| The investigation of the Eocene-Oligocene aridification record and detailed climate evolution in the Asian inland region is of significant importance for understanding the tectonic uplift of the northeastern margin of the Tibetan Plateau and global climate change.Previous studies have progressively identified aeolian deposits in the northwestern region of China.However,there is currently no explicit report on the presence of aeolian deposits in lacustrine sediments.The Wuwei Basin is located on the northeastern margin of the Tibetan Plateau.In the southern part of the basin,a relatively complete and continuous sequence of Paleogene-Neogene lacustrine deposits has been preserved,providing important materials for understanding the tectonic uplift processes and climate evolution in the northeastern part of the Tibetan Plateau.In this study,we focus on the Tiantishan section,which is constrained by magnetostratigraphic dating.By employing climate proxy indicators such as grain size,chromaticity,magnetic susceptibility,Ca CO3 content,Cl-and SO42-concentrations,as well as scanning electron microscope（SEM）analysis of quartz particles,the climatic and environmental information recorded in the section and the mechanisms of its changes were investigated.The main results and findings obtained are as follows:1.We established high-resolution climate change sequences for sediment grain size,chromaticity,magnetic susceptibility,and Ca CO3 content in the Huangyanghe Formation（40.1～36 Ma）and Xianshuihe Formation（27.8～26.4 Ma）sections,as well as the Cl-and SO42-concentrations in the upper part of the Huangyanghe Formation（37.8～36 Ma）.The ranges of median size（Md）,redness（a*）,low-frequency mass susceptibility（χ）,and Ca CO3 content were 3.82～366.89μm,1.58～14.33,1.16～25.12×10-8 m3/kg,and 0.33～57.24%,respectively.The Cl-and SO42-concentrations ranged from 0.01 to 1.81 mg/g and from 0.02 to 7.54 mg/g,respectively.2.The results of end-member analysis of grain size indicate that the sediment in the section consists of three end-member（EM）components.EM1（grain size range:0.89～56.37μm）represents clay particles transported by long-distance suspension by high-altitude winds,as well as fine sand components transported by regional wind processes.This component is most abundant in the lower part of the Xianshuihe Formation,ranging from 311 to 427.1 m（27.8～27 Ma）,followed by the upper part of the Huangyanghe Formation,ranging from 148.5 to 272.5 m（37.8～36 Ma）.EM2（grain size range:44.77～355.66μm）represents fine to medium particles transported into the lake by surface runoff.EM3（grain size range:89.34～796.21μm）represents coarse particles that have been reworked by flood events or re-deposited due to secondary disturbances in the lake environment.3.The aeolian component,i.e.,EM1 submember,was generally found in this section.SEM showed that quartz particles present typical aeolian sedimentary features,including butterfly-shaped impact craters,round pitting,and elongated streamlined ridges.Those characteristics indicated that the southwestern part of the Wuwei Basin had already undergone aridification as early as 40.1 Ma.4.The climate environment recorded in the section exhibits two states:a relatively cold and dry condition in the lower part of the Huangyanghe Formation（40.1～36 Ma）and a cold and dry condition in the upper part of the Xianshuihe Formation（27.8～26.4Ma）.The former is primarily influenced by global cooling and the tectonic uplift of the northeastern margin of the Qilian Shan during the Eocene,with global cooling playing a major role.The latter is primarily influenced by the tectonic uplift of the northeastern margin of the Qilian Shan.Those two climate states underwent four stages of evolution:a relatively cold-dry climate from 40.1 to 38.6 Ma,a relatively cold-humid climate from37.8 to 36 Ma,a relatively dry-hot climate from 27.8 to 27 Ma,and a relatively cold-dry climate from 27 to 26.4 Ma. |
