Abstract Climate change in the Asian interior during the early Cenozoic remains poorly constrained due to difficulties in distinguishing the impacts of global cooling, the early uplift of the Tibetan Plateau and the retreat of the Paratethys. A quantitative estimation of paleolake salinity enables a better understanding of the regional hydrological cycle and contemporaneous climate change. Here, we present a quantitative record of paleolake salinity in the Qaidam Basin between ~42 and 29 Ma using detailed mineralogical investigations and clay boron content data. This paleolake salinity record generally covers a tectonically less active period in the Asian interior characterized by continuous global cooling, the abrupt Eocene–Oligocene climate transition and the Paratethys transgression/regression cycles, thus offering an opportunity to explore the roles of global cooling and the Paratethys retreat in regulating the regional hydrological cycle. The results of two boron-derived paleosalimeters, equivalent boron and Couch's salinity, collectively indicate a two-stage paleolake salinity evolution, from an oligohaline–mesohaline environment in the middle–late Eocene (~42–~34 Ma) to a mesosaline environment in the early Oligocene (~34–~29 Ma). This transition is also supported by qualitative chloride-based and ostracod-based paleosalinity estimates in the Qaidam Basin. Our quantitative paleolake reconstruction between ~42 and 29 Ma in the Qaidam Basin yields a generally good match with the hydroclimate change in the neighboring Xining Basin, thus indicating a comparable regional drying trend. The synchronous changes in quantitative paleolake salinity and silicate weathering processes derived from the illite weathering index and chlorite content in the studied section suggest climate control of silicate weathering. Global cooling is speculated to have been the first-order driving factor in regulating long-term climatic evolution and weathering responses in the Asian interior between ~42 and 29 Ma. Superimposed on this trend, the Paratethys transgression/regression cycles served as an important factor regulating wet/dry fluctuations in the Asian interior between ~42 and ~34 Ma.

中文翻译：

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柴达木盆地（青藏高原东北部）古湖盐度在~42~29 Ma之间的演变：与全球变冷和帕特提斯海入侵的联系

摘要 由于难以区分全球变冷、青藏高原早期隆升和副特提斯海退的影响，早期新生代亚洲内陆地区的气候变化仍然缺乏约束。古湖盐度的定量估计有助于更好地了解区域水文循环和同期气候变化。在这里，我们使用详细的矿物学调查和粘土硼含量数据提供了柴达木盆地~42 至 29 Ma 之间古湖泊盐度的定量记录。这种古湖泊盐度记录通常涵盖亚洲内部构造不那么活跃的时期，其特征是全球持续变冷、始新世-渐新世气候突然转变和帕拉提斯海侵/回归循环，从而提供了一个机会来探索全球降温和帕拉提提斯撤退在调节区域水文循环方面的作用。两个硼衍生的古盐度计、等效硼和 Couch 盐度的结果共同表明古湖泊盐度演化为两阶段，从始新世中晚期（~42~~34 Ma）的贫盐-中盐环境到中盐酸盐环境。渐新世早期（~34-~29 Ma）。这种转变也得到柴达木盆地基于氯化物和基于介形虫的古盐度定性估计的支持。我们在柴达木盆地约 42 至 29 Ma 之间的定量古湖泊重建与邻近的西宁盆地的水气候变化总体上很好地匹配，因此表明具有可比的区域干燥趋势。研究断面伊利石风化指数和绿泥石含量引起的定量古湖盐度和硅酸盐风化过程的同步变化表明硅酸盐风化的气候控制。据推测，全球变冷是调节亚洲内陆约 42 至 29 Ma 长期气候演变和风化响应的一级驱动因素。叠加在这种趋势上，Paratethys 海侵/回归循环是调节亚洲内部~42 到~34 Ma 之间干湿波动的重要因素。据推测，全球变冷是调节亚洲内陆约 42 至 29 Ma 长期气候演变和风化响应的一级驱动因素。叠加在这种趋势上，Paratethys 海侵/回归循环是调节亚洲内部~42 到~34 Ma 之间干湿波动的重要因素。据推测，全球变冷是调节亚洲内陆约 42 至 29 Ma 长期气候演变和风化响应的一级驱动因素。叠加在这种趋势上，Paratethys 海侵/回归循环是调节亚洲内部~42 到~34 Ma 之间干湿波动的重要因素。