Eocene Climate Change and Its Driving Mechanism in the Nangqian Basin of the Central-eastern Qinghai-Tibetan Plateau.

[Objective] Since the Paleogene, with the uplift of the Qinghai-Tibetan Plateau and the large-scale thrust and strike-slip of the strata, many inland basins were formed in the northern part of the Qinghai-Tibetan Plateau. Be-cause of their particular location, the evolution of paleoclimate and paleo-vegetation in the basins is very significant for clarifying the climatic evolution in the Paleogene in Eurasia. [Methods] In this study, the evolution history of paleoclimate and paleo-vegetation in the Nangqian Basin in central and eastern Qinghai-Tibetan Plateau (51.8-37.5 Ma) was reconstructed from biomarkers and total organic carbon (TOC). [Results and Conclusions] The evolution history of Paleogene climate index in Nangqian Basin is in three stages, each with different changes of sedimentary facies. Stage I (51.8-46.4 Ma) : The sedimentary facies are mainly fluvial and short-term lacustrine, with lower CPI and TOC values and higher Paq values. ACL and nC₂₇/nC₃, show an obvious change trend in this period, which may have been affected by the mixing of terrestrial sediments brought by rivers. Most of the main peak carbon was nC22 and nC23; the vegetation type was predominantly woody plants, and the climate was relatively humid. Stage II (46.4-42.7 Ma): The Paq value and nC₂₇/nC₃, ratio gradually decreased, ACL gradually increased, the carbon number of the main peak gradually changed from low to medium or high, the vegetation gradually changed from woody to herbaceous plants, and the climate gradually became drought alternating with wet conditions. Stage III (42.7-37.5 Ma) : Paq, nC₂₇/nC₃, and ACL changed little, the carbon number of the main peak increased significantly, n-alkanes with high carbon numbers increased significantly, and drought conditions increased. TOC also significantly increased, reflecting increasing total vegetation productivity, which may have been a response to the warm Middle Eocene Climatic Optimum (MECO). In addition, from comparisons with previous reports of climate change in the study area and adjacent basins, it is considered that the climate change in the Nangqian Basin during 51.8-37.5 Ma was mainly affected by global climate change and the retreat of the Tethys Sea. The uplift of the Qinghai-Tibetan Plateau and its higher elevation greatly influenced water vapor in the Nangqian Basin, whereas monsoons have little influence on water vapor. [ABSTRACT FROM AUTHOR]