An Eocene–Miocene Continuous Rock Magnetic Record from the Sediments in the Xining Basin, NW China: Indication for Cenozoic Persistent Drying Driven by Global Cooling and Tibetan Plateau Uplift

messier, william f. jr.;

Tibetan Plateau uplift and global cooling have generally been thought to have caused the drying of the Asian inland, but how and when these factors drove the aridification is unknown. The Xining Basin at the NE Tibetan Plateau received continuous Eocene–Miocene fine-grained sediments, providing an excellent opportunity to address this question. Here we present detailed rock magnetic and diffuse reflectance spectroscopy (DRS) analyses for a well-dated Cenozoic sedimentary record from the Xie jia section in the basin. Magnetic susceptibility (χ), saturation magnetization (Ms) and saturation isothermal remanent magnetization (SIRM) in this section show a long-term decreasing trend from∼52 to∼25 Ma, well coinciding with global cooling and drying in the region, and an increasing trend since ∼25 Ma, which is in contrast to the further progressing aridification of the basin. Thermomagnetic results and DRS-determined hematite contents suggest that the relative content of magnetite and hematite is the main control on the χ, Ms and SIRM values. We argue that the long- term Eocene– Oligocene global cooling increased the drying of the Asian inland, lowering the lake level and exposing a larger area to low temperature oxidation for longer times, thus producing more hematite and leading to the decreasing trend of magnetic concentration parameters from ∼52 to ∼25 Ma. An intensive uplift of the NE Tibetan Plateau since∼25 Ma, associated with a change in the sedimentary source, might be responsible for the increase of χ, Ms and SIRM after 25 Ma.