Ostracod palaeoecology and environmental change in the Laptev and Kara seas (Siberian Arctic) during the last 18 000 years

Ostracod palaeoecology and environmental change in the Laptev andKara seas (Siberian Arctic) during the last 18 000 years

ANNA STEPANOVA, EKATERINA TALDENKOVA AND HENNING A. BAUCH

Stepanova, A., Taldenkova, E. & Bauch, H. A. 2012: Ostracod palaeoecology and environmental change in theLaptev and Kara seas (Siberian Arctic) during the last 18 000 years. Boreas, 10.1111/j.1502-3885.2012.00254.x.ISSN 0300-9483.

Fossil ostracod assemblages were investigated in five AMS 14C-dated cores from various water depths of theLaptev and Kara seas ranging from the upper continental slope (270 m) to the present-day shelf depth (40 m). Sixfossil assemblages were distinguished. These represent the varying environmental conditions at the North Siberiancontinental margin since about 18 ka. In the cores from the shelf the ostracod assemblages reflect the gradualtransition from an estuarine brackish-water environment to modern marine conditions since 12.3 ka, as inducedby the regional early Holocene transgression. The core from the upper continental slope dates back to c. 17.6 kaand contains assemblages that are absent in the shelf cores. The assemblage older than 10 ka stands out as aspecific community dominated by relatively deep-water Arctic and North Atlantic species that also containseuryhaline species. Such an assemblage provides evidence for past inflows of Atlantic-derived waters from as earlyas c. 17.2 ka, probably facilitated by upwelling in coastal polynyas, and a considerable riverine freshwaterinfluence with enhanced surface water stratification owing to the proximity of the palaeocoastline until earlyHolocene times. In all studied cores, relative increases in euryhaline species dominant in the inner-shelf regions arerecorded in the mid–late Holocene sediments (<7 ka), which otherwise already contain modern-like ostracodassemblages with relatively deep-water species. This observation suggests euryhaline species to be largely sea-ice-and/or iceberg-rafted and therefore may provide evidence for a climate cooling trend.

Anna Stepanova (e-mail: [email protected]), Borisyak Paleontological Institute RAS, Profsoyuznaya,123, Moscow, 117997, Russia; Ekaterina Taldenkova (e-mail: [email protected]), Geographical Faculty,Lomonosov Moscow State University, Leninskie Gory, 1, Moscow, 119991, Russia; Henning A. Bauch (e-mail:[email protected]), Mainz Academy for Science, Humanities and Literature, c/o GEOMAR Helmholtz Centre forOcean Research, Wischhofstrasse 1-3, 24148 Kiel, Germany; received 7th February 2011, accepted 27th January2012.

The late Quaternary palaeoceanographic record of theArctic is characterized by marked shifts in climate andsea level (Bauch et al. 1999, 2001; Spielhagen et al.2004). These changes are strongest at the interfacebetween marine and terrestrial systems, owing to thehigh instability of this environment related to the inputof terrigenous material, high organic productivity, pro-nounced seasonality and large changes in salinity (Steinet al. 2001, 2004).

Ostracods represent an important group of benthicorganisms inhabiting a wide range of aquatic environ-ments, from freshwater pools to marine deep-seabasins. Ostracods rapidly invade newly created habitatsand are able to track their preferred environmental andclimatic optima (Griffiths 2001). The taxonomic com-position of ostracod assemblages is strongly dependenton environmental conditions. Certain index taxa arerestricted to a specific range of environmental param-eters and can reflect changes of water-mass propertieseven on decadal time scales, making ostracod assem-blage analysis a very precise instrument (AlvarezZarikian et al. 2000; Cronin et al. 2002; Cronin &Vann 2003). For this reason they are frequentlyused as palaeo-environmental indicators of bottom-water salinity and temperature (Cronin 1977, 1981;McDougall et al. 1986; Stepanova 2006).

Previous studies of recent ostracods from the Laptevand Kara seas have covered environments ranging fromfreshwater lagoons and brackish-water estuaries to rela-tively deep-water parts of the upper continental slope(Stepanova et al. 2003, 2004, 2007). Distribution pat-terns of ostracods in these river-influenced marginal seasshow a clear relationship with salinity and water depth.

Using these data on recent Arctic ostracod assem-blages, we carried out a detailed study of fossil ostra-cods from five AMS 14C-dated sediment cores from theLaptev and Kara seas (Bauch et al. 2001; Simstich et al.2004; Taldenkova et al. 2005, 2008, 2010) in order toanalyse the transition from Lateglacial to modern envi-ronments in the High Arctic continental marginal area.These regions underwent a transformation from terres-trial to marine environments under postglacial sea-levelrise and faced climate-induced changes in water-masscirculation, freshwater influence and extent of sea icecover (Bauch et al. 1999, 2001; Polyakova & Stein2004; Stein et al. 2004; Polyakova et al. 2005;Taldenkova et al. 2005, 2008, 2009, 2010; Darby et al.2006). Our inferences about past environmentalchanges are based on the analysis of ostracod abun-dance and taxonomy, the ecology of index and domi-nant species, and the range of individual speciestolerance to changing salinity and water depth.

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DOI 10.1111/j.1502-3885.2012.00254.x © 2012 The AuthorsBoreas © 2012 The Boreas Collegium

Materials and methods

Materials and sampling

Five core sections were analysed, four from differentparts of the Laptev Sea and one from the Kara Sea(Fig. 1; Table S1 in Supporting Information). Coresfrom the Laptev Sea were obtained in 1998 duringthe TRANSDRIFT V expedition aboard the R/V

‘Polarstern’. Three cores from the Laptev Sea shelf arelocated in the palaeovalleys of the biggest rivers of theregion. The eastern cores include core PS51/138-12from the Lena palaeovalley (water depth 45 m) andcore PS51/135-4 from the Yana palaeovalley (waterdepth 51 m). Core PS51/159-10 was retrieved fromthe Khatanga palaeovalley (water depth 60 m) on thewestern outer shelf, whereas core PS51/154-11 origi-nates from the western continental slope (water depth

aimyr PeninsulaK o t e l n y y

Lena Delta

Tiksi

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Taimyr PeninsulaK o t e l n y i

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LenaDelta

PS51/138-12

PS51/135-4PS51/159-10

PS51/154-11

154

Barents-Kara Ice Sheet

Laptev Sea

Fram Strait

Arctic Ocean

Kara Sea

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159

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135

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RussiaB

C5

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2

787674 E

N

Ob’

Yenisei

BP00/07-5

Kara SeaB

1 5

Severnaya Zemlya

100 km

100 km100 km

Fig. 1. A. Distribution of Atlantic-derived waters and ice-sheet boundaries during the Last Glacial Maximum (Svendsen et al. 2004). Numbersin the figure correspond to the studied core sites: 07=BP00/07-5; 154=PS51/154-11; 159=PS51/159-10; 135=PS51/135-4; 138=PS51/138-12. B.Site locations in the Kara Sea. C. Site locations in the Laptev Sea. This figure is available in colour at http://www.boreas.dk.

2 Anna Stepanova et al. BOREAS

270 m). Core BP00/07-5 was obtained from the easternKara Sea shelf (water depth 43 m) during expeditionSIRRO-2000 aboard the R/V ‘Akademik BorisPetrov’. It originates from a topographic depression, aformer Yenisei valley now filled with sediments (Sim-stich et al. 2004). Hereafter the cores will be referred toas PS138, PS135, PS159, PS154 and BP07.

Cores were sampled in 2–3 cm thick slices at differentintervals: cores PS138 and BP07 were sampled at2–10 cm step intervals, whereas cores PS159 and PS154were sampled continuously. Samples from these coreswere freeze-dried and weighed. Core PS135 was origi-nally sampled onboard at 30-cm intervals. These large-size samples (approximately 400 mL) were washedonboard without weighing. Later, the core wasre-sampled at 10-cm intervals (~100 mL). These lattersamples were freeze-dried, weighed and washed over a63-mm mesh-size sieve. Ostracods were picked fromsample residues, identified and counted. All ostracodvalves are stored at the Borisyak Paleontological Insti-tute of the Russian Academy of Sciences (Moscow);counts and distribution data are available at:doi:10.1594/PANGAEA.735539.

Core chronologies

The chronology of all cores is based on radiocarbondates of marine biogenic carbonate of bivalves, ostra-cods and foraminifers (Table S1) (Bauch et al. 2001;Simstich et al. 2004; Taldenkova et al. 2008, 2009,2010). Dates were obtained by accelerator mass spec-trometry (AMS) at the Leibniz Laboratory in Kiel.Reservoir age corrections were applied as follows:-370 years for the Laptev Sea (Bauch et al. 2001) and440 years for the Kara Sea (Simstich et al. 2004). Forconverting the AMS 14C dates to calendar years BPwe used the Fairbanks 0107 program (Fairbanks et al.2005). Application of the Fairbanks 0107 program forthe dates from the Kara Sea showed insignificant agedifferences from the originally obtained age estima-tions of Simstich et al. (2004) based on the calibrev.4.3 program (Stuiver & Reimer 1993) (Table S1).All ages noted in the text are calculated calendarages.

Ages between dated samples were estimated by linearinterpolation and assuming a modern age for the sedi-ment surface (Fig. 2). When calculating the age modelfor core PS154 we excluded the two reversed dates from31 and 39 cm depth obtained on bivalves and gastro-pods (Table S1), as these are probably the result of theburrowing of molluscs into the sediment accumulatedunder low sedimentation rates. The sediment sectionbetween 51 and 115 cm is strongly bioturbated andshows almost the same age (Table S1). Therefore, onlythe youngest and the oldest dates were taken for agemodel calculations, and the date obtained on foramini-fers from 73 cm was assigned to 51 cm core depth.

Micropalaeontological analysis

For ostracod assemblage characterization we comparethe number of species per sample, the total abundanceof specimens, and the abundance of different ecologicalgroups and individual taxa (Figs 3–7). For core PS135(Fig. 3) we produced species percentages instead of thespecies abundances because of the two sets of samplesused (see ‘Materials and sampling’): the large-sizesamples lack dry bulk weight information, while thesmall-size samples contain few ostracod valves, pre-sumably because of a certain degree of dissolution insediment during storage. For the relative percentage,samples with fewer than 10 valves were excluded fromthe calculation. Since the number of specimens in mostof the small-size samples was less than 5 valves persample, when compared with the original large-sizesamples that average 50–70, we produced species per-centage plots only from the latter set of samples.

In general, ostracod occurrence in marine sedimentsis rare compared with other fossil groups. Ostracodsgrow discontinuously, and their juvenile stages are pre-served. It has been shown, for instance, that the numberof juvenile valves (population-age structure) is ameasure characterizing the energy type of an environ-ment together with the preservation of an ostracodassemblage (Whatley 1983) (Figs 3–7). Whatley (1983)distinguished three types of assemblages, interpretationof which can vary: (i) with up to 80% of juveniles ofdifferent instars (life assemblage in a low-energy envi-ronment); (ii) with at least 25% of juveniles of olderinstars (either a life assemblage in a high-energy envi-ronment when smaller juveniles and dead valves arewashed out or a thanatocoenosis with current velocitiessufficient to carry larger allochtonous valves); and (iii)100% juvenile percentage (mass burial indicating acatastrophic event or low-energy environment wherethe current velocity is low and smaller allochtonousvalves could be introduced). Although the percentageof juveniles in our samples is quite variable, indicatinga rather high-energy environment, ostracod valve pres-ervation was generally good, without traces of corro-sion. The exception is in core PS154, where theextremely poorly resolved interval corresponding to9–7 ka contained only few ostracods, and those were ofrelatively poor preservation.

The Shannon–Weaver diversity index (Shannon &Weaver 1963) was calculated for all cores (Fig. 8). It is

defined as H S p pi ii

S

( )=− ( )=∑ ln

1

, where pi is the pro-

portion of the ith species and S is the total number ofspecies. The Shannon-Weaver diversity index is rela-tively insensitive to sample size and low total abun-dance (Alvarez Zarikian et al. 2009). Low index values(typically around 1.5) indicate low diversity andhigh dominance in association with stressful condi-tions, whereas high values (around 3.5) correspond to

Ostracod palaeoecology and environmental change in the Laptev and Kara seas, last 18 000 years 3BOREAS

5908(CB)

1061(CT)

- ,Core Base inter/extrapolated age

- ,Core Top extrapolated age

- Core Catcher, extrapolated age

0

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(cm

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2778

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5598

6925

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120

Age (cal. ka)

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BP00-07/5

PS51/154-11

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Fig. 2. Age–depth relationships in the studied cores. This figure is available in colour at http://www.boreas.dk.


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10

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10

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10

30

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5Cytheropteron occultum

5Cytheropteron inflatum

10Bythocythere constricta

10

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Rabilimis mirabilis

10

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Sarsicytheridea punctillata

10Pteroloxa sp.

5Elofsonella concinna

5Cytheropteron elaeni

10Cytheropteron arcuatum

10

20


5Cytheropteron montrosiense

10Cytheropteron discoveria

5Rabilimis septentrionalis

5Palmenella limicola

10Cytheropteron sulense

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low-dominance, high-diversity assemblages and morestable conditions (Shannon & Weaver 1963).

According to the distribution of modern ostracods indifferent environments and their ecological preferences,primarily salinity and water depth, we divided ostracodspecies into six groups: freshwater, brackish water,euryhaline, marine shallow-water, marine relativelydeep water, and a specific group of marine relativelydeep-water North Atlantic species restricted toAtlantic-derived waters (Table S2; for details see alsoStepanova et al. 2007). Generally, these ecologicalgroups correspond to the following salinity ranges:marine relatively deep-water taxa are found in watersof >30 psu; marine shallow-water taxa in waters of>18 psu; euryhaline species tolerate a wide salinityrange between 5 and 35 psu; brackish-water speciescorrespond to the salinity range 5–26 psu; and, finally,freshwater species are found in waters with salinitylower than 7 psu (Stepanova et al. 2007).

Regional setting

Modern environments of the Laptev and Kara seas

The Kara and Laptev seas are marginal Arctic seaslargely represented by vast shallow shelves gentlysloping northwards down to water depths of 100 m. Aremarkable feature of the shelf topography in both seasis the presence of several palaeo-river valleys formedduring the Pleistocene sea-level lowstand and stret-ching towards the shelf break. These palaeovalleyswere the major areas of sediment accumulation duringthe Holocene transgression and contain evidence forthe transformation of the shelf from a terrestrial to amarine environment (Bauch et al. 1999, 2001; Dittmerset al. 2003; Stein et al. 2004).

Bottom-water temperature in the seas is below 0°Cnearly all year round. Only in the outer shelf troughsand at the continental slope does the temperature riseto 1.0–1.5°C owing to the inflow of warmer Atlantic-derived waters (Dobrovol’skii & Zalogin 1982; Pavlovet al. 1996). Atlantic surface waters enter the ArcticOcean north of Spitsbergen (Fram Strait BranchWater, FSBW) and through the Barents Sea troughs(Barents Sea Branch Water, BSBW), submerge beneathcolder polar waters and flow eastwards along the Eura-sian continental slope (Rudels et al. 2004; Fig. 1A). Inthe Laptev and Kara seas these waters remain at theslopes, although periodically reversed bottom currentsmay bring them onto the shelf along the river palaeo-valleys (Dmitrenko et al. 2001).

The seas are ice-covered for 9–10 months of the year,with a continuous fast ice cover bounding the shallowinner-shelf zone approximately along the 20–25 misobaths. Further to the north, in the middle shelf area,the fast ice edge is often separated from the drift iceby wind-induced open-water areas termed polynyas

(Pavlov & Pfirman 1995; Dmitrenko et al. 2000). Insummer, the seas are largely ice-free with the drift icemargin located far northwards, although in some yearsthe remnants of fast ice and pack ice can persist duringthe summer in the western Laptev Sea close to theTaimyr Peninsula (Reimnitz et al. 1995). The importantmechanism of sedimentation in the near-shore zone isthe resuspension of sediments by autumn storms andreversed currents, entrainment into the newly formed icein the freshened coastal waters, and ice-rafting to theseasonal drift ice margin in distant offshore regions(Eicken et al. 1997; Wegner et al. 2005).

The eastern parts of the Kara and Laptev seas aresubject to strong runoff influence by the Yenisei, Ob’and Lena rivers. As a result of the considerable riverrunoff in summer and ice formation in winter, salinityis the most variable feature affecting the spatial distri-bution of ostracods. Surface water salinities rangefrom less than 5 psu in the estuaries down to 28–30 psuin the river-distal areas. Because of the prevention ofvertical mixing by the low-density freshwater outflow,bottom-water salinities are less variable. They rangefrom 10–15 psu in the outer river estuaries in the KaraSea up to 34–35 psu on the outer shelf and upper con-tinental slope (Polyak et al. 2002; Stepanova et al.2003, 2007).

Recent ostracod assemblages from surface sediments

Modern ostracod assemblages from the Laptev andKara seas show large changes in composition withincreasing distance from the coast and decreasingriver influence as a result of changes in water depth,food supply and water mass properties (Table S2;Stepanova et al. 2003, 2007).

River estuaries of the Ob’ and Yenisei rivers areinhabited by freshwater species in their inner parts,where summer bottom water salinities are less than2 psu, and by a brackish water assemblage in theirouter parts, where salinity increases up to 26 psu.

The mixed euryhaline–marine assemblage is domi-nated by the euryhaline species Paracyprideis pseudop-unctillata, and Heterocyprideis sorbyana occurs in theinner-shelf zone of the Kara and Laptev seas down to20–30 m water depths, where bottom-water salinitiesrange between 26 and 32 psu.

A taxonomically diverse marine assemblage com-prises shallow-water marine taxa. These characterizenorthern parts of the Kara and Laptev shelves andupper continental slope with stable bottom environ-ments and a salinity higher than 32 psu.

In the middle-shelf zone, shallow-water marinespecies could reach up to 100%, while the relative abun-dance of euryhaline species and the total abundancedecrease.

Taxonomic diversity and percentages of relativelydeep-water species including the North Atlantic ones


increase on the outer-shelf and upper continentalslope. Atlantic-derived waters influence the taxonomiccomposition of modern assemblages in the easternKara Sea and western Laptev Sea, but they do notreach the eastern Laptev Sea. We found North Atlan-tic species only at sites from the Kara Sea and fromthe Khatanga valley of the Laptev Sea (Stepanovaet al. 2003, 2007).

Abundant euryhaline species that dominate theinner-shelf assemblage are also found at greater waterdepths, where they are identified as part of an ice-raftedassemblage (Cronin et al. 1994; Jones et al. 1998, 1999;Stepanova et al. 2003, 2007). They are possiblyentrained into the newly formed ice during autumnstorms and the freeze-up period and transported to thedistal open-sea areas (Nürnberg et al. 1994; Eickenet al. 1997).

Fossil ostracod assemblages from marinesediment cores: abundance, taxonomyand age ranges

Six fossil assemblages have been distinguished in thestudied cores: Estuarine brackish water (1), Inner-shelfeuryhaline–marine (2), Middle-shelf shallow-watermarine (3), Outer-shelf marine (4), Pre-Holocene uppercontinental slope marine (5), and Holocene upper con-tinental slope marine (6a, b). A sequence of assem-blages from estuarine to modern normal marine ones(assemblages 1–4 in Figs 3–5, 7–9) is observed in allstudied sediment cores from palaeoriver valleys on themiddle-outer shelf, namely PS138, PS135, PS159 andBP07. Relatively deep-water ostracod assemblages 5and 6 from the deepest core PS154 on the western uppercontinental slope of the Laptev Sea (Figs 6, 8, 9) reflecta transition from a glacial-like near shore environmentto a modern offshore relatively deep-water marine one.In Table S3 we list the most abundant and ecologicallycharacteristic taxa for all six assemblages and give their(palaeo)ecological interpretation together with taxo-nomic notes based on our own data and publishedevidence.

Some minor disadvantages of studying ostracods inshelf–upper slope environments arise from the fact thatthey usually occur in lower abundance compared with,for instance, benthic foraminifers, and that in theseenvironments they may represent mixed faunas withallochthonous elements. Marginal marine environ-ments are known for a number of post-mortem tapho-nomic processes (Boomer & Eisenhauer 2002) and theycommonly contain allochthonous elements either fromland or from open-marine habitats. It is important todistinguish between these elements in the assemblage(Whatley 1983; Boomer & Eisenhauer 2002; Frenzelet al. 2010). For the Kara and Laptev seas, staining ofsurface samples with Rose Bengal showed that most

samples contain 90–100% of live organisms, and at thesame time they have a juvenile portion of about25–50%, which implies washout of valves of dead andsmaller juvenile valves (Whatley 1983; Stepanova et al.2007). In the studied core samples the preservation ofvalves is generally good, and most samples hold a juve-nile percentage of 25–50%; this indicates either a high-energy environment from which smaller juvenile oradult dead valves were washed out or a thanatocoenosiswith current velocities sufficient to carry larger valves(Whatley 1983). A direct comparison with modern shelfassemblages with a high ratio of live organisms andsimilar juvenile percentage (25–50%) convinces us of theautochthoneity of fossil assemblages and high-energyenvironments. Another pattern is periodically observedin assemblage 5 on the upper continental slope core.High fluctuations of juvenile ratio of up to 100% corre-spond to periodic sudden events, such as slides, whenostracods were overwhelmed and entombed in thesediment (Whatley 1983).

Shelf cores

Lithologically, all shelf cores obtained from palaeorivervalleys (see ‘Modern environments of the Laptev andKara seas’) represent relatively uniform sedimentsequences composed largely of bioturbated marine siltyclay to clayey silt, with the weight percentage of thecoarse fraction (>63 mm) not exceeding 4–5%, exceptfor the middle part of core PS138 where it rises to10–20% (Stein et al. 2004; Taldenkova et al. 2005,2008).

Although the general trend in ostracod assemblageevolution from estuarine to modern normal marine(assemblages 1–4) is the same for all studied shelf cores(Table S3; Figs 8, 9), there are certain differencesbetween similar assemblages from different locations.Therefore, our attention was also drawn to the corre-lation between variations in the taxonomic composi-tion of assemblages and geographical locations of thecores.

At the end of the Late Pleistocene and in the earlyHolocene, as a result of the fast sea-level rise and pro-ximity of the coastline, accumulation rates in the riverpalaeovalleys on the outer shelf were high (Bauch et al.1999, 2001; Taldenkova et al. 2005, 2009), and environ-mental changes were fast and abrupt, as indicated bythe succession of several fossil ostracod assemblages inthe studied shelf cores. None of these cores reveals atransition from terrestrial to marine deposits. Thelower age of the oldest estuarine brackish-water assem-blage (1), which is present in all cores from the LaptevSea, ranges in accordance with the water depth of thesites, from 12.3 ka in core PS159 (60 m), to 11.4 ka incore PS135 (51 m) and to 11.3 ka in core PS138 (45 m).The estimated duration of the estuarine phase is uncer-tain and ranges from 200 to 1000 years (Fig. 8). In the


Kara Sea core this assemblage is missing, as the age ofthe oldest studied sediments is 8 ka.

Assemblage (1) is characterized by a low taxonomicdiversity (Fig. 8). It is represented mainly by euryha-

line and brackish water species, as well as by someshallow-water marine ones (Table S3; Figs 3–5, 8, 9).Total abundance is relatively low, on average 10–40specimens per 100 g of sediment (Figs 3–5). The

Fossil assemblages

inner-shelf, marineeuryhaline-2

estuarine, brackish water1

middle-shelf, marineshallow-water3 Holocene upper continental slope, marine6

outer shelf, marine

Pre-Holocene upper continental slope, marine

4

5

Laptev SeaKara Sea

outer-shelfPS159 (60 m)

Western

upper continentalslope PS154

(270 m)

outer-shelfPS135 (51 m)

middle-shelfPS138 (45 m)

Eastern

middle-shelfBP07 (43 m)

11 200

10 300

1

2

3

11 400

6200

6

12 000

11 2002

4

3

12 3001

6200

Age (

cal. a

)

4

3

5

Eastern

8

0

0

0

1

0

0

0

0

3300

0

2000

4000

6000

8000

12 000

14 000

16 000

10 000

2

10 200

1

2

3

11 300

8200

0

2000

4000

6000

8000

12 000

14 000

16 000

10 000

8000

7300

0.5

1.5

2.5

Shannon-Weaver diversity

10 000

1160

18 000

17 200

1650

18 000

0.5

1.5

2.5

1.5 2.50.5

0.5

1.5

2.5

0.5

1.5

2.5

500

Fig. 8. Comparison of fossil ostracod assemblage evolution in different parts of the Laptev and Kara seas based on species diversity definedby the Shannon–Weaver index.


Fig. 9. Comparison of the changes in the relative representation of different ecological groups during the Postglacial sea-level rise in differentparts of the Laptev Sea. North Atlantic species belong to the group of deep-water species, but their percentage is also shown separately in orderto highlight the periods of enhanced influence of Atlantic-derived waters at the continental slope. LG=Lateglacial; B-A=Bølling-Allerød;YD=Younger Dryas; EHol=early Holocene; MHol=middle Holocene; LHol=late Holocene. This figure is available in colour athttp://www.boreas.dk.


Shannon–Weaver diversity index is generally slightlylower for sediments older than 10 ka, averaging 0.5–1in all cores from the Laptev Sea shelf (Fig. 8). Thepercentage of juvenile valves in lower parts of the coresaverages 25–50%. According to Whatley (1983), thismay imply a high-energy environment. Altogether,this indicates a harsh and unstable environment of theestuarine zone with very strong freshwater influence.

Assemblage (1) is replaced upcore by assemblage (2),which is practically devoid of any brackish water taxa.This transition occurred at around 12 ka in core PS159,at 11.2 ka in core PS135, and at 10.2 ka in core PS138(Fig. 8).

The inner-shelf, euryhaline–marine assemblage (2) istaxonomically more diverse than assemblage (1),reflecting a gradual transition to normal marine condi-tions (Table S3; Figs 3–5, 7–9). It is dominated byshallow-water marine species and the euryhalinespecies P. pseudopunctillata. In cores PS159 and BP07from the west some relatively deep-water species arealso present (Figs 5, 7). They reach up to 30–50% incore PS159 (Fig. 9) and represent Arctic (Krithe glacia-lis, Argilloecia spp., Rabilimis mirabilis) as well asNorth Atlantic (Cytheropteron biconvexa, C. porterae,Bythocythere spp.) taxa (Table S3). The high percent-age of relatively deep-water Arctic and North Atlanticspecies in the early Holocene inner-shelf assemblagecould be explained by the proximity to the shelf breakand the occasional penetration of Atlantic-derivedwaters from the west into the shelf palaeovalleys withwind-induced reversed currents (Dmitrenko et al.2001). The percentage of juvenile valves in the easternpart of the Laptev Sea is very low, less than 25%, whilefarther west and in the Kara Sea it is 50–80%. Lowjuvenile ratios in the east reflect low total abundanceand indicate a high-energy environment. In the westernLaptev Sea, the higher ratios evidence a lower energyenvironment, while in the Kara Sea, we probablyobserve a life assemblage with up to 80% represented byjuveniles of different instar stages (Whatley 1983).

The average total abundance of ostracods is low,fewer than 50 specimens per 100 g of sediment(Figs 3–5, 7, 9). The diversity index is highly variableand ranges between 0 and 1.5 (Fig. 8). Similar to itsmodern analogue from surface sediments (Stepanovaet al. 2003, 2007), the inner-shelf assemblage character-izes river-affected coastal regions down to depths of20–30 m, with considerable seasonal changes includingfast ice cover in winter and the formation of a thicksurface-freshened water layer in summer owing to riverrunoff and ice melting.

The time duration of the existence of assemblage (2)ranges from 800 to 900 years in cores PS159 and PS135,to 2000 years in core PS138 (Fig. 8). This regional dif-ference could be attributable to the stronger fresheninginfluence of the Lena River discharge compared withthe Khatanga and Yana rivers. In core BP07, the age of

the initiation of this assemblage is unknown, but theduration of its existence exceeds 700 years.

A relatively poor middle-shelf shallow-water marineassemblage (3) occurs in all four studied cores from theLaptev and Kara shelves. It is dominated by variousshallow-water marine species, while euryhaline andbrackish-water species are rare or absent (Table S3). Inthe eastern Laptev Sea, the transition to assemblage 3 ismarked by the increasing proportion of shallow-watermarine species and indicates the establishment ofmodern-like conditions (Figs 3, 4). Taxonomic differ-ences within assemblage (3) from the studied cores aredefined largely by the distribution of Atlantic-derivedwaters in the study area. Unlike the case for the easternLaptev Sea, relatively deep-water species includingNorth Atlantic species occur in the western Laptev andeastern Kara sea shelf sediments and include some ofthe taxa found in the underlying sediments (assemblage2) together with some other Arctic (Sarsicytherideapunctillata) and North Atlantic (Cytheropteron arcua-tum) forms. These species may indicate an influence ofAtlantic-derived waters, probably brought via reversedbottom currents to these two sites during the Holocene.

The total abundance is in general low, fewer than 50specimens per 100 g of sediment. However, we observethe evident increase in ostracod abundance during thetransition to middle-shelf conditions, which is mostpronounced in the continuously studied core PS159,where the peak total abundance of 250 specimens per100 g of sediment is recorded at 10.2 ka (Figs 7, 9). Thispeak abundance suggests a high seasonal productivityand probably a short time period when the core sitewas located at the fast sea-ice margin. Juvenile ratiosrise in the east, compared with the underlying layer,to 25–50%, and fluctuate a lot in the west, rangingbetween 20 and 100%. Such a distribution of juvenileratios reflects the stabilization of environments in theeast and fluctuating parameters in the west, such asAtlantic water inflow and food supply. The diversityindex for assemblages 2–3 is less than 1.5. In general,the tendency in all cores is towards an increase ofdiversity upcore, which indicates a stabilization of theenvironments.

In the western cores PS159 and BP07, assemblage (3)is replaced upcore by the outer-shelf marine assemblage(4), at 7.2 ka in the Khatanga palaeovalley and at3.3 ka in the eastern Kara Sea shelf (Figs 5, 7–9). Theassemblages from the two cores are almost identical,although the core from the Kara Sea actually comesfrom a shallower environment than PS159. The taxo-nomic composition of assemblage (4) is somewhat con-tradictory. On one hand we observe an increase oftaxonomic diversity owing to the introduction of rela-tively deep-water species, including North Atlanticforms (Table S3; Figs 5, 7). On the other hand, wenotice a significant increase in euryhaline species,primarily P. pseudopunctillata (Table S3). In fact,


this represents the second, recurring introduction ofthis species, as it almost disappeared in the previousassemblage (3).

The total abundance in core PS159 is fewer than 50specimens per 100 g of sediment, but abruptly risesto 250 specimens in the uppermost layers, possiblybecause of the better preservation of valves (Figs 5, 9).In the Kara Sea, this assemblage is characterized by anincreased total abundance of up to 80 specimens per100 g of sediment (Fig. 7). The juvenile ratio in bothcores averages 25–50% and indicates a rather high-energy, modern-like environment (Figs 5, 7). TheShannon–Weaver diversity index for assemblage 4 isrelatively high (1.5–2), reflecting water-depth increaseand the increasing influence of Atlantic-derived waters.

Western Laptev Sea upper continental slope

Owing to its deeper location on the western upper con-tinental slope of the Laptev Sea, core PS154 (270 m) isdistinguished from the other studied shelf cores. Corelithology together with the occurrence of ice/iceberg-rafted debris (IRD) and authigenic concretions are dis-cussed in Taldenkova et al. (2008, 2010). The coresequence younger than 15 ka consists largely of biotur-bated clayey silt. Sandy lenses occur in the older partof the core, with a prominent sandy layer (up to 70 wt%of >63-mm fraction) at about 15 ka. IRD peaks arerecorded in the oldest (>c. 16.2 ka) and youngest(<7 ka) parts of the core. Prior to shelf flooding, thissite was located relatively close to the coast at a dis-tance of about 50 km (Taldenkova et al. 2008). Water-mass distribution and topography of the sea floor weredifferent from those on the shelf, and thus the fossilassemblages (5) and (6) recognized here (Figs 6, 9) havedifferent components compared with the shelf cores.

The oldest ostracod-bearing sample at 673 cm coredepth dates back to c. 17.2 ka and contains only fourvalves belonging to Cluthia cluthae, Cytheropteronlaptevensis and C. paralatissimum. The overlying nearly1-m-thick sediment sequence is barren of ostracods.Ostracods are constantly present since c. 15.6 ka.

The taxonomic composition of the oldest pre-Holocene upper continental slope assemblage (5)(Table S3; Figs 6, 8, 9), dating back to 15.6–10 ka, issimilar to that of assemblage (4), but has many distin-guishing features. It includes two deep-water Arcticspecies (Krithe glacialis, Rabilimis mirabilis), which areespecially abundant around 15.6–15 ka, when fewNorth Atlantic species are present. Various NorthAtlantic species are constantly present, and becomequite abundant after c. 15 ka, evidencing the continu-ous presence of Atlantic-derived waters. We presumethat the bottom-water temperature did not change sig-nificantly, as even at present Atlantic-derived watersarrive in the Laptev Sea considerably chilled, down to1°C northwards of New Siberian Islands (Dmitrenko

et al. 2010). The highest relative abundances of NorthAtlantic species, implying the strongest inflows ofchilled Atlantic-derived waters, are observed at 15–14.5and 11.5–10 ka (Fig. 9). The second and third mostabundant ecological groups of ostracods in assemblage(5) are represented by two euryhaline species andshallow-water middle-shelf species (Figs 6, 9). At about15 ka, an evident slide event is suggested by a sedimentunit with a 70% sand fraction (Taldenkova et al. 2008,2010). This sediment layer contains valves of brackishwater and even freshwater ostracod species (Table S3;Fig. 6). Periodic smaller peaks in the occurrence ofeuryhaline and brackish-water species might be indi-cative of smaller, unidentified slide events and/orenhanced freshwater influence.

The total abundance within this assemblage is dis-tributed very irregularly, but is typically fewer than 50specimens, with several peaks of up to 120 specimensper 100 g of sediment at about 14.5, 13 and 12 ka(Figs 6, 9). Juvenile ratios exhibit high-amplitude fluc-tuations of 0–100%, giving additional evidence for ahigh-energy environment and high mortality ratesowing to catastrophic slide events (Fig. 6). TheShannon–Weaver diversity index is higher (1–2) in sedi-ments older than 10 ka (Fig. 8), reflecting a highly vari-able total abundance and taxonomic diversity causedby an active hydrodynamic regime, the proximity of thecoast, and temporal catastrophic events such as slides.The exception is the oldest time interval of this section(15–15.6 ka), when the diversity index is below 1, andthe ostracod association is composed almost entirely ofK. glacialis.

The transition to the Holocene upper continentalslope marine assemblage (6) occurred at about 10 ka.The overall composition of this assemblage is less taxo-nomically diverse than that of assemblage (5) owing tothe absence of many shallow-water taxa (Table S3;Fig. 6). It was subdivided into two intervals: 10–5.4 ka(sub-assemblage 6a) and 5.4–2 ka (sub-assemblage 6b).The first sub-assemblage is dominated by the relativelydeep-water species R. mirabilis and the North Atlanticspecies Cytheropteron tumefactum, and some shallow-water marine taxa are also present (Cytheropteronsuzdalskyi and C. montrosiense). The second sub-assemblage is characterized by a combination of thedeep-water marine species R. mirabilis, K. glacialis andSarsicytheridea punctillata, the euryhaline species Het-erocyprideis sorbyana and the North Atlantic taxonC. tumefactum. Other species are present in minor quan-tities. Sub-assemblage 6b is very similar to the modernostracod assemblage of this area (Stepanova et al. 2003).The total abundance of ostracods here is slightly higherthan in the underlying assemblage and averages 30–60specimens per 100 g of sediment (Fig. 8). Juvenile ratiosare very low in sub-assemblage (6a), reflecting theoverall low total abundance. They rise to 10–50% insub-assemblage (6b) and indicate the establishment of a


modern-like environment. The Shannon–Weaver diver-sity index varies between 1.5 and 2 for assemblage (6),indicating stabilization of the environment.

Besides the possibility of ice-rafting from the innershelf areas (see ‘Recent ostracod assemblages fromsurface sediments’), the abundance of euryhalinespecies in assemblage (6) can be explained by thehigh tolerance of these species to environmentalchanges (Figs 6, 9). The range of their salinity/depth/temperature/nutrition requirements is very wide whencompared with purely marine species. That is why wefind euryhaline species abundant both in shallowinner-shelf environments and on the outer shelf /uppercontinental slope.

Reconstruction of past environmental changesbased on ostracod assemblage analysis

Whereas there are several investigations of Quaternarydeep-sea ostracod assemblages from the Central ArcticOcean (Cronin et al. 1994, 1995, 2010), records fromArctic shelves are rare. So far, the most detailed data onfossil ostracods from Arctic shelf sediments come fromborehole sections in northern Alaska (McDougall et al.1986). However, that investigation lacks sufficient agecontrol, as only three radiocarbon dates were availablefor these up to 8-m-long borehole sediment sequences.In this respect, our data on the timing and character ofostracod assemblage variations in the Laptev and Karaseas represent the first record that allows both the trendand the rate of environmental changes in these mar-ginal Arctic seas to be addressed under the last postgla-cial sea-level rise.

Lateglacial and deglaciation (c. 17.6–11 ka)

Siberian shelves to the east of Taimyr Peninsula aswell as the southeastern Kara Sea were sub aeriallyexposed during the Last Glacial Maximum (LGM). Atthis time, an ice sheet partly covered the Kara Seashelf and the vast territory farther west, affecting theinfluence of North Atlantic water on this region(Svendsen et al. 2004; Fig. 1A). Sea level during theLGM was about 120 m lower than modern valuesprior to the shelf flooding (Fairbanks 1989), and theLaptev Sea upper continental slope was located veryclose to the palaeocoastline, giving rise to conditionscharacterized by a cold-water marine environmentsubject to meltwater and fluvial influence (Bauch et al.2001; Spielhagen et al. 2005; Taldenkova et al. 2008,2010). Enhanced meltwater inputs resulted in strongwater stratification, as evidenced by abundant authi-genic concretions of vivianite and rhodochrositeformed under anaerobic conditions and the generalabsence of benthic remains during the time periodbetween 17.6 and 15.4 ka (Taldenkova et al. 2010).

At the same time, ostracod species with North Atlan-tic affinities were identified here as early as c. 17.2 ka,giving clear evidence for the presence of Atlantic-derived waters in the Arctic during lateglacial times. Itfurther implies that ostracods were able to settle andsurvive in the harsh environments of the early deglacialperiod with considerable meltwater input and strongwater stratification. Scarce findings of ostracods arerecorded simultaneously with extremely high abun-dances of small-sized subpolar planktic foraminifersand the occurrence of some benthic foraminiferalspecies, including the species Cassidulina neoteretisindicative of Atlantic-derived waters (Taldenkova et al.2010). Rare subpolar planktic foraminifers were iden-tified in the sediment sequence interval of c. 17.6–15.6 ka, giving evidence for subsurface inflows ofAtlantic-derived waters to the site, but the event at c.17.2 ka probably corresponds to enhanced upwelling ofAtlantic-derived waters in a coastal polynya main-tained in winter by offshore winds from the Siberianmainland (Taldenkova et al. 2008, 2010).

The continuous presence of North Atlantic species inthe record since c. 15.6 ka suggests that the westernLaptev Sea slope was constantly under the influence ofthe subsurface inflow of Atlantic-derived waters(Figs 6, 9). The most abundant occurrence of thesespecies corresponds to the transition from the Lategla-cial to the Bølling–Allerød period (Fig. 9). It furtherindicates a strong penetration of subsurface inflow ofAtlanic-derived water along the Eurasian continentalslope, as is also supported by the occurrence of thehighest relative abundances of C. neoteretis in corePS154 (Taldenkova et al. 2008, 2010); the latter arecomparable to the reported values of this speciesaround Svalbard (Slubowska et al. 2005; Slubowska-Woldengen et al. 2007, 2008).

The total abundance of ostracods within assemblage(5) is distributed very irregularly, with several peaksfound at about 14.7–14.5, 13 and 12 ka (Figs 6, 9).These peaks may correspond to enhanced surface-water productivity owing to climate amelioration and astronger fluvial influence possibly resulting in surface-water warming. Interestingly, the maximum totalabundance of ostracods as well as benthic foraminifersin core PS154 centred at 13 ka corresponds to the localfreshwater event indicated by the low d18O spike inplanktic foraminifers identified from the eastern LaptevSea continental slope by Spielhagen et al. (2005). Incore PS154, the abovementioned abundance peaks arelargely represented by shallow-water marine andeuryhaline/brackish-water taxa. This confirms the pos-sibility of enhanced freshwater influence at the expenseof the open-sea water masses, including Atlantic-derived waters (Figs 6, 9). Unlike the 15-ka slide event(Fig. 6), when the valves of freshwater, brackish waterand euryhaline ostracods were probably displaceddownslope from a nearby coast, the younger deglacial


peaks of brackish water and euryhaline species at 13and 12 ka after the onset of shelf flooding are a result ofice-rafting processes. Valves of these species can beentrained into the newly formed ice in the freshenednear shore zone during autumn storms and then trans-ported to the seasonal drift-ice limit (cf. Eicken et al.1997). The fact that these valves were abundant in corePS154 during deglacial times may imply that thesummer drift-ice margin was located close to the site.Because the latter was still not far from the formercoastline prior to extensive shelf flooding in the earlyHolocene, it would mean that the overall climate con-ditions remained relatively cool at this time.

On the outer shelf, the transition into the earlyHolocene (12.3–11 ka) was marked by rapid inunda-tion and the occurrence of brackish-water ostracodassemblages typical of unstable estuarine and inner-shelf environments (Stepanova 2006). The studiedsites were located in river estuaries at water depths ofabout 5–10 m and with a bottom-water salinity ofaround 18–20 psu (Taldenkova et al. 2005). In thewestern Laptev Sea, deep-water ostracod species andspecies with North Atlantic affinities were alsopresent, where they accounted for up to 50% ofassemblage (2) in core PS159 (Fig. 9). We supposethat the deeper water species were transported to shal-lower sites with wind-induced reversed bottom cur-rents (Dmitrenko et al. 2001; Wegner et al. 2005).None of these ostracod taxa were found on theeastern Laptev Sea shelf (Figs 3, 4, 9), probably owingto the dominance of fresh water discharged from theLena River. It is noteworthy that at about 11.3 kaseveral planktic foraminiferal species were found incore PS135 (Taldenkova et al. 2005), which probablymeans that the Yana River runoff was less strongthan that of the Lena around this time.

Early–middle Holocene (11–6 ka)

On the western continental slope, the major changefrom a fluvial-affected to modern marine environmentoccurred at around 10 ka. This change was accompa-nied by a sharp reduction in sedimentation rates fromabout 120 to 5 cm ka-1 owing to the onset of extensiveshelf flooding (Fig. 2; Bauch et al. 2001), and resulted ina less good preservation of ostracod valves, of whichonly a few are found between 9 and 7 ka (Figs 6, 9).Because the number of shallow-water and euryhalineostracods is clearly lower, their absence on the conti-nental slope may indicate climate warming with reducedsea-ice cover in summer, and a northward displacementof the seasonal drift-ice margin. Because the sea-icemarginal zone is known to be an area of high seasonalproductivity (Polyak et al. 2002), a spatial change couldhave contributed to the drastically diminished sedimen-tation rates. The offshore shift of the sea-ice marginalzone is supported by the almost complete absence of

ice-rafted debris in the sediments of core PS154 duringthe early Holocene (Taldenkova et al. 2010).

As is evidenced by the high percentage of NorthAtlantic species, a continuous influence of Atlanticwater is observed on the western Laptev Sea slopeduring the early Holocene (Fig. 9). These data gener-ally agree with other interpretations indicating anenhanced influence of modified Atlantic watersbetween 11 and 9 ka (e.g. Stein et al. 2001, 2004). Awarmer climate at this time has also been reconstructedby various authors using proxy records for terrestrial(MacDonald et al. 2000; Naidina & Bauch 2011),coastal marine (Duplessy et al. 2001; Lubinski et al.2001; Ivanova et al. 2002) and deep Arctic Ocean(Cronin et al. 2010) environments.

However, the taxonomic diversity of Atlantic water-indicative ostracod species was higher during deglacialtimes than in the early Holocene. Certain species of thisgroup (Pseudocythere caudata, Cytheropteron dimling-tonensis, C. nodosoalatum, C. paralatissimum, C. bicon-vexa) disappeared from the record around 10–11 ka,whereas other species (C. tumefactum, C. occultum,C. pseudomontrosiense, C. arcuatum) were continu-ously present also in the Holocene record, and the onlynewly invading North Atlantic species was C. perlaria(Fig. 6). Slightly earlier, at around 12 ka, the Atlanticwater-indicative benthic foraminifer C. neoteretisalso disappeared from the record of core PS154, onlyto re-appear in the mid-Holocene at about 5 ka(Taldenkova et al. 2008, 2010). The observed changesin the taxonomic composition of microfossils in theearly Holocene might possibly be related to the vari-ability in the modified Atlantic water masses enteringthe Laptev Sea (Fig. 1A); that is, to a possibly strongerinfluence of the BSBW after the flooding of the for-merly glaciated over-deepened Barents Sea shelf(Lubinski et al. 2001) at the expense of the FSBW.

On the shelf, changes in ostracod assemblage com-position, which are similar in all studied cores fromboth the Laptev and the Kara seas, reflect rapid trans-gressive inundation of the region and the transitionfrom estuarine–brackish-water to normal marine con-ditions (Figs 3–5, 7–9; Bauch et al. 2001; Taldenkovaet al. 2005, 2008, 2009). Age–depth relationship plotsfor the studied cores (Fig. 2) show that there is anevident drop in sedimentation rates owing to the south-ward migration of coastline and major depocentres(Bauch et al. 2001) that occurred, depending on the sitelocation and water depth, between 10.2 and 8.2 ka(Figs 1, 2). In core BP07 from the Kara Sea, sedimen-tation remained steady and relatively high during thelast 8 ka, because the core spans a shorter time intervalthan the other shelf cores, and also the site is locatedin a sea-floor depression that was gradually filledwith sediments during the Holocene. Along with thedrop in sedimentation rates in the western Laptev Seacore PS159, relatively deep-water species that were


abundant during early stages of inundation becamealmost absent later (Figs 5, 9). Between 10.5 and 6 kathey gave way to shallow-water marine species notwith-standing the water depth increase (Fig. 9). This couldbe due to the relatively weak estuarine-like water circu-lation on the shelf in the early Holocene when thestrength of reversed bottom currents was probablyreduced. An increase in the percentage of the euryha-line species P. pseudopunctillata between 9 and 6 ka inthe eastern Laptev Sea (Figs 3, 4, 9) might be explainedby the enhanced influence of river runoff from the Lenaand Yana initiated by climate warming.

Late Holocene (6 ka–recent)

Because global and regional sea level stabilized ataround 5–6 ka (Fairbanks 1989; Bauch et al. 1999,2001), the ostracod assemblages at all studied sitesacquired their modern-like composition at this time(Figs 3–7, 9; Stepanova et al. 2003, 2007; Stepanova2006).

On the western upper continental slope and the outershelf of the Laptev Sea, the youngest ostracod assem-blage is characterized by a significant amount ofshallow-water and euryhaline species co-occurring withdeep-water species, including the North Atlantic ones(Table S3; Figs 6, 9). Given the greatest distance fromthe coast and water depths since the beginning of therecord at about 17.6 ka, the re-introduction of ostracodspecies characteristic of inner-shelf environments to theouter shelf and upper continental slope must be linkedprimarily to rafting by sea ice and probably also byicebergs, which implies climate cooling resulting inthe onshore shift of the seasonal drift-ice limit andthe growth of local ice caps (e.g. Severnaya Zemlya).The latter assumption is supported by a considerableincrease in IRD in cores PS154 and PS159 since 7.2 ka,pointing to this local source of iceberg-rafted material(Taldenkova et al. 2010). Interestingly, the high-resolution record of IRD in core PS159 shows a sub-millennial periodicity with the peaks centred at about7.2, 6.4, 5.4, 3 and 2 ka (Taldenkova et al. 2010), whichis roughly similar to the timing of the peaks in euryha-line ostracod species abundance (Fig. 9).

Not only in the cores from the western Laptev Seaouter shelf and slope (influenced by iceberg-rafting fromSevernaya Zemlya) does the percentage of euryhalinespecies increase, it is also true for the Kara Sea mid-shelfcore BP07 and cores from the eastern Laptev Sea outerand mid-shelf. We assume that this points to a south-ward shift of the summer drift-ice limit owing to climatecooling. As a result, more ostracod valves from thenear-shore regions that had been entrained in the newlyformed ice during autumn storms and then transportedto the drift-ice margin (cf. Eicken et al. 1997) later foundthemselves released into the sediments of the outershelf and upper continental slope rather than being

transported farther offshore to the Arctic Ocean andNorth Atlantic. The increase in the reintroduced near-shore species is especially pronounced in core BP07(Fig. 7). Here, the share of euryhaline P. pseudopunctil-lata in the upper part of the core (assemblage 4) is evenhigher than in the lowermost assemblage (2), whichcorresponds to an otherwise more fluvially affected envi-ronment, as is also clearly indicated by the lower valuesof the d18O composition of calcareous microfossils fromthe same core (Simstich et al. 2004, 2005).

In general, the tendency in all studied cores from theshelf is towards the increase of diversity upcore, whichindicates stabilization of environments, depth increaseand the establishment of diverse and stable ecolo-gical niches during the course of the Holocene. TheShannon–Weaver diversity index remains relativelylow for all identified assemblages. For late Holoceneostracod assemblages it ranges between 1.5 and 2 forassemblages (3)–(4) (outer shelf) and further decreasesfor the upper continental slope assemblage (6) (from1.5–2 down to 0) (Fig. 8). Such a difference may beexplained by variable nutrient supply. The latter, prob-ably, decreased on the continental slope owing to thecombination of the considerable distance from thecoastline and the southward migration of the seasonaldrift-ice margin induced by cooling.

Conclusions

The late Pleistocene–Holocene shelf history of theLaptev and eastern Kara seas is reconstructed based onostracod assemblages from four sediment cores recov-ered from mid outer shelves and a core from thewestern Laptev Sea upper continental slope. The fossilrecords show large faunal changes associated withrapid inundation of the shelves and coastal retreatduring postglacial sea-level rise, together with variabil-ity in freshwater runoff. In sediments younger than7 ka we observe climatic-driven rather than sea-level-driven water-mass changes, as reflected in the increasedrepresentation of ice-rafted euryhaline taxa in other-wise modern-like assemblages, along with an increasingIRD content.

Ostracod assemblage analysis indicates three majorperiods of environmental change characterized by sixecologically different assemblages: estuarine brackish-water assemblage (1), inner-shelf euryhaline–marineassemblage (2), middle-shelf shallow-water marineassemblage (3), outer-shelf marine assemblage (4), pre-Holocene upper continental slope marine assemblage(5), and Holocene upper continental slope marineassemblage (6a, b):

• The Lateglacial and deglaciation (c. 17.6–11 ka) arecharacterized by assemblage (5) on the upper con-tinental slope of the Laptev Sea, indicative of a


cold-water marine environment strongly influencedby both a subsurface inflow of Atlantic-derivedwater and riverine freshwater input. Ostracods withNorth Atlantic affinities occur as early as c. 17.2 kaand are continuously present after c. 15.6 ka, indi-cating that Atlantic-derived water upwelled to thesite facilitated by coastal polynyas and offshorewinds. After about 14.7 ka, the increased totalabundance of ostracods with peaks at 14.7–14.5, 13and 12 ka probably reflects enhanced surface-waterproductivity as a result of climate amelioration, agrowing fluvial influence and offshore shifts in theposition of the seasonal sea-ice marginal zone. Onthe outer shelf, this period (starting from 12.3 ka) ischaracterized by the transition from estuarinebrackish water (with water depths of about 5–10 mand bottom-water salinities of 18–20 psu) to inner-shelf environments.

• The early–middle Holocene (11–6 ka) is dominatedon the western upper continental slope of theLaptev Sea by sub-assemblage (6a), introduced ataround 10 ka and indicative of climate warming,reduced sea-ice cover and the continuous presenceof Atlantic-derived waters. On the shelves, assem-blages (2) and (3) indicate a rapid inunda-tion process and the transition to normal marineconditions.

• The late Holocene (6 ka–recent) corresponds tosub-assemblage (6b) on the western upper continen-tal slope of the Laptev Sea, and to assemblages (3)and (4) on the shelves. They reflect the stabilizationof depositional environments and the establishmentof modern-like conditions. The simultaneousincrease in the percentage of shallow-water marineand euryhaline species recorded in all cores suggeststhat these species are mostly sea-ice- and/or iceberg-rafted. This observation probably provides evi-dence for a cooling trend that was accompanied bythe southward migration of the seasonal sea-icemargin and the regional re-growth of ice caps.

Acknowledgements. – This research was supported by the BMBF andRussian Ministry for Education and Science (OSL fellowshipprojects) and the Russian Foundation for Basic Research (projects08-05-00849 and 11-05-01091). It is a contribution to the APEX(Artic Paleoclimate and its Extremes) program. We are grateful to theIntegrated Ocean Drilling Program at Texas A&M University forhosting the first author and providing office support and analyticalinstrumentation during the completion of this paper. The paper ben-efited greatly from comments made by T. M. Cronin (USGS) on anearlier version. The helpful remarks and suggestions of E. M. Brou-wers (USGS) and the two anonymous reviewers are very muchappreciated.

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Supporting information

Additional Supporting Information may be found inthe online version of this article:

Table S1. Radiocarbon (AMS) and calendar ages ofthe studied cores.

Table S2. Some characteristic ostracod species fromdifferent environments in the Laptev and Kara seas(Stepanova et al. 2003, 2004, 2007; Stepanova 2006).I=index species; d=dominant species; r=rare species

Table S3. Description of ostracod assemblages and(paleo)ecological references for ostracod taxa based onour own data and published evidence. f=freshwater;br=brackish water; eury=euryhaline; sm=shallow-watermarine; dm=deep-water marine; Atl.=deep-watermarine with North Atlantic affinities.

Please note: Wiley-Blackwell are not responsible for thecontent or functionality of any supporting materialssupplied by the authors. Any queries (other thanmissing material) should be directed to the correspond-ing author for the article.


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Ostracod palaeoecology and environmental change in the Laptev and Kara seas (Siberian Arctic) during the last 18 000 years