Characteristics and Significance of C-O Isotopes of Oligocene Miocene Lacustrine Carbonate Rocks in Zhangxian, Northern Margin of West Qinling Mountains

Zhangxian basin, located in the northern margin of the west Qinling mountains, deposited a large number of lacustrine carbonate rocks during the Oligocene-Miocene. Its C and O isotopic characteristics are of great significance to reveal the paleoenvironment and Paleoclimate of the northeastern margin of the Qinghai Tibet plateau during this period δ¹³C_V-PDB values ranged from-7.6 ‰ to-2.0 ‰, with an average of-5.04 ‰, δ¹⁸O_V-PDB values ranged from-9.11 ‰ to 1.12 ‰, with an average of-2.10 ‰. δ¹⁸O_V-PDB. value is consistent with that of the modern closed lake, which indicates that Zhangxian basin is a closed lake in Oligocene-Miocene. The average paleotemperature is 17℃, which is about 10℃ higher than the modern temperature, and the profile contains gypsum and mirabilite, which indicates that Zhangxian basin is in a warm and arid climate environment in Oligocene-Miocene, and the average paleosalinity Z value is 116, Therefore, Zhangxian basin was a closed semi saline continental lake in this period. adopt δ¹³C_V-PDB value shows that the productivity is high, but the reduction reaction affects the preservation of organic matter because of the sulfate bacteria in the ancient lake water δ¹³C_V-PDB was negative. The Oligocene-Miocene paleoenvironment and paleoclimate changes in Zhangxian basin are basically consistent with the Oligocene-Miocene records of Xining basin, Xunhua basin, Linxia basin and Tianshui basin, all of which are gradually arid. The main reason for this change is the global climate change, which has little to do with the rapid uplift of the Qinghai Tibet plateau.