Middle to late Eocene chemical weathering history in the southeastern Tibetan Plateau and its response to global cooling

Abstract

Continental weathering is a crucial factor to maintain the balance of the carbon cycle and thus the livability of the Earth. The Tibetan Plateau (TP), known as the Third Pole of the Earth, has undergone strong uplift since the Cenozoic, thus making it an ideal area to study continental weathering processes and associated driving factors. However, early Cenozoic weathering records are rare in this region. Here, we report high quality mineralogical and geochemical records of clay fraction (< 2 μm) of a middle to late Eocene sequence retrieved from the Qujing Basin, southeastern TP, to track the early Cenozoic chemical weathering history in this area. The results show that the main clay mineral assemblages contain illite-smectite mixed layers, illite and palygorskite. The chemical weathering intensity indicators (e.g., clay mineral assemblages and the chemical index of alteration (CIA)) collectively indicate a clear long-term modulation of global cooling on regional chemical weathering processes during middle to late Eocene, as indicated by high correlation between global temperatures and chemical weathering indicators and the similarity in their periodicity characteristics. Moreover, a sudden decrease of the chemical weathering intensity with large fluctuations was identified at 42–39.9 Ma, which may be linked to rapid transport of clay minerals caused by tectonic activity and/or an increased precipitation in the southeastern TP. This study indicates a control of global cooling on chemical weathering of the southeastern TP area during middle to late Eocene, while tectonic activity or other factors might exert catalytic influences at some certain periods.