青藏高原沱沱河盆地始新世—中新世湖相碳酸盐岩碳氧同位素特征与古环境意义OA北大核心

The Tuotuohe Basin,located in the Qingzang(Tibet)Plateau,is one of the core areas that attested to predominant environmental changes during the plateau's uplift.The Cenozoic sedimentary successions in this region record the evolution of the plateau's topography and geomorphology,as well as regional climatic changes.Based on the petrological and mineralogical analysis of Cenozoic lacustrine carbonate rocks in the Tuotuohe Basin,the characteristics of carbon and oxygen isotopes are studied to explore their paleoenvironmental significance.The results show that the Cenozoic lacustrine carbonate rocks in the Tuotuohe Basin are dominated by micritic limestone,with a small amount of dolomite and bioclastic limestone.The vertical carbon and oxygen isotopic compositions reveal four stages of paleoenvironmental evolution in this area:The first stage corresponds to the period of 38.5-30.5 Ma,when lacustrine carbonate rocks formed in an open lake during a relatively humid climate,likely influenced by increased rainwater input to the lake basin in a flood plain setting under the background of alluvial fan-fluvial drought climate.The second stage is from 30.5 Ma to 23.6 Ma,during which there were significant changes in the paleoenvironment around 26.5 Ma.From 30.5 Ma to 26.5 Ma,the climate was relatively humid,but regional precipitation decreased,and evaporation intensified,related to the local uplift in the northern part of the plateau and changes in the lake basin's hydrological status.From 26.5 Ma to 23.6 Ma,increased evaporation and a dry,cold climate resulted from the change of geomorphic framework in the northern Qingzang(Tibet)Plateau and the reduced westerly rain input.The third stage is between 23.6 Ma and 22.3 Ma,when evaporation decreased,and the climate became relatively humid,related to the development of ancient lakes in the hinterland of the Qingzang(Tibet)Plateau.In the fourth stage(22.3 Ma-19.7 Ma),the climate became drier and colder,and the lake transitioned to a closed saltwater lake,caused by the integration of Hoh Xil into the plateau system and the drying of the interior of Asia.The changes in lake hydrology and climate,revealed by the carbon and oxygen isotopes of the Eocene-Miocene lacustrine carbonate rocks in the Tuotuohe Basin,are closely related to the paleogeographic pattern and geomorphologic evolution in the northern part of the plateau.