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The Qaidam Basin in the northeastern part of the Tibetan Plateau preserves a continuous and thick Cenozoic sedimentary succession, which is of great significance to studying the aridification of inland Asia. Based on the normalized concentrations (element/Al) and chloride content records of the lacustrine sediments in deep drilling well SS9 in the western Qaidam Basin, the change in the aridity of inland Asia during the late Middle Eocene was investigated. The results show that the Al-normalized concentrations of Na and Cl, which are indicator elements of saline lake sediments, increased sharply at ~38 Ma. The peaks in the Ga/Al and chloride values indicate that the salinity of the lake was extremely high at ~38 Ma. Based on these results, the sudden decrease in the sedimentation rate, the chemical weathering index, the hematite content, and the (smectite + illite/smectite)/illite ratios, the Qaidam Basin experienced abrupt aridification during this period. The decrease in water vapor transport caused by the final retreat of the Paratethys Sea from the southern Tarim Basin may have been the main reason for the changes in the arid environment in the Qaidam Basin during the late Middle Eocene. Long-term global cooling and the uplift of the Altyn Tagh may have played a role in the background conditions. • Normalized concentrations were introduced as indicators of climatic evolution. • A continental record of abrupt aridification that occurred at ~38 Ma was provided. • Final retreat of the Paratethys Sea in the southern Tarim Basin was the main factor. [ABSTRACT FROM AUTHOR]