METHODS Ostracodes species assemblages from two sediment cores Nam Co 8 and Nam Co 08/1 each 2.70 m and 11 m long, were taken from 31 m and 91 m water depth during September 2005 and Our ostracode-based transfer function for paleo water depth estimation was applied to the species assemblages of core Nam 08/1. Species assemblages and oxygen and carbon Isotopes of the valves of selected ostrcode species (Leucocytherella sinensis, see Tab. 1) of core Nam Co 8 were measured to obtain information about past hydrological conditions. The German Research Foundation (DFG) sponsors our work within the priority program 1372 “Tibetan Plateau: Formation – Climate – Ecosystems (TiP)”. We also want to thank Steffen Mischke helping with taxonomical issues. Lake System Response to Late Quaternary Monsoon Dynamics on the Tibetan Plateau: Microfossils as Indicators of Lake Level Changes Antje Schwalb 1, Peter Frenzel 2, *, Claudia Wrozyna 1, 3, Anne Lödige 1,Gerd Gleixner 4, Gerhard Daut 5, Roland Mäusbacher 5, Liping Zhu 6 1,* Institut für Umweltgeologie, Technische Universität Braunschweig, Langer Kamp 19c, Braunschweig, Germany 2 Institut für Geowissenschaften, Friedrich Schiller Universität Jena, Burgweg 11, Jena, Germany 3 Institut für Erdwissenschaften, Karl Franzens Universität Graz, Heinrichstr. 26, 8010 Graz, Austria 4 Max Planck Institut für Biogeochemie, Hans Knöll Str. 10, Jena, Germany 5 Institut für Geographie, Friedrich Schiller Universitöt Jena, Löbdergraben 32, Jena, Germany 6 Institute of Tibetan Plateau Research, Chinese Academy of Sciences, No.18, Shuangqing Rd., Beijing, China OBJECTIVES The Tibetan Plateau (fig.1) is a key region for global atmospheric circulation and for investigations of the Asian Monsoon. Fossil ostracode assemblages, paleo-water depth estimations using an ostracode- based transfer function, as well as stable oxygen and carbon isotopes signatures of calcitic ostracode valves from two sediment cores from Lake Nam Co, Southern Tibetan Plateau, covering the last approx. 23 and 7.3 cal. ka BP, respectively, are used to reconstruct lake level changes (fig. 4 A,B) ACKNOWLEDGMENTS CONCLUSIONS Our results suggest that (1) the LGM may not have been as dry as generally anticipated, the (2) onset of glacier melting and/or monsoon strengthening occurred possibly earlier than the Late Glacial Northern Hemisphere warming, and (3) a reduction in melt water supply and/or monsoonal precipitation started before the onset of the Younger Dryas. Our data confirm the general view of a (4) decrease in melt water supply and/or monsoonal precipitation during the early to mid- Holocene leading to a rise in lake level prior to ~5.4 cal. ka BP. The mid-Holocene is characterized by a further increase in aridity and a decrease in lake level (fig. 6) that is consistent with results from other climate records from Lake Nam Co and adjacent areas. This trend is interrupted by a short period of wetter climate and slowly rising lake levels terminated by the onset of the ‘Little Ice Age’ (Wrozyna et al. 2010). RESULTS AGU Fall meeting 2010 December 13-17, 2010 San Francisco, US Fig. 1 Monsoonal circulation affecting the Tibetan Plateau including the study area Lake Nam Co (red circle). Fig. 2 Study area Lake Nam Co, south-central Tibetan Plateau (Data source Landsat ETM 2007) with core locations (red dots). Stable isotopes More details of Holocene hydrological conditions are provided by ostracode species assemblages and ostracode δ 18 O/δ 13 C signatures from core Nam 08 (fig. 4 B, 5). The early Holocene is characterized by an abundance maximum and relatively low species diversity dominated by L. sinensis. Lowest δ 18 O values of the entire sequence (fig. 5) indicate large amounts of freshwater input into the lake and/or lower evaporation; lower lake productivity is indicated by high δ 13 C values prior to ~5,4 cal. ka BP. Around ~5,4 cal. ka BP a distinct increase in δ 18 O values reflects stronger evaporation, and a slight increase in δ 13 C points to reduced inflows. Higher species diversity is caused by higher numbers of shallow water species. The late Holocene (last ~3,8 cal. ka BP) is characterized by higher variability in species distribution with increasing numbers of deep-water indicators for the most recent part suggesting rising lake level. Further increase in δ 18 O possibly reflects an increase in precipitation and a decrease in melt water input. The distinct shift to more negative δ 13 C values suggests higher lake productivity. The variability ranges of the last ~1,3 cal. ka BP are confirmed by data from Recent ostracodes from water depths ranging between 3.7 and 53 m. 30°30’-35’ N, 90°16’-91°03’ E 4719 m a.s.l. max. water depth: 95 m salinity: 1,3 ‰ precipitation: mm a -1 Lake Nam Co 15 km Fig. 5 δ 18 O and δ 13 C signatures of Leucocytherella sinensis from core Nam 8. deep-watershallow water above the thermocline (<20-30m)adabtable ?L. dorsotuberosa f. postilirata ?L. dorsotuberosaL. sinensisF. gyirongensisI. cf. mongolicaE. gyirongensis 500 µm C. xizangensis Tab. 1 Typical ostracode species from Lake Nam Co and their preferred water depths. Diversity is low (fig. 4 A,B); except for the cosmopolitan species C. candida and L. inopinata the species are endemic to the Tibetan Plateau and adjacent areas. Pleistocene-HoloceneHolocene B A Fig. 4 Ostracode species distribution of core Nam 08 (A) and reconstructed lake levels by the application of an ostracode-based transfer function for water depth and of core Nam 08-1 (B). The presence of ostracodes (tab. 1) suggest the existence of Lake Nam Co for at least the last 21.5 cal. ka BP. The Pleistocene in Lake Nam Co is characterized by low abundances and low diversity of ostracode species. Preliminary calculations of lake levels (fig. 4 A) provide values of ~40m higher than today for the Last Glacial Maximum (LGM) followed by decreasing lake levels between ~15 to ~12.5 cal. ka BP. LGM YD Fig. 6 Compilation of proxy records from Lake Nam Co showing moisture availability and lake level changes Lake levels similar to today occurred around ~12 cal ka BP and correspond to the timing of the Younger Dryas (YD). Generally low lake levels occurred during the Holocene up to 60m below present level which might be overestimated due to higher species variability (see fig. 4 B). Fig. 3 Nyainqentanglha Mountains south of Lake Nam Co. References: Wrozyna, C., Frenzel, P., Mackensen, A., van Geldern, R., Steeb, P., Zhu, L., Schwalb, A., Stable Isotope and Ostracode species assemblage evidence for lake level changes of Nam Co, southern Tibet, during the past 600 years. Quaternary International 212: 2-13.