Expanded lacustrine sedimentation in arid Inner Asia under Oligocene icehouse

Abstract:

Inner Asia has long been considered as an arid region undergoing continuous aridification in Cenozoic driven by global cooling, Paratethys Sea retreat, and Tibetan Plateau uplift. However, during Oligocene, widespread lacustrine facies dominated the Qaidam Basin in the western Inner Asia, which is in contrast to the fluvial or delta dominated strata in this basin in Miocene and Eocene. The Oligocene lacustrine expansion contrasts with the conventional viewpoint of continuous aridification. To reveal the factors driving lacustrine evolution, we consider the paleoclimate and tectonism. For the paleoclimate, we apply compound-specific isotope analysis of hydrogen (δ²H) to leaf wax n-alkanes in two sections in Qaidam Basin from 51 to 12 Ma. The first-order trend of δ²H is characterized by relatively low values during Oligocene, which is synchronous with the lacustrine expansion. During Oligocene, our δ²H values follow the decreasing trend of published carbonate δ¹⁸O in the westerlies dominated Inner Asia, but contrast with the increasing trend of δ¹⁸O in eastern Inner Asia dominated by East Asian Summer Monsoon. The west-east contrast in isotopic records suggests different climatic responses to the Oligocene icehouse. We interpret with a composite paleoclimate record that the wet climate was dominant in the west but arid climate in the east. For tectonism, previous studies show that during Oligocene, large scale strike-slip (ca. 400 km) occurred in Altyn-Tagh Fault, which closed the westward outlet of drainage in Qaidam Basin and contributed to the lacustrine expansion. In Middle Miocene, uplift of the surrounding mountains of Qaidam Basin, such as Altun Shan and Qilian Shan, created a rain shadow effect and aridification, which contributed to the shrink of Qaidam lake. Our study shows that both climatic and tectonic factors control the lacustrine evolution. Also, our paleoclimate reconstruction reveals a dynamic climate of Inner Asia in Cenozoic instead of continuous aridification.