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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, 27Al 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 (∼1.1–82.0%, averaging 14.8%) in the Early Eocene Climate Optimum (∼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. • Both beidellite and montmorillonite were identified in the Qaidam Basin. • A continuous drying trend is shown by the beidellite content at ∼53–43.9 Ma. • Global climate change mainly regulated regional climate change during ∼53–43.9 Ma. • Smectite monomineral index reveals climate change in complex sedimentary settings. [ABSTRACT FROM AUTHOR]

The tectonically active NE Tibetan Plateau, which is located at the confluent area of the humid Asian monsoon region and dry Asian inlands, provides excellent archives of drainage pattern evolution in response to regional changes in topography and climate. However, the uplift of the NE Tibetan Plateau and formation of modern drainage patterns are still disputed. In this study, based on the Sr Nd isotopic distribution in modern surface water and sediments, we present integrated records of 87Sr/86Sr ratios in carbonate and 87Sr/86Sr-ε Nd (0) values in silicate of the Huaitoutala section in the northern Qaidam Basin to reveal the source-to-sink history of particulate and dissolved loads since ∼15 Ma. Our results show that changes in Sr Nd isotopes at ca. 10 Ma and ca. 5 Ma are indicative of two stages of source alteration and reorganization of the drainage system. Specifically, a remarkable increase in 87Sr/86Sr ratios in carbonate and silicate together with a decrease in ε Nd (0) values since ca. 10 Ma suggests a tectonic denudation event of Qilian Shan at ∼10 Ma. The subsequent declines in 87Sr/86Sr ratios in carbonate and silicate together with an increase in ε Nd (0) values since ca. 5 Ma, which approach the modern 87Sr/86Sr ratios near the study section site, suggest that a modern-like drainage pattern in the northern Qaidam Basin might have formed in response to tectonic uplift of Qilian Shan at ca. 5 Ma. Using coupled carbonate Sr-silicate Sr Nd isotopes, we provide reliable provenance tracers of drainage reorganization and the evolution of source-to-sink systems caused by tectonic deformation. • Carbonate and silicate Sr-Nd isotopic records since ∼15 Ma in the North Qaidam Basin. • Sr-Nd isotopic records show a three-phase evolution with shifts at ∼10 Ma and ∼ 5 Ma. • Particulate and dissolved loads with distinct Sr-Nd isotope changes synchronously. • Drainage reorganization caused by uplift and denudation of the Qilian Shan. • Modern-like drainage patterns in the North Qaidam Basin may have formed after ∼5 Ma. [ABSTRACT FROM AUTHOR]