全球变化知识资源中心

Carbon dioxide drawdown resulting from enhanced chemical weathering during orogenesis has been invoked to explain late Cenozoic global cooling. Establishing chemical weathering records from the India-Asia collision zone is important to test this hypothesis because uplift of the Tibetan Plateau is thought to be responsible for Cenozoic cooling. However, proxies for the intensity of chemical weathering can be affected by additional factors, such as sediment grain size and provenance. Here we report major element compositions and calculated chemical weathering intensity records of three size fractions (0-5, 5-20, 20-63 mu m) from the Dahonggou section of the Qaidam Basin in the northeastern Tibetan Plateau, and compare those records with published provenance data from the same section. Results show that the indices of the fine (0-5 mu m) fraction vary in coordination with provenance shifts, but variations of the 5-20 and 20-63 mu m fractions are less affected by provenance variations. Comparison with Upper Continental Crust reveals that some labile elements are not leached but instead are enriched in the fine fraction, indicating that it does not faithfully record chemical weathering intensity. In addition, weathering can result in clay mineral transformation instead of elemental variations, complicating the relationship between element-based parameters and weathering intensity. This work suggests that changes in sediment provenance must be accounted for when inferring variations in chemical weathering intensity on the basis of element-based weathering intensity indices of the clay fraction.

Plain Language Summary Understanding the relationships among climate change, chemical weathering, and uplift of the Tibetan Plateau is significant for predicting future climate change. However, this relationship is unclear due to the lack of accurate nonmarine chemical weathering records. In this contribution, we compare geochemical records of multiple grain-size fractions from Cenozoic strata in the Qaidam Basin with changes in sediment provenance. The weathering indices of the 5-20 and 20-63 mu m fractions show less influence of provenance changes than the finest (0-5 mu m) fraction. We conclude that the silt-sized fraction provides a better proxy to reconstruct weathering history than a finer fraction. We also recommend that weathering studies be accompanied by sediment provenance data to screen for provenance-driven changes.