Moisture variability in northeast Tibet following the middle Miocene Climate transition

After the middle Miocene Climate transition, global cooling caused substantial hydrological changes in East Asia. Today, many lines of evidence indicate that inland East Asia experienced persistent aridification after the middle Miocene Climate transition. However, chemical weathering records from the South China Sea as well as late-Miocene paleolake expansions in northeastern Tibet contrast with the idea of long-term aridification after the middle Miocene Climate transition. Here, we present manganese-to-iron ratios and clay mineral contents of sediments formed approximately 14 to 10 million years ago from the Guide Basin, northeastern Tibetan Plateau, combined with previously published records (lithology, magnetic susceptibility, redness, and strontium content). Our results show that a paleolake in the Guide Basin expanded between approximately 11.4 and 10.5 million years ago, opposite to the long-term drying trend after the middle Miocene Climate transition. On the 100-kilo-year eccentricity scale, our results demonstrate alternating moisture source variability in northeast Tibet, with precipitation predominantly delivered via the westerlies and the East Asian summer monsoon. This variability could potentially be related to the movement of the westerly jet over the Tibetan Plateau. After comparison to climate model simulations, we attribute this movement to astronomical forcing and the waxing and waning of the Antarctic ice sheet. A rise in lake levels occurred after the mid-Miocene climate transition, challenging the notion of persistent long-term aridity and indicating a significant influence of westerly winds and East Asian summer monsoons, according to paleorecords and climate models from the northeastern Tibetan Plateau.