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 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.

中文翻译：

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贝得石作为单矿物风化指数揭示青藏高原东北部柴达木盆地始新世气候变化

青藏高原新生代盆地沉积的共同特点是沉积相变化频繁、岩性粗大，这在很大程度上限制了我们对青藏高原隆升与其环境影响之间关系的认识。由这些沉积物得出的单矿物指数可以有效消除粒度和沉积相变化的影响。在这里，我们对柴达木盆地红柳沟剖面采集的蒙脱石进行了详细的矿物学、光谱和微形态研究，以探讨气候变化与始新世早中期青藏高原隆起之间的关系。 X射线衍射、透射电镜、铝核磁共振波谱、傅里叶变换红外光谱等多种方法综合表明，红柳沟剖面的蒙脱石为蒙脱石和贝得石的混合物。贝得石的起源很可能是伊利石的风化产物，伊利石-贝得石混合层（伊利石向贝得石蚀变的中间产物）的存在表明了这一点。贝得石/蒙皂石比率的长期下降趋势从始新世早期气候最佳值（∼52.0–49.5 Ma）的最高值（∼1.1–82.0%，平均14.8%）到49.5–43.9 Ma时的总体<2%表明长期气候干燥。这段由贝得石含量变化推导出来的气候变化历史与其他气候记录一致，支持全球气候变化是调节区域气候变化的驱动因素。因此，我们的研究表明，蒙脱石的单矿物指数可以作为重建区域气候变化的有效方法，特别是在复杂的沉积环境中。