The Carbon And Oxygen Analysis Of The Miocene Deposits In Tianshui Basin And It’s Paleoclimate Signification

A series of significant climatic and environmental change events occurred in the Cenozoic, of which Miocene is a critical period for the formation and evolution of the East Asian monsoon and Asian aridification. The Tianshui basin is located in the intersection of three great natural zones (eastern monsoon area, northwest arid area and Qinghai-Tibet Plateau Cold and Arid Regions) is very sensitive to climate change. Continuous, widely distributed Neogene depositional strata was deleveloped in this area, which provide favorable natural carriers to study the past climatic and environment evolution of the eara. Although the evolution of the Eastern Asian monsoon and aridification in Asian have been gained some achivements through a large number of studying of magnetic stratigraphy、mammal magnetic, sedimentology, sedimentry particle size, magnetization and so on, however, its remains not unanimous. In this paper, we select Miocene sedmentry stratum of Yawan section in Tianshui basin controlled by high resolution stratigraphic chronology and mammal fossils, to restore the past climatic and environment evolution of the area and to discuss the evolution of the Asian monsoon, to understand aridification in Asian inland through using inorganic carbon and oxygen isotope, organic carbon isotope, C/N and TOC indexs, We obtain the following conclusions:1):the climatic evolution can be divided into five stages during this period:17.02～12.26Ma, 12.26～10.92Ma,10.92～8.50Ma,8.50～7.050Ma,7.05～6.05Ma. During the 17.02～12.26Ma period, the precipitation was relative abundant, the climate was very warm and humid; in the interval from 12.26 to 10.92Ma, the climate was cold and humid; from 10.92 to 8.50Ma, with less precipitation, however, the chimate was not dry; in the phase 8.50～7.05Ma, the climate was relative warm and humid; since 7.05 to 6.05Ma, the climate was relative cold and dry. Global cooling may be the cause of the relative cold and wet climatic condition during 12.26～10.92Ma.2):Theδ13Corg Values are relatively stable throughout the 17.02 to 6.05Ma interval ranging from-24 to-28.3%o VPDB, suggesting that C4 grasses were either absent or insignificant in the Tianshui region prior to 6.05Ma.4):The Tianshui climate records reveal that the East summer monsoon was strong in 17.02-12.26Ma and weak in 12.26-8.50Ma, since 8.50 to 7.05Ma the summer monsoon was strong again, in the Late Miocene after 7.05Ma, the winter monsoon bagan strong. In the Late Miocene after 7.05Ma, representing the starting of aridification in the Asian inland,against the starting of aridification in the Asian inland form the Late Oligocene or the early Micocene.