Provenance Control on Chemical Weathering Index of Fluvio-Lacustrine Sediments: Evidence From the Qaidam Basin, NE Tibetan Plateau

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 μm) 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 μm) fraction vary in coordination with provenance shifts, but variations of the 5–20 and 20–63 μm 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.