Lake Level Reconstruction Using the Water Depth Related Distribution of Ostracoda in two Tibetan Lakes, Nam Co and Donggi Cona

Changes in the Asian Monsoon are reflected in paleo-lake level changes that can be reconstructed from ostracode associations of lake sediment cores. Ostracodes are one of the main micropaleontological proxies in paleolimnological research. They show high abundance and continuous presence in the often slightly saline high-altitude lakes on the Tibetan Plateau (TP). A comprehensive knowledge about the ecology and present day depth distribution of species, however, is a prerequisite for the use of this proxy. Therefore, we studied Recent ostracode associations from two lakes on the Tibetan Plateau for actualistic reference. Nam Co is situated on the south-eastern TP (30.5° N; 90.7° E) at 4719 m altitude. The surficially closed lake covers an area of 1961 km2 and has a maximum depth of 105 m. The water is slightly brackish (0.8 psu). The other lake, Donggi Cona, is located on the north-eastern TP (35.3° N; 98.9° E) at 4144 m altitude. Covering 250 km2; Donggi Cona is much smaller and shallower (38 m maximum depth) than Nam Co. Donggi Cona is a through-flow system and thus characterized by prevailing freshwater conditions. Ostracode diversity and species composition of both lakes are different; we identified six species in Nam Co and thirteen species in Donggi Cona sediments; four of the species we found in Nam Co were found in Donggi Cona as well. The most abundant species (70-95%) in Nam Co is Leucocytherella sinensis Huang, 1982, which is absent from Donggi Cona. There, Eucypris gyirongensis Huang, 1982 (shallow water indicator) and Leucocythere mirabilis (Kaufmann, 1892) (deep water indicator) are dominant. Despite these faunistic differences, our results suggest that Limnocythere inopinata (Baird, 1843) prefers shallow water and Leucocythere dorsotuberosa Huang, 1982, deep water below the thermocline in both lakes. The depth dependent distribution of ostracode species provides, together with autecological information on preferences and tolerances of species a database to establish a transfer function for the reconstruction of water depths and thus past lake levels. These transfer functions cover water depths between 3.7 m and 64 m in Nam Co, and between 0.2 m and 35 m in Donggi Cona. The standard error is approximately 7 m. Ostracodes of a 2.5 m long core and two short cores from Nam Co, for example, indicate lake levels 10 m to 20 m lower than today before 4 ka BP followed by a high stand of initially more than 10 m above the modern lake level. A distinct drop of approximately 25 m is reconstructed for the Little Ice Age. Since then until today, the lake level has been rising. Faunistic and also some ecological differences of species associations stress the need of regional modern data sets necessary for the establishment of transfer functions and their application to long sedimentary sequences from Tibetan Plateau lake systems.