THE OXYGEN AND CARBON ISOTOPIC RECORDS OF MIOCENE SEDIMENTS IN THE TIANSHUI BASIN OF THE NORTHESTERN TIBETAN PLATEAU AND THEIR PALEOCLIMATIC IMPLICATIONS

The Tianshui basin of the northeastern Tibetan Plateau is located in the intersection of the eastern monsoon area, the northwest arid area and the Qinghai-Tibet Plateau cold and arid regions, so it is very sensitive to climatic change. Based on the δ¹⁸O, δ¹³C and TOC data along the Xiashan section, this paper deals with the Miocene climatic evolution of the Tianshui basin. The climatic change in the region can be divided into five stages in 17.02~12.26 Ma, 12.26~10.92 Ma, 10.92~8.50 Ma, 8.50~7.050 Ma, 7.05~6.05 Ma respectively. During the period of 17.02~12.26 Ma, the climate was very warm and humid under high precipitation; in the interval from 12.26 to 10.92 Ma, the climate was relative cold and humid; from 10.92 to 8.50 Ma, with less precipitation, the climate was relative cold and dry; in the phase 8.50~7.05 Ma, the climate was relative warm and humid; from 7.05 to 6.05 Ma, the climate was relative cold and dry. Global cooling may be the reason for the relative cold and wet climate during 12.26~10.92 Ma. The average δ¹⁸O values during the 10.92~8.50 Ma period are 1.5‰ more positive than that of the period prior to 10.92 Ma, probably due to the elevation of the Qinghai-Tibet Plateau that was sufficient to change the atmospheric circulation, by blocking the moisture from the Indian Ocean or south Pacific into the Tianshui basin. Since 7.05 Ma, the climate has been relatively cold and dry, marking the initiation of aridification in the inland Asian. All of the information is important to the understanding of the history of up-lifting of the Tibetan Plateau in-depth, the aridification in Asia interior as well as the climatic evolution in this region.