Beidellite as a monomineral index of weathering reveals Eocene climate change in the Qaidam Basin, northeastern Tibetan Plateau

Cenozoic basin sediments on the Tibetan Plateau are commonly characterized by frequent changes in sediment facies and coarse lithologies, which largely limits our understanding of the relationship between the Tibetan Plateau uplift and its environmental impacts. The monomineral index derived from those sediments can effectively eliminate the effects of grain size and sedimentary facies changes. Here, we present detailed mineralogical, spectroscopic, and micromorphological investigations of smectites collected from the Hongliugou section of the Qaidam Basin to explore the relationship between climate change and the Tibetan Plateau uplift in the early-middle Eocene. Multiple methods involving X-ray diffraction, transmission electron microscopy, Al-27 nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy collectively show that the smectites in the Hongliugou section are mixtures of montmorillonites and beidellites. The origin of the beidellites was likely the weathering product of illite, as indicated by the presence of illite-beidellite mixed layers (an intermediate product of the alteration of illite to beidellite). The long-term decreasing trend of beidellite/smectite ratios from the highest values (similar to 1.1-82.0%, averaging 14.8%) in the Early Eocene Climate Optimum (similar to 52.0-49.5 Ma) to generally <2% at 49.5-43.9 Ma suggested a long-term climatic drying. This climate change history deduced from the variation of beidellite content is consistent with other climate records and supports that global climate change was the driving factor regulating regional climate change. Our study thus suggests that the monomineral index of smectite may serve as an effective method for reconstructing regional climate change, especially within complex sedimentary environments.