Global cooling controls Paleogene (54-26 Ma) paleolake hydrological evolution and regional chemical weathering processes in the Qaidam Basin, NE Tibet

Abstract

The thick and continuous lacustrine-fluvial sediments in the Qaidam Basin (paleo-Qaidam Lake), NE Tibetan Plateau, serve as excellent archives to reconstruct the Asian inland climate evolution history and explore the driven factors involving Tibetan Plateau uplift, Paratethys retreat, and global climate change. However, the debated elevation history of the Tibetan Plateau and the overlapping climate effects of the Tibetan Plateau uplift and Paratethys retreat makes it difficult to assess the driving mechanism on regional climate change in a particular period. Fortunately, some recent progress suggests that precisely dated Paratethys transgression/regression cycles appear to have fluctuated over broad regions with low relief in the northern Tibetan Plateau before 26 Ma, when the global climate was characterized by generally continuous cooling, thus offering an opportunity to distinguish between the climatic effects of the Paratethys retreat and those of global cooling.Here, we present detailed mineralogical and geochemical investigations of clay minerals collected from the Hongliugou section, northern Qaidam Basin, to reconstruct paleolake salinity variation and basin weathering history in the Paleogene (~54-26 Ma), aiming to obtain a complete regional climate change record and its controlling factors. The paleolake salinity, indicated by two clay-fraction paleosalimeters, equivalent boron and Couch’s salinity, collectively present a long-term lake salinization, from slightly mesohalobic environment (averaging ~4.4‰) in middle Eocene (42-~34 Ma) to mesohalobic/polysaline environment (averaging ~8.3‰) in late Eocene-early Oligocene (~34-~29 Ma). Regarding regional weathering history, the clay mineral assemblage, the clay-fraction geochemistry index (e.g., CIA and Mg/Al ratios) and monomineral indices involving illite chemistry index, beidellite content and chlorite chemistry index and Mg/Al ratios collectively suggest a long-term decreasing trend in 54-26 Ma. The history of both paleolake hydrological evolution (salinity) and basin weathering matches well with the global climate change as indicated by benthic oxygen isotope. We thus reasonably speculate that the secular trend of regional climate change is primarily controlled by global cooling, which regulates regional climate change by influencing the evaporation capacity in the moisture source of Qaidam Basin and the westerly wind strength. Superimposed on this trend, the Paratethys transgression/regression cycles served as an important factor regulating wet/dry fluctuations in the Asian interior.