Global cooling controls Eocene environmental change in the Lunpola Basin, central Tibetan Plateau: Evidence from salinity and weathering reconstructions

Detailed investigations of sedimentary rocks from the Tibetan Plateau interior, which has been shaped by plateau development and climate change, will provide new insights into its uplift history. However, few continuous environmental records with clear climatic significance and precise age controls from the Tibetan Plateau hinterland exist, particularly in the early Cenozoic. This shortage hinders a full understanding of the impact of uplift on regional climate change. In this study, we present detailed mineralogical, micromorphological and geochemical investigations of the middle-upper Eocene (similar to 39.6-35.3 Ma) lacustrine sediments from the welldated Dayu section in the Lunpola Basin, central Tibetan Plateau, to explore the paleoenvironmental evolutionary history of the middle-late Eocene from paleoweathering and paleohydrological perspectives. The results reveal that regional weathering process continuously weakened, as indicated by a long-term decrease in the (smectite + mixed-layer illite-smectite)/(illite + chlorite) ratio. The quantitative reconstruction of salinity variations via clay boron-based paleosalimeters reveals long-term salinization of paleolakes from freshwater-mesosaline conditions (averaging 9.0 psu) at similar to 39.6-37.8 Ma to freshwater-polysaline conditions (averaging 13.4 psu) at similar to 37.8-35.3 Ma. The long-term continuous weakening of the weathering intensity and lake salinization collectively suggests a secular drying trend in the central Tibetan Plateau. This drying was consistent with the climatic evolution of the southeastern and northern Tibetan Plateau, which is attributed primarily to global cooling, with secondary contributions from the increasing distance from moisture sources in the Paratethys and regional tectonics.