Eocene Climatic And Chemical Weathering Records In Nangqian Basin,Eastern Tibetan Plateau

Since the Cenozoic,the most significant change of global climate is the periodic sharp decline of global temperature.Under the background of global cooling,Asia has also experienced a series of important environmental changes and regional tectonic events,among which the formation and evolution of Asian monsoon and the aridity of the Asian interior are closely related to the uplift of the Tibetan Plateau.Continental weathering denudation and tectonic events and climate environment has close relation,the uplift of Tibetan Plateau makes a lot of fresh bedrock exposed to accelerate the process of silicate rock erosion and weathering,and led to the reduction of atmospheric CO₂,which is considered as an important factor of the Cenozoic global cooling.However,due to the lack of weathering records of silicate rocks in the internal plateau,this hypothesis has been debated so far.More continuous weathering and denudation records from key regions of the Tibetan Plateau are needed to understand the interaction between uplift of the Tibetan Plateau and climate change and weathering denudation.The Nangqian Basin of eastern Tibetan is located at the intersection of the eastern Qiangtang landmass and the northern Hengduan Mountains,in the important transition zone from the east-west tectonic belt to the north-south tectonic belt of the Tibetan Plateau,and at the same time at the intersectional area of the Indian monsoon(South Asian monsoon)region and the arctic-alpine region of the Tibetan Plateau,which is an ideal area for the study of the coupling relationship between tectonic uplift,weathering-erosion and climate change.In this paper,based on the study of clay mineral composition and element geochemical characteristics of the clay component(<2?m)of Cenozoic sediment in Nangqian Basin,supplemented by the analysis of sediment chromaticity,magnetic susceptibility and calcium carbonate content,the Eocene chemical weathering history and climate evolution process of the Nangqian Basin of the eastern Tibetan Plateau were restored,and the main controlling factors of chemical weathering intensity change of the study area during the Eocene was discussed.The main research results are as follows:(1)The chemical weathering intensity sequence of silicate rocks during the Eocene in Nangqian Basin of eastern Tibetan Plateau have been established.(2)During the period of 51.8?50 Ma,the climate of Nangqian Basin was relatively humid and hot,and the chemical weathering is strong,which was related to the Early Eocene Climate Optimum(EECO)and the tectonic uplift of the Tibetan Plateau caused by the collision of India and Eurasia plate.From 50 to 45.7 Ma,the climate changed to dry and cold,and the intensity of chemical weathering continued to weaken,mainly due to global cooling.During 45.7?35 Ma,the climate was arid totally and the chemical weathering intensity was relatively weak,of which 42.5?40Ma appeared brief warm-humid of climate and enhanced of chemical weathering,which was related to the Middle Eocene Climatic Optimum(MECO).(3)During the Eocene,the climate in the Nangqian Basin of the Tibetan Plateau was generally relatively dry(controlled by the arid climate of the subtropical zone of the planetary wind system),indicating that the South Asian monsoon did not appear in the area of Nangqian Basin of the Tibetan Plateau before at least 35 Ma.And the change of chemical weathering intensity was controlled by the dual factors of global climate change and the tectonic uplift of the Tibetan Plateau,and the global climate change was the main control factor.This study reveals the evolution process of Eocene paleoclimate and chemical weathering in Nangqian Basin,which provides a basic basis for the study of the relationship between plateau tectonic uplift,weathering denudation and climate change,and has a certain reference value for the study of the formation and evolution of South Asian monsoon.