The Holocene paleolimnology of Lake Issyk-Kul, Kyrgyzstan: trace element and stable isotope composition of ostracodes

Abstract

The stable isotope and trace element geochemistry of ostracode shells from Lake Issyk-Kul, Kyrgyzstan, provide an important new paleoclimate record for central Asia. The δ¹⁸O, δ¹³C, Sr/Ca, Mg/Ca and U/Ca composition of shells from the species Candona neglecta found in two piston cores constrains the paleohydrology of the lake from 8700 to 1000 calendar yr before present. These data, combined with robust chronologies based on 16 AMS ¹⁴C dates, suggest that the region went from a relatively moist climate in the early Holocene to a more arid climate in the late Holocene. Intense freshwater input, presumably glacial runoff, flowed into the lake from 8700 to 8300 cal yr BP as evident from the δ¹⁸O and δ¹³C values. From 8300 to 6900 cal yr BP δ¹⁸O and Sr/Ca values indicate that the lake was relatively fresh and an open basin, and U/Ca values suggest that the lake was well-mixed with well-oxygenated bottom waters. There is a transition zone between 6900 and 4900 cal yr BP when δ¹⁸O and Sr/Ca values rapidly increase and Mg/Ca and U/Ca values rapidly decrease. During this period the lake evolved from an open, freshwater, well-mixed lake to a closed, more saline, less well-mixed lake. For the remainder of the record δ¹⁸O, Sr/Ca, Mg/Ca and U/Ca are relatively constant, implying conditions similar to modern conditions, although the slight increasing trend in Sr/Ca may suggest an increase in salinity over this period. The δ¹³C values were approximately in equilibrium with atmospheric CO₂ in the early part of the record but became relatively negative in the later part of the record. This indicates that the dominant mechanism controlling the isotopic composition of dissolved inorganic carbon shifted from exchange with the atmosphere to variations in biological productivity.

Introduction

Paleoclimate records from central Asia have not been fully exploited even though the potential for sensitive climate records in this region is high. Two major pressure cells, the Siberian High to the north and the Southwest Asian Low to the southwest, interact over the region and changes in their intensity or location would strongly affect central Asian climate. These changes should be preserved in paleoclimate proxy records. Previous work (e.g. Lake Baikal: Colman et al., 1995; Lake Qinghai: Lister et al., 1991; Lake Manas: Rhodes et al., 1996; central China Loess deposits: Kukla and An, 1989; Lakes Sumxi and Bangong: Gasse et al., 1996) indicate that while there are some similarities in the paleoclimate records from the region, there are also differences in the perceived patterns of climate change throughout central Asia.

The current study is an analysis of sediments from Lake Issyk-Kul, a deep lake with a potentially long and continuous record of paleoclimate change. Its location in the heart of the Tien Shan mountains offers a unique opportunity to study the climate history of a region in central Asia that is dominated by a continental climate regime and is relatively far from the monsoons to the south and southeast.

Lake Issyk-Kul is located in Kyrgyzstan, a former Soviet Republic (Fig. 1), and is the 11th largest lake in the world by volume and the 5th deepest (Herdendorf, 1990). It has a volume of 1730 km³, a surface area of 6247 km², and a maximum depth of 668 m (Fig. 2). Issyk-Kul is set in a compressional basin (Klerkx et al., 1999) between two mountain ranges, the Kungey Alatoo to the north and the Terskey Alatoo to the south, both of which are part of the Tien Shan. The depression that the lake fills has existed since the Miocene, although the initial formation of the Tien Shan dates from the Early Paleozoic (Grosswald et al., 1994).

Atmospheric circulation over Lake Issyk-Kul is controlled by the Siberian High Pressure Cell and the Southwest Indian Low. During winter months, the southwest branch of the Siberian High extends south into the Tien Shan, bringing cold, dry air from Mongolia into the Issyk-Kul basin. During the summer months, frontal cyclonic circulation develops when cold air masses from the north occlude southern cyclones from the southwest, generating precipitation. This circulation pattern brings maximum, albeit minor, precipitation to the basin during the summer months and driest conditions during the winter months (Dando, 1987, Aizen et al., 1995).

The lake is in a semi-arid setting and local meteorological data suggest that moisture is mostly derived from west of the basin (Romanovsky, 1990, Aizen et al., 1995). A strong gradient of rainfall exists from west to east across the lake: the western shore of the lake near Balykchy receives 120 mm/yr of rainfall, while the eastern shore near Karakol receives 720 mm/yr (Romanovsky, 1990). The surrounding mountain ranges contain alpine glaciers that extended nearly to the present lakeshore during the last glaciation (Grosswald et al., 1994). The sources of freshwater for the major rivers that flow into the lake are rainfall, snowmelt, groundwater, and glacial melt (Aizen et al., 1995).

The seasonal air temperature range in the region (−23 to 41°C) is reduced near the lake (−2 to 18°C; Romanovsky, 1990). Issyk-Kul never freezes due to its brackish salinity (6‰) and high-altitude insolation (Romanovsky, 1990). Surface water temperatures range from 4.4°C in winter to 21.4°C in summer, and there is a permanent thermocline located above 100 m (Romanovsky, 1990; E. Ralph, personal communication). The lake is highly oligotrophic; transparency measured by Secchi disk depths in mid-lake typically fall in the range of 13–22 m (Romanovsky, 1990). Convective winter turnover of the lake water occurs every 2 out of 3 yr, and oxygen concentrations range from 6.8 to 9.6 mg O₂/l (Romanovsky, 1990). Oxygen concentration decreases with depth, with the deep bottom waters (600 m) at about 70% saturation (E. Ralph and M. Vollmer, personal communication).

Geologic maps of the basin indicate that granitic, metamorphic, folded sedimentary rocks, and glacial debris dominate the watershed and contribute to the composition of the lake. Limestone and marble are uncommon. Lake Issyk-Kul waters have a pH of about 8.6–8.9 and are oversaturated with respect to calcite (Alekin, 1946). They are dominated, in order of abundance, by the cations Na⁺, Mg²⁺, and Ca²⁺ and the anions SO₄²⁻, Cl⁻, HCO₃⁻, and CO₃²⁻ (Romanovsky, 1990).

The lake is currently a closed basin lake and would overflow at an elevation of 1620 m, 13 m above the current lake level. During overflow lake waters would extend westward into the Boom Gorge and flow down the Chui River (Fig. 2). The Chui River flows to the west and is ultimately a part of the internal drainage of the Aral–Caspian hydrographic region (Aizen et al., 1997).

Fluctuations in Issyk-Kul’s level, resulting from variable evaporation and precipitation in the basin, are common in the recent and ancient past. Instrumental records indicate that lake level has declined about 3 m since 1926. While Soviet era hydrological projects caused some of this decline, natural processes such as increased evaporation caused by increased temperatures generated part of the decline (Romanovsky, 1990). Preceding the 20th century fall the lake had been documented at just below the outflow level in 1856 AD by the direct observations of Semenov (Semenov, 1858). According to maps of Jesuit missionaries Issyk-Kul may have surpassed the outflow level and become open as early as 1755 AD (Romanovsky, 1990). Remnants of orchard trees, soils and settlements 3–6 m below the current lake level have been dated to the 13th through 15th centuries (Schnitnikov, 1980); lacustrine macrophytes elevated 7.7 m above present lake level have been recently dated at 580±40 ¹⁴C yr BP, suggesting a rapid mid-14th century rise (Rasmussen et al., 2000). Radiocarbon dating of lacustrine macrophytes from 1623 m elevation indicates that there was another highstand, with overflow, between 1200 and 1400 yr BP (Aleshinskaya et al., 1971, Trofimov, 1978, Berdovskaya and Egorov, 1986).

Pleistocene terraces indicate ancient lake levels much higher than those of the Holocene. A late Pleistocene erosive terrace, with an estimated date of occurrence of 26 ka to 10 ka (based on radiocarbon dates of associated lacustrine sediment) is found 33 m above present lake level (Aleshinskaya et al., 1971). An early or middle Pleistocene wavecut terrace is observed 68 m above the present lake level (Trofimov and Grigina, 1979). Depth profiles of the lake reveal a network of submerged relic river valleys (western and eastern bays, Fig. 2) suggesting a Late Quaternary lake level about 110 m below the present surface (Trofimov, 1978).

Surface sediments in the lake consist of terrigenous muds with abundant carbonates (Alekin, 1946, Korotaev, 1967). Beach-zone lithification of nearshore granitic materials via inorganic calcite and organic monohydrocalcite precipitation forms semi-continuous lake-margin pavements and microbialites (Rasmussen and Romanovsky, 1995, Rasmussen et al., 1996). Deep-basin sediments consist of fine-grained terrigenous muds and silts along with endogenic calcite and ostracodes, as well as material exported from the shallow shelves (Rasmussen et al., 1998, Rasmussen et al., 1999).

The existence of large fluctuations in lake level in the basin’s past, and of abundant organic and inorganic carbonate makes Lake Issyk-Kul an excellent target for paleoclimate reconstruction. Inorganic and organic carbonates (ostracodes) have been used in numerous lake basins to study past climate (e.g. Stuiver, 1970, Johnson et al., 1991, Curtis and Hodell, 1993). This work focuses on sediment from two cores, IK97-10P and IK97-11P, collected from the 240 meter-deep western portion of the basin in 1997 (Fig. 2). These two cores were the longest of 11 recovered from the lake and were relatively undisturbed by turbidites, indicating that they may contain a long and continuous paleoclimate record. This paper will describe the trace element and stable isotope geochemistry of ostracodes from these two cores, while another paper will concentrate on the sedimentology of the core materials (Rasmussen et al.,in preparation).