Preliminary results for ostracod-based salinity reconstruction over the last 16800 years of Lake Nam Co, Tibet

Totally 72 adult ostracod valves belonging to three taxa (Leucocytherella sinensis, Leucocythere dorsotuberosa, Leucocythere dorsotuberosa f. postilirata) were picked from the top 4 m section of sediment core NC08/01 recovered from Lake Nam Co on the central Tibetan Plateau. AMS ¹⁴C dating shows that this core section covers the last 16,800 yr. We developed a new analytical technique for in-situ Laser-ablation-ICP-MS trace element analysis of single ostracod valves. Here we report the first rare earth element content measurements on single ostracod shells showing PRIMA-normalized REE patterns consistent with those of the upper continental crust. A combination of the molar ratios of Mg/Ca, Sr/Ca, U/Ca and Ba/Ca in the shells of L. sinensis, the most abundant taxon in the core, was used to constrain the paleohydrologic and paleosalinity changes in Lake Nam Co. High values of Mg/Ca and Sr/Ca indicate dry climate conditions and high salinity between 16,800 and 10,400 yr BP. This period of relatively dry climate has been interrupted by short humid phases. In the subsequent period, between 10400 and 5400 yr BP, increasing precipitation may have led to elevated lake level and reduced salinity. This period was followed by a decrease in lake volume between 5400 and 4700 yr BP. In contrast to previous studies, lower element/Ca ratios indicate relatively wet conditions from 4700 to 1300 yr BP, which is probably due to precipitation of high-Mg calcite and aragonite in the sediment reducing the Mg/Ca and Sr/Ca ratios of the lake water. From 1300 yr BP, ostracod trace element chemistry indicates very dry conditions interrupted by some short-term wet climate oscillations. U/Ca and Ba/Ca ratios show a good agreement with Mg/Ca and Sr/Ca ratios in the whole profile and may also serve as proxies for ostracod-based paleoclimatic reconstructions.