LAKE QINGHAI, CHINA - CLOSED-BASIN LAKE LEVELS AND THE OXYGEN ISOTOPE RECORD FOR OSTRACODA SINCE THE LATEST PLEISTOCENE

In 1985 a Sino-Swiss limnogeological expedition to China's largest inland lake recovered seismic profiles and piston cores to help calibrate palaeoclimatic models of monsoon and insolation fluctuations over the northern Tibet-Qinghai Plateau since the Last Glacial Maximum. Lake Qinghai, a perennial lake with an area of 4278 km2 (1985) at 3194 m above sea level, receives drainage from 30,000 km2 of catchment near the northwest corner of the Plateau. The 26.5m deep terminal lake (1985), with brackish, alkaline, magnesium-sulphate rich waters (14 g l-1 TDS), has not overflowed since the last Glacial, but its level have responded to climatic and hydrologic changes. Seismic-reflection profiles for the sub-bottom sediment structures revealed a complex topography with channels, relict terraces, tectonic tilting and deformation in deposits buried under a 4-6 m packet of mainly flat-bedded sediments from which core samples were recovered. The lowermost cored material (approximately 14,500 yr B.P.) comprises mainly water-lain loess deposited in a smaller flood-plain lake of up a few metres depth under cold arid climatic conditions. AMS radiocarbon dating indicates average sedimentation rates for latest Pleistocene and Holocene deposits in central basin regions of approximately 0.3-0.5 mm yr-1. The delta-O-18 values of fossil ostracod valves provide a record of changes in the isotopic chemistry of the lake waters that can be related to the precipitation: evaporation budgets which have controlled the palaeo-levels for Lake Qinghai. The precipitation/evaporation budget for the Lake Qinghai region is largely determined by the Asian monsoon system. The Lake Qinghai sediment record shows that there was no large influx of glacier melt-waters into the lake basin during the post-Glacial. The regional climate remained arid until at least approximately 12,500 yr B.P., when an incipient monsoon was possibly initiated. River-transported silts first reached the basin centre at approximately 11,800 yr B.P., when the lake depth may have increased to approximately 5 m. Possibly a monsoonal weakening for several centuries after approximately 10,800 yr B.P. (Europe, Younger Dryas) caused a further period of aridity. At approximately 10,200 yr B.P. the basin began filling towards modern lake dimensions in response to a permanent strengthening of the Asian monsoon. Substantial lake-level rises probably characterised the periods approximately 10,200-9800, 9500-8500, 8300-7200 yr B.P. and through perhaps a century at about 6800 yr B.P. Decreasing levels probably characterised the intervening intervals. Short-term lake-level fluctuations through the mid-Holocene climatic optimum were minimal, indicating a very stable climatic / hydrologic regime. An isotopic steady-state for the lake waters since 3000 +/- 500 yr B.P. obscures the climatic / hydrologic regime dependent delta-O-18 gradient, but there is little stratigraphic evidence to suggest that the lake level has been significantly below its present level since that time. Three negative delta-O-18 excursions for the lake water may correspond to known glacier advances on the Plateau at approximately 3000, 2000 and 1500 yr B.P. during China's neoglacial.