Holocene hydrographical changes of the eastern Laptev Sea (Siberian Arctic) recorded in δ18O profiles of bivalve shells

Quaternary Research 61 (2004) 32 – 41 www.elsevier.com/locate/yqres

Holocene hydrographical changes of the eastern Laptev Sea (Siberian Arctic) recorded in y18O profiles of bivalve shells Thomas Mueller-Lupp, a,* Henning A. Bauch, b and Helmut Erlenkeuser c a

b

GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1-3, 24148 Kiel, Germany Mainz Academy of Sciences, Humanities and Literature, c/o GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany c Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, Kiel University, 24098 Kiel, Germany Received 14 October 2002

Abstract Oxygen isotope profiles along the growth axis of fossil bivalve shells of Macoma calcarea were established to reconstruct hydrographical changes in the eastern Laptev Sea since 8400 cal yr B.P.. The variability of the oxygen isotopes (y18O) in the individual records is mainly attributed to variations in the salinity of bottom waters in the Laptev Sea with a modern ratio of 0.50x/salinity. The high-resolution y18O profiles exhibit distinct and annual cycles from which the seasonal and annual salinity variations at the investigated site can be reconstructed. Based on the modern analogue approach oxygen isotope profiles of radiocarbon-dated bivalve shells from a sediment core located northeast of the Lena Delta provide seasonal and subdecadal insights into past hydrological conditions and their relation to the Holocene transgressional history of the Laptev Sea shelf. Under the assumption that the modern relationship between y18Ow and salinity has been constant throughout the time, the y18O of an 8400-cal-yr-old bivalves would suggest that bottom-water salinity was reduced and the temperature was slightly warmer, both suggesting a stronger mixture of riverine water to the bottom water. Reconstruction of the inundation history of the Laptev Sea shelf indicates local sea level f27 m below present at this time and a closer proximity of the site to the coastline and the Lena River mouth. Due to continuing sea level rise and a southward retreat of the river mouth, bottom-water salinity increased at 7200 cal yr B.P. along with an increase in seasonal variability. Conditions comparable to the modern hydrography were achieved by 3800 cal yr B.P. D 2003 University of Washington. All rights reserved. Keywords: Arctic shelf; Laptev Sea; Oxygen isotopes; Bivalve shells; Paleohydrography; Holocene

Introduction The Arctic Ocean and its hydrographic structure play an important role in influencing the global thermohaline circulation through the export of freshwater and sea ice to the Nordic Seas (Aagaard and Carmack, 1989). Changes in the export rates of Arctic freshwater and sea ice could result in a perturbation of the thermohaline circulation (Aagaard and Carmack, 1994; Broecker, 1997) thereby effecting heat transport toward northern latitudes (Rahmstorf, 1995; Werner et al., 1999). The low-salinity layer of the Arctic Ocean is fed by riverine freshwater from Siberia (Bauch et al., 1995), where the Laptev Sea is regarded as one of the key region which affects the Arctic Ocean’s freshwater budget. The shallow Laptev Sea is influenced by large quantities of freshwater

* Corresponding author. Fax: +49-431-6002941. E-mail address: [email protected] (T. Mueller-Lupp).

supplied during summer by several rivers, especially by the Lena River. The Lena River contributes, in fact, the second largest freshwater discharge among Arctic rivers, with a mean annual discharge of 532 km3 per year (Global Runoff Data Center, 1998) and reflects central to northern Siberian precipitation regime. This freshwater runoff is subject to strong seasonal and annual variations and causes strong stratification in the shallow Laptev Sea (Dmitrenko et al., 1995, 1999). Given the variability on seasonal, annual and, in particular, longer timescales, the dispersal and fate of the river discharge and its influence on the hydrographical settings are a central task in understanding environmental changes on Siberian shelves. The major objective in this study is to use stable oxygen isotopes from living and fossil carbonates to reconstruct past hydrographical changes. The oxygen isotopic composition of marine carbonates is controlled by the isotopic composition of the water from which the carbonates precipitated and the temperature of the surrounding water (Epstein et al., 1953; Grossman and Ku, 1986). By establishing y18O profiles along

0033-5894/$ - see front matter D 2003 University of Washington. All rights reserved. doi:10.1016/j.yqres.2003.09.003

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the axis of maximum growth of bivalve shells it is possible to obtain hydrographical information of the bivalves’ habitat (Andreasson and Schmitz, 1998; Bemis and Geary, 1996; Israelson et al., 1994; Jones et al., 1998; Khim et al., 2001; Krantz et al., 1988). Using oxygen isotope profiles from modern and fossil bivalves in a sediment core northeast of the Lena Delta, this study investigates past variations in the bottom water hydrography during snapshot views of the past 8400 cal yr and demonstrates how these temporal variations are related to overall environmental change.

Material and methods Bivalves Modern and fossil bivalve specimens of Macoma calcarea were collected from site PS51/92 northeast of the Lena Delta (Fig. 1). While the modern shell was collected alive

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from the near surface, fossil shells were well preserved with no obvious signs of reworking. The fossil bivalves were found in living position with paired valves in place, implying no significant lateral transport. Macoma calcarea shows a panarctic distribution and is widespread in the Laptev Sea between water depths of 10 to 300 m (Gukov, 1999; Richling, 2000). A serial sampling technique similar to that used in other studies (Bemis and Geary, 1996; Erlenkeuser and Wefer, 1981; Krantz et al., 1987, 1988; Mueller-Lupp, 2002) was applied to derive high-resolution isotope records from the shells. Prior to taking carbonate samples, the exterior of each shell was cleaned to remove any surface contamination. Individual carbonate powder samples (>15 Ag) were millcut under the microscope sequentially from the outer layer along the axis of maximum growth with a spatial resolution of approximately 0.15 to 0.3 mm. Sample positions are reported as distance from the umbo to the ventral margin along the sampling profile. To avoid contamination of the sample with

Fig. 1. Bathymetric map of the Laptev Sea shelf (isobaths in meters) showing the position of core PS51/092-12. The chronology of core PS51/92-12, the derived age model, and the bivalves of Macoma calcarea, which were used for the oxygen isotope analyses, are also indicated.

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material from subjacent shell layers, the sample was milled only from the surface of the outer layer. During sampling, the carbonate powder was vacuumed on a fiberglass filter. For isotope analysis, the carbonate powder on the filter was reacted with 100% orthophosphoric acid under vacuum at 73jC in the KIEL carbonate device, which was coupled online to a Finnigan MAT 251 gas isotope mass spectrometer. Isotope values are reported as parts per mil (x) in the usual ynotation relative to the PDB standard (defined via NBS 20). The external error amounts to less than F 0.08x. Experiments carried out on sample replicates showed that the average (n = 38) difference between replicates was F 0.17xon the y18Oscale. Sediment core The chronology of core PS51/92-12 is based on radiocarbon ages measured on bivalve shells using an accelerator mass spectrometer (AMS) at the Leibniz Laboratory in Kiel (Table 1). Assuming linear sedimentation rates between the age tiepoints, an age model was constructed by interpolating between the age tiepoints (Fig. 1). The conversion of the given 14C years B.P. into calendar years (cal yr B.P.) was carried out using the intercept method of the marine data set (Stuiver et al., 1998) from the program CALIB rev. 4.3 (Stuiver and Reimer, 2000). A reservoir effect for the Laptev Sea shelf of 370 F 49 yr was taken into account (Bauch et al., 2001a). A total of seven bivalves were used for the AMS Table 1 Bivalve species, radiocarbon age estimates, calibrated calendar years used for the chronology, and age model of core PS51/92-12 from the Laptev Sea shelf (Kasten Corer; 130.140 E; 74.592 N; 32 m water depth) Depth Lab# [cm] 0 2 64 120 160 210 300 402 500 14

Bivalve species

Macoma calcarea KIA-6877 Leionucula bellotii KIA-6878 Leionucula bellotii Macoma calcarea KIA-6879 Macoma moesta KIA-12931 Macoma calcarea KIA-6880 Macoma calcarea KIA-6881 Leionucula bellotii KIA-6882 Macoma calcarea

Age

Age range [14C yr B.P.] [cal yr B.P.] 1 Sigma [cal yr B.P.] 0

—

collected alive 590 F 25

273

301 – 246

1505 F 35

1078

1164 – 1009

not 1188a AMS-dated 1680 F 35 1267

1302 – 1223

3810 F 35

3809

3866 – 3695

6725 F 40

7270

7333 – 7230

7280 F 45

7754

7830 – 7681

7950 F 55

8408

8515 – 8361

—

C years were converted (intercept method) into calendar years using the marine data set of Stuiver et al. (1998) in the program CALIB rev.4.3 (Stuiver and Reimer, 2000). A local reservoir age of 370 F 49 years was used (Bauch et al., 2001a). The bold marked specimens were used for the oxygen isotope analyses. a Interpolated age from the age model.

dating, including the species of M. calcarea, Leionucula bellotii, and M. moesta. The radiocarbon-dated bivalves of M. calcarea from 210-, 300-, and 500-cm core depths were also used for oxygen isotope profile analysis (Fig. 1). The age of the specimen at 120-cm core depth was calculated from the resulting age model (Table 1).

Results Application of oxygen isotope profiles from bivalve shells in the Laptev Sea An accurate reconstruction of environmental conditions from the y18O records of fossil bivalves requires measuring or constraining as closely as possible the analogous modern setting of several parameters. These include the seasonal range of temperature and salinity and the relationship between salinity and y18O of the water. Further, knowing the shell mineralogy is important because calcite and aragonite have slightly different fractionation factors as a function of temperature (Grossman and Ku, 1986; Horibe and Oba, 1972). Since XRD analyses of shells of M. calcarea reveal that they consist of aragonite we used the paleotemperature equation of Grossman and Ku (1986): y18 OBivalve ¼ 4:65  0:21T þ y18 Owater : Measurements of y18Ow (Mueller-Lupp et al., 2003) in surface and bottom-water samples from the Laptev Sea document a linear relationship with salinity (y18Owater = 0.5Salinity 18.86x), from which a y18Ow to salinity ratio of 0.50x/salinity1 was calculated. This relationship implies a freshwater y18O end member of 18.86x[SMOW], which is consistent with measured y18Ow values of 18.9x (Le´tolle et al., 1993) for the Lena River water, the dominant freshwater source in the Laptev Sea. Based on the y18Ow to salinity ratio mentioned above and the equation of Grossman and Ku (1986), the effect of seasonal salinity and temperature variations on the y18O signal in the bivalves can be calculated. While a mean annual winter to summer salinity variation of 0.89 (Table 2) would cause a minimum y18Oaragonite shift 0.45x, the bottom-water temperature remains relatively constant throughout the year between 1.2 and 1.5jC with a seasonal variation of 0.36jC which would cause an oxygen isotope shift of only 0.08x. Therefore, the main forcing factor of the within-shell isotope variations is change in bottom-water salinity. Studies that deal with the dependency of carbonate y18O on temperature, salinity, and y18Ow have indicated that both calcitic and aragonitic mollusks deposit shell carbonate near oxygen isotopic equilibrium (Wefer and Berger, 1991). Nevertheless, to more accurately interpret our oxygen isotope profiles, we also quantified the species-dependent 1 In the Practical Salinity Scale salinity is defined as a pure ratio, and has no dimensions or units [UNESCO, 1985].

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fractionation offset of M. calcarea (Table 3). We compared the measured y18OBivalve at the ventral margin, which represents the youngest part of the bivalve and therefore the conditions before collection, with an expected y18Oaragonite value. For calculating the expected y18Oaragonite the actual bottom-water temperature (Bude, S.O., personal communication, 1999) on the collection day and the actual y18O signature of the ambient porewater in the upper two centimeters of the sediment were used (taken from a multicorer with an undisturbed surface). The resulting offset of 0.01x reveals that M. calcarea appears to calcify near the equilibrium (Table 3). For the hydrographic reconstruction from the fossil bivalves we therefore assume that the species-dependent fractionation offset for modern M. calcarea is equivalent to the offset of the fossil specimens. Bivalve oxygen isotope profiles As shown by previous studies (Mueller-Lupp et al., 2003), oxygen isotope profiles of modern bivalve specimens from the Laptev Sea exhibit amplitude changes that are interpreted as annual cycles with more negative y18O values indicating summer and more positive y18O values indicating winter. In the modern bivalve from site 92, 2.5 seasonal cycles were identified with mean summer to winter variations of 1.30x(Fig. 2, Table 4). The light y18O values at 18 mm, which is equivalent to the ventral margin, represents the collection date in August 1998. Employing the equation of Grossman and Ku (1986), together with an in situ bottom-water temperature of 1.24jC and a y18Ow to salinity relationship of 0.50x/salinity, we calculated a salinity record from the oxygen isotope values of this bivalve (Fig. 2). The variability in the calculated salinity record is remarkably consistent when compared with the mean summer to winter salinity range obtained for the period 1950 – 1990 (Environmental Working Group [EWG], 1998) (Fig. 2). The oxygen isotope record of the specimen collected from 120 cm core depth shows two seasonal cycles. A very prominent winter to summer variation of f3xand a second, Table 2 Average modern water temperature and salinity obtained from the ‘‘Joint U.S. Russian Atlas of the Arctic Ocean for winter and summer period’’ (1950 – 1990) of the environmental working group (EWG 1998) in a square with 50 km side length around station PS51/92 (130.140 E; 74.592 N) D sala

Water depth [m]

Salinity Winter

Summer

0 5 10 15 30

26.27 26.21 26.24 30.20 32.57

15.78 17.97 23.20 27.34 31.68

10.49 8.24 3.04 2.86 0.89

Winter

Summer

D tempa [jC]

 1.29  1.28  1.30  1.59  1.66

2.22 1.81 0.76  0.52  1.30

 3.51  3.09  2.06  1.07  0.36

Temperature [jC]

a Difference (D) between winter and summer data (w  s). If the temperature (D temp) is negative, the water temperature is higher in summer than in winter. If the salinity (D sal) is positive, the salinity is higher in winter than in summer.

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Table 3 Offset calculation by subtracting the y18Oar measured from the y18Oar expected Station

T bottom water [jC]

y18O porewater [xSMOW]

y18O expected [xPDB]

y18O measured [xPDB]

Offset [x PDB]

92

 1.24a

 3.26

1.45

1.46

0.01

18

The Oar expected is calculated solving the equation of Grossman and Ku (1986) to y18Oaragonite, where temperature is replaced by the actual bottomwater temperature and y18Ow by the y18O of the porewater. a Bude, S.O. unpublished data.

less pronounced cycles with an amplitude of f1.2x(Fig. 2). In the shell profile from 210-cm core depth 2.5 annual cycles are discernible from the y18O profile with a mean of 2.08x(Table 4). The bivalve y18O profile from 300 cm core depth with a mean y18O of 1.78xalso shows 2.5 seasonal cycles but has a relatively high seasonal amplitude of 2 to 3.5x. The oxygen isotope profile of the lowermost bivalve at 500 cm reveals rather reduced seasonal variations and its mean y18O value of 0.58xis significantly depleted compared to the records of the bivalves in the upper section of the core (Fig. 2, Table 4). Comparing all y18O shell profiles (Fig. 2) it is obvious that the average y18O value of the shell profiles at 3800 cal yr B.P., at 1200 cal yr B.P., and of the modern bivalve are rather similar, while the average y18O value of the shell profile at 7300 cal yr B.P. is slightly depleted. Furthermore, a distinctive increase of the seasonal amplitudes in the y18O profile is observed in this bivalve, implying more pronounced hydrographical variations between the summer and the winter regime at this time. In contrast to all younger bivalves, the isotope shell profile at 8400 cal yr B.P. reveals a mean value of 0.58xand only a small seasonal amplitude of 0.36x(Table 4). When comparing the mean y18O value of the modern and of the oldest bivalves, a depletion of 1.39xis discernible (Fig. 2).

Paleohydrographical implication Because it is uncertain whether this depletion of 1.39xis induced by changes in bottom water temperature and/or salinity, a conceptual model in which possible salinity and temperature combinations were calculated to obtain the mean y18O value of the investigated bivalve shell profiles is presented (Fig. 3a). Our temperature and salinity model (Fig. 3a) suggests two extreme interpretations of the observed isotopic shift of 1.39xbetween the modern bivalve and the 8400-yr-old one: It was caused either by a salinity reduction of 2.8 or by bottom water that was 6.3jC warmer. Both extremes require that the other factor, temperature or salinity respectively, remain constant. According to the salinity – temperature model an increased bottom-water temperature of more than 6jC, i.e., a bottom-water temperature of f5jC, requires a water

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Fig. 2. Mean y18O of the shell profiles and their ages. The frames show the detailed oxygen isotope shell profiles of a modern bivalve shell of site PS51/92 from a water depth of 32 m and fossil bivalve shells collected from a sediment core (PS51/92-12). The record of the modern bivalve also shows the calculated salinity record, which is compared to mean winter and summer salinity data at station 92. Sample positions [mm] are measured as distance from the umbo toward the ventral margin along the axis of maximum growth.

salinity of 32.5. It is obvious from the modern distribution of the water temperatures and salinities in the Laptev Sea (Fig. 3b) that water temperatures of more than 5jC do occur in the Laptev Sea but not in combination with a salinity of 32. Although this time (8400 cal yr B.P.) marks the onset of the Holocene climatic optimum in the Laptev Sea region (Laing et al., 1999; Pisaric et al., 2001), it is hard to reconcile a bottom-water temperature of more than 5jC with salinities of 32.5. It is conceivable that during the Holocene climate optimum, warmer freshwater can be the reason for warmer shelf waters, but if we have a closer look at the salinity – temperature relation it is evident that a Table 4 Core depth, age, mean y18O, and mean seasonal variations of the investigated bivalves Core depth [cm]

Age [cal yr B.P.]

Mean y18O [x]

Mean seasonal y18O variation [x]

0 120 210 300 500

0 1200 3800 7300 8400

1.97 2.06 2.08 1.78 0.58

1.3 1.3 1.0 1.6 0.7

warming of the bottom water is only possible when the salinity is reduced at the same time. Because the hydrography of the Laptev Sea is mainly controlled by the input of freshwater, higher bottom-water temperatures in general are the result of freshwater input; i.e., they are only found in connection with lower salinity (Fig. 3b). Therefore, if the freshwater is the cause for higher bottom-water temperature, a salinity of 32.5 is not conceivable. The second extreme interpretation of the depletion of 1.39xis the assumption that salinity was reduced by 2.8. Taking into consideration the modern temperature and salinity data, this is a more consistent interference. If salinity was reduced by 2.8 before 8400 cal yr B.P. the model leads to a conceivable bottomwater temperature of 1.2 to 1.4jC. These temperatures are rather typical modern bottom-water temperatures (Fig. 3b; Environmental Working Group [EWG], 1998). In fact the isotopic depletion of 1.39xof the 8400-year-old bivalve has to be interpreted as a combination of both interpretations: slightly higher bottom-water temperature combined with reduced salinity. However, both a warmer and less saline bottom water means that site 92 was more strongly influenced by riverine freshwater at 8400 cal yr B.P. than it is at present.

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Fig. 3. (a) Conceptual temperature and salinity model for the interpretation of the different y18O values in the bivalve shells. The lines are representing possible salinity and temperature combinations to obtain the mean y18O value of the dated bivalves. The gray shaded box indicates reliable temperature/salinity combinations that were found today in the Laptev Sea. (b) Salinity and temperature data from several expeditions to the Laptev Sea, carried out in winter and summer since 1993 (Kassens and Dmitrenko, 1995; Kassens and Karpiy, 1994; Kassens et al., 1997). The black dashed line represents possible combinations of salinity and temperature values to obtain the mean y18O value of the 8400-yr-old bivalve.

Because the Laptev Sea region evolved into a modern shelf sea only during the postglacial sea level rise, the southward transgressing sea had a major impact on the shelf environment (Bauch et al., 1999). On the basis of major changes in the average sedimentation rate in sediment cores and other sedimentological parameters, Bauch et al. (2001b) reconstructed time slices of the postglacial-transgressional history of the Laptev Sea shelf and estimated that the inundation of the present 31-m isobath was concluded by about 8900 cal yr B.P. while the Holocene sea-level maximum was reached around 5000 cal yr B.P.. Using this time frame of the sea-level rise between 8900 and 5000 cal

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yr B.P., a sea level 27 m below that of today is estimated for site 92 at 8400 cal yr B.P. (Fig. 4b). This suggests that site 92 was located close to the paleocoastline in a valley which is identified as the main paleovalley of the Lena River (Fig. 5). Based on the Holocene inudation history of the Laptev Sea shelf (Bauch et al., 2001b) and on continuous sedimentation since 8400 cal yr B.P., a paleo water depth of f10 m is estimated for site 92 at 8400 cal yr B.P. (Fig. 4b). For verification we compared the reconstructed salinity with modern bottom-water salinities at stations of 10-m water depth in the proximity of the Lena Delta. These modern bottom-water salinities do not exceed values of 26 to 27 (Kirillov, S., personal communication, 2001) and are thus lower by 2.5 to 3.5 as compared to our reconstructed salinity. A paleohydrographical interpretation on the basis of reconstructed bottom-water salinities remains incomplete without discussing the possible changes in surface-water salinities. On the basis of a correlation between freshwater diatom assemblages in core-top sediments and summer surface water salinities, Bauch and Polyakova (2003) reconstructed surface-water salinities for core 92 (Fig. 4a). Their reconstructed surface-water salinity of 9 to 10 in combination with our estimated bottom-water salinity of 29.5 indicates a near-coastal environment influenced by river runoff at 8400 cal yr B.P., but they also reveal that the water column of site 92 was probably under the influence of strong stratification, which was more intensive than that found for similar water depths in the Laptev Sea nowadays. In arctic estuarine systems like the southern Kara Sea with its major rivers of Ob and Yenisey, surface and bottom-water salinity data (Stein and Stepanets, 2001) show a stronger stratification of the water column than the one observed in the delta-influenced area east of the Lena Delta (Wegner et al., 2002; Kirillov, S., unpublished data). Since the modern shelf topography in the area of site 92 is clearly recognized as a submerged Lena paleoriver channel formed during times of lowered sea level (Holmes and Creager, 1974; Kleiber and Niessen, 1999), we concluded that there did not exist a delta system at those times when the sea level was lower. This is in accordance with the relatively young sedimentation history of the Lena Delta (Schwamborn et al., 2002). From the submarine topography near site 92 it is obvious that at 8400 cal yr B.P. the Lena paleoriver mouth resembled an estuarine system. Such an estuarine system has a different hydrography than a delta in that a reversed bottom current with higher salinity exists below a low-salinity river plume. This scenario could explain the more pronounced stratification of the water column and the relatively high bottom-water salinity reconstructed by us for this time. The distinct increase in the mean y18O of the shell profiles from 8400 to 7300 cal yr B.P. gives evidence for significant changes in the bottom-water salinity due to increasing distance of the site 92 relative to the Lena River mouth. According to our reconstruction, mean bottom-water

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Fig. 4. (a) Reconstructed surface and bottom water salinity in core PS51/92-12 for the past 9000 cal yr. Surface salinity reconstruction were obtained from Bauch and Polyakova (2003) and based on freshwater diatoms. (b) Profile of the modern shelf topography along 130.1jE and the reconstructed postglacial sealevel rise in the Laptev Sea from Bauch et al. (2001b).

salinity changed from 29.5 to 32, which is similar to the modern mean bottom-water salinity found at site 92. In this time interval, increasing bottom-water salinity is also coeval with an increase in surface-water salinity (Fig. 4a) and with major changes in the depositional environment, recognized in other cores from similar water depths (Bauch et al., 2001b). In dependence of the southward retreat of the coastline, the depositional realm of the river also shifted, both leading to a stepwise decrease in sedimentation and accumulation rates and in riverine influence. between 8000 and 7000 cal yr B.P. (Bauch et al., 1999; Mueller-Lupp et al., 2000). On one hand, the reconstructed mean bottom-water salinity of 32 at 7300 cal yr B.P. gives clear evidence of the end of this transitional phase, as this mean value is already similar to the modern one. On the other hand, the higher summer-to-winter

variation in the bottom-water salinity at 7300 cal yr B.P. probably provides an indication of a stronger freshwater discharge during summer. Evidence from lake sediments (Laing et al., 1999) and palynological records (Andreev et al., 2002; Pisaric et al., 2001) from near-coastal areas of the Laptev Sea with the treeline reaching its northernmost position in Eurasia (Mac Donald et al., 2000), all suggesting the wettest and warmest conditions during the Holocene. Although no isotope shell profile exists for the time between 6000 and 4000 cal yr B.P., when the sea level reached its Holocene maximum, paleohydrological reconstruction at 3800 cal yr B.P. reveals that modern hydrographical conditions were fully established at this time. This is in accordance with paleoclimatic reconstructions that also indicate the establishment of modern-type conditions in the Laptev Sea region during the last 3000 –4000 cal yr B.P. (Laing et

T. Mueller-Lupp et al. / Quaternary Research 61 (2004) 32–41

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Fig. 5. Paleoenvironmental scenario of the Laptev Sea region at 8400 cal yr B.P. The sea level was 27 m below that of today. Note that the topographical data obtained from the 2001 IBCAO do not reflect the actual paleosurface prior to inundation.

al., 1999; Mac Donald et al., 2000; Naidina and Bauch, 2001; Pisaric et al., 2001).

Summary Paleohydrographical changes on the eastern Laptev Sea shelf during the past 8400 cal yr B.P. are reconstructed from oxygen isotope profiles of bivalve shells collected from a well-dated sediment core from northeast of the Lena Delta. Detailed profiles of fossil shells are compared with an isotopic record of a modern specimen of M. calcarea, which reflects the modern hydrographical conditions of the bottom water at the investigated site. The isotope profile of oldest studied specimen (8400 cal yr B.P.) shows y18O values that are on average depleted by 1.39xin comparison with the modern specimen from the same site. Assuming a constant y18Ow-to-salinity relationship in the Laptev Sea since 8400 cal yr B.P., this depletion is interpreted mainly as the result of a reduced salinity at 8400 cal yr B.P., indicating a more coastal and river-influenced environment. A reconstructed bottom-water salinity of 29.5

in comparison with a surface-water salinity of 9 inferred from freshwater diatoms provides clear evidence for enhanced influence of riverine freshwater as well as for a more pronounced stratification of the water column at 8400 cal yr B.P. Because of the Holocene transgressional history of the Laptev Sea shelf, the increasing sea level was the most influencial factor on the paleoenvironment, with prominent impacts on the locality of the paleoriver mouth and thus on the hydrographical conditions. Due to the continuing southward retreat of the coastline and the Lena River mouth relative to the study site an increase in the bottom water salinity at 7300 cal yr B.P. is reconstructed. The oxygen isotope shell profile at 7300 cal yr B.P. gives evidence that bottom-water salinities similar to modern values occurred at this time but with significantly higher summer-to-winter amplitudes than today. The time slices at 3800 and 1200 cal yr B.P. reveal that modern hydrographical conditions were already fully established. Although the high-resolution isotope profiles from fossil bivalves of the Laptev Sea shelf cover only brief time intervals, they offer new important insights into

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