SPELEOTHEM RECORD OF THE LATE PLEISTOCENE CLIMATE ON LOESS PLATEAU AND QINGHAI-TIBETAN PLATEAU TRANSITION ZONE,CHINA

In order to fully understand the causes and mechanisms of recent environmental change, it is necessary to develop a highly detailed and reliable record of past climatic fluctuations. Unfortunately, there is a paucity of high-resolution paleoclimate data from the Chinese loess plateau and the Qinghai-Tibetan plateau transition zone. As the physical and isotopic hydrologic patterns in these regions are very complex, and thus very hard to model, there is a great necessity for a more accurate and thorough paleorecord. Speleothems collected from Wanxiang Cave, a limestone cave located at 33°19' N, 105°00' E contain a detailed record of climate change. The climate of this region is dominated by the rainy summer monsoon and the dry winter monsoon. The intensity of the Asian monsoon has undergone dramatic changes throughout the Pleistocene which are recorded in the geochemistry of the speleothems. Geochemical variations along speleothem growth axes combined with precise U-series dating provide a high temporal resolution record of the Southeast Asian monsoon intensity over the last 300,000 years. The d¹⁸O of precipitation at this site is inversely related to temperature due to the dominance of the amount effect over the temperature effect. Speleothem calcite formed during stadial periods, when monsoon intensity was weak should, therefore, exhibit more positive d¹⁸O values than interstadial periods, assuming the calcite precipitated in isotopic equilibrium from waters which closely reflected precipitation. To test these assumptions, we have performed a pilot study on cave waters and modern calcite collected from Wanxiang Cave to determine the principle controls on the calcite isotopic composition. Measurement of d¹⁸O of cave waters, precipitation, and modern calcite shows that cave waters reflect the mean composition of precipitation at this site and that modern calcite is forming in isotopic equilibrium. Variations in d¹³C of speleothem calcite may reflect variations in the amount of CO₂ degassing as well as changes in the ratio of C₃ to C₄ vegetation, both of which respond to changes in monsoon intensity. The high-resolution time series obtained from these speleothems may aid in understanding the natural variability of the climatic, hydrologic, and ecologic systems in China on long time scales.