Expanded lacustrine sedimentation in the Qaidam Basin on the northern Tibetan Plateau: Manifestation of climatic wetting during the Oligocene icehouse

The Qaidam Basin in the core area of arid Inner Asia has been considered undergoing continuous aridification over the Cenozoic. However, the Qaidam Basin is marked with expanded lacustrine sedimentation during the Oligocene, which contrasts with the fluvial or deltaic facies stratigraphically below (Eocene) and above (Miocene-present). The Oligocene lacustrine expansion challenges the idea of persistent aridification. To solve the conundrum, we reconstruct a long-term compound-specific hydrogen isotope (delta H-2) record from sedimentary leaf wax n-alkanes to evaluate the paleoclimatic context before, during, and after the Oligocene lacustrine expansion. The delta H-2 results reveal three shifts at ca. 40 Ma, 34 Ma, and 24 Ma. The leaf wax delta H-2 values range from -176.8 parts per thousand to -166.7 parts per thousand from 51 to 40 Ma, followed by an abrupt increase of 23.9 parts per thousand at 40 Ma. We interpret this rapid increase as enhanced aridification due to the coeval retreat of the Paratethys Sea from the region. At 34 Ma, the delta H-2 plunges across the Eocene-Oligocene transition (EOT). Post-EOT delta H-2 values are the lowest, vary with high amplitude from -187.1 parts per thousand to -153.2 parts per thousand, and are associated with the lacustrine facies expansion, indicating a wetter climate. By compiling the regional isotopic proxy studies, we observe the contrasting patterns in paleohydrology conditions since the EOT: the relaxation of aridity in the westerlies region versus the enhanced aridification in the East Asian summer monsoon region. We interpret that the west-east contrasting patterns represent the different climatic responses to global cooling: wetting in the west as a result of the enhanced moisture transport via westerlies replacing the subtropical high, and drying in the east due to the reduction in moisture content associated with weakening East Asian summer monsoon. Wetting in Inner Asia is synchronous with cooling in the ocean (North Atlantic) and on land (Xining Basin). Since 24 Ma, delta H-2 increases in response to warming during the latest Oligocene to the early Miocene when the subtropical high re-occupied Inner Asia, causing the aridity. This study reveals a dynamic climate in Inner Asia with different mechanisms responding to global change. (C) 2021 Elsevier B.V. All rights reserved.