Early-Middle Eocene Hydroclimate Variations Recorded by Environmental Magnetism in the Linxia Basin, NE Tibetan Plateau

Although Neogene climate change reconstruction in central Asia is important for understanding global cooling, Asian monsoon evolution, and the Tibetan Plateau stepwise uplift during the Cenozoic, the paleoenvironmental evolution during the Paleogene in this region is still ambiguous due to a lack of corresponding continuous and complete terrestrial records with precise age constraints. Recently, a new fluvial sedimentary sequence with magnetostratigraphic ages spanning from similar to 53 to 40 Ma was found in the Linxia Basin, NE Tibetan Plateau, thereby providing a window to understand Eocene climate change in the Asian interior. From this fluvial sedimentary sequence, we present early-middle Eocene hydroclimate variations based on a combined method of rock magnetism and diffuse reflectance spectroscopy. Our results show magnetic enhancement of fluvial sediments at 51.7 Ma resulted from abundant single-domain magnetite inputs from the surrounding mountains through strong erosion and transport in a relatively wet environment. Subsequently, at 47.6 Ma, magnetic weakening of floodplain sediments corresponds to an increase in hematite concentrations produced via low-temperature oxidation in a prolonged dry environment. Comparisons of paleoclimatic proxies, such as organic geochemistry and rock magnetism from neighboring basins, tectonic deformation of the NE Tibetan Plateau, and sea level change in the Paratethys Sea suggest that the hydroclimate variation from relatively wet to dry climate in the early-middle Eocene in this area was mainly controlled by global climatic change and probably superimposed by the uplift of the Tibetan Plateau.