Late Miocene reorganization of lake hydrological systems on the northern Tibetan Plateau linked to topographic growth

Abstract

The hydrological cycles of lakes on the Tibetan Plateau (TP) have had profound impacts on the climatic and environmental evolution of the Asian interior and hence they are crucial for the well-being of the large human population of this region. The late Miocene is regarded as the most recent interval when the TP was extensively occupied by large paleolakes. However, it is unclear when and how these mega paleolakes shrank and the environment evolved into its present domination by desert and semi-desert. To address these issues, we conducted detailed analyses of diffuse reflectance spectroscopy, sedimentology, paleomagnetic-based tectonic rotation, and biogeochemistry of a Middle to Late Miocene sedimentary sequence in the Xining Basin in the North-eastern (NE) TP, supplemented by a spatiotemporal comparison of our results with equivalent records from adjacent regions. The results reveal a clear transition of the lacustrine system in the Xining Basin at ∼ 11 Ma, characterized by decreased water level and an increase in the oxidation state of the depositional environment. Combined with a summary of sedimentary evidence from seven different basins in the NE TP, we propose that a fundamental reorganization of lake hydrological systems occurred in the NE TP at ∼ 11 Ma. This reorganization was synchronous with tectonic deformation and topographic growth in the NE TP, as revealed by our new tectonomagnetic and other available tectonic evidence. Therefore, tectonic activity—rather than changes in climate or solar insolation, as previously proposed—was the principal factor responsible. This hydrological reorganization was of profound significance for the regional hydrological cycle and the climatic and ecological environment of Asia, because the elevated topography prevented the NE TP from regaining its Middle Miocene paleohydrological configuration.