Paleo-monsoon dynamics on the Tibetan Plateau from LGM to the beginning of the Holocene

In this contribution we will present one of the longest continous high-resolution environmental archives on the Tibetan plateau. Lake sediments from Nam Co (Central Tibetan Plateau) date back to ~23 ka cal BP.During that period,a multi proxy approach reveals oscillations of different duration and magnitude. Sedimentation rates are highest ~21,2 ka cal BP(>5 mm/a)with rapidly decreasing values to <1 mm/a at ~20,5 ka cal BP and <0,5 mm/a after ~18,5 ka cal BP. A slight increase is observed between 13,2 and 11,3 ka cal BP. In the geochemical record (TIC, TOC, TN, TS, Ca,Mg, Sr, Al, Ti, Fe, Mn) the time span between ~23 and 13,8 ka cal BP is characterized by the lowest values of TOC and TN and only small variations of the other elements. This points to a slowly rising lake level of Nam Co with decreasing sediment transport to the coring location and a low bioproductivity. This is supported by subbottom profiling showing submerged beach remains up to 60 m below the present lake level. Marine records, originating from the Arabian Sea,give evidence for a first prominent post glacial climate change in South-East Asia around ~14 ka cal BP (SIROCKO ET AL.1993). Prior to this date, an intense moisture transport from the Arabian Sea to the Tibetan Plateau by monsoonal air masses seems to be unlikly. Therefore we assume that sedimentation rates and geochemical signatures from ~23 to 13,8 ka cal BP reflect the input of melt water from glaciers caused by rising air temperatures after the LGM. At ~13,8 ka cal BP the system seems to change. A sharp increase in TOC is accompanied by rising concentrations of TN and TS and a significant lowering of the TIC content, indicating an increase in bioproductivity and a rising lake level, leading to an incomplete mixing of the water body and lower ionic concentrations. A second sharp rise in TOC can be observed ~12,2 ka cal BP. In contrast to the 13,8 ka cal BP event, levels of TN and TS remain low, leading to high TOC/TN molar ratios(up to 21) within this core section. An enhanced input of terrestrial organic matter at ~12,2 ka cal BP is a possible explanation for these high values. During this period allochthonous input, represented by Al, Ti and K is slighly decreasing, most probable associated with the increasing lake level influencing the sediment transport to the coring location. After 12,2 ka cal BP a lowering of the TOC can be observed, reaching lowest values ~11,4 ka cal BP, indicating a drop in bioproductivity. Around 11 ka cal BP smaller peaks in Al, Ti K and Rb/Sr-ratio are visible, pointing to a higher input of physically weathered material from the catchment. Both signals might correspond to the Younger Dryas. From the Arabian Sea strong monsoonal pulses, dated to 13,5, 13,0 and 13,0-12,5 ka cal BP are reported (SIROCKO ET AL. 1993, OVERPECK ET AL. 1996).These are associated with an intensified moisture transport from the Arabian Sea to the Tibetan Plateau. The changes in lake Nam Co between ~13,8 and 12,2 ka cal BP are therefore obviously directly related to the strengthening of the monsoon in the Arabian Sea, but the differnt pulses are not detected in the Nam Co until now. An event corresponding to the Younger Dryas is not reported from these marine records.