic record from Lake Gerdizty shows thatan open tundra and steppe vegetation persisted throughout MIS 3(65e25 ka), reflecting a persistent cool summer climate (Fig. 14).

The first significant warming occurred at the onset of the Late-glacial interstadial (BøllingeAllerød) at around 15e14 ka. Dwarfshrub communities then re-established in the landscape thatremained treeless.

The first dramatic change in vegetation cover happened soonafter the Holocene transition when birch and spruce trees reachedthe area around Lake Gerdizty.

Acknowledgements

The field investigations formed part of the former researchproject “Ice Age Development and Human Settlement in NorthernEurasia (ICEHUS)”, funded by the Research Council of Norway(project nos. 176048 and 167131). The coring of the lake floorsediments in Lake Gerdizty was executed by the Vorkuta basedgeotechnical drilling company “SEVERSTROY”, supervised by AlexeiMatioushkov, VSEGEI (National Geological Institute). HerbjørnPresthus Heggen helped with processing and describing the sedi-ment cores. Alexei Matiouchkov, Tore Dolvik and Torgeir Hjelleparticipated in the field work in the Urals. Brent Goehring recal-culated the 10Be ages. Eva Bjørseth prepared many of the illustra-tions. Two anonymous reviewers provided useful comments and

suggestions that helped to improve the quality of the manuscript.We offer our sincere thanks to all who have contributed to thisstudy. This study is also a contribution to the Centre for ClimateDynamics at the Bjerknes Centre, Bergen, Norway.

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