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The Qaidam Basin, located on the northeastern Tibetan Plateau and containing Cenozoic sediments with a maximum thickness of ~ 12,000 m, is an ideal place to study the phased uplift of the NE Tibetan Plateau and regional climate change. The estimation of the paleosalinity of sedimentary environments not only helps to evaluate the evolution of lakes in this region but offers insights into contemporaneous climate change. We present detailed geochemical and mineralogical investigations from the lacustrine interval of the Hongliugou section in the northern Qaidam Basin to reconstruct salinity fluctuations in the paleolake during the late Eocene era (~ 42.0–35.5 Ma). The clay mineral assemblages mainly contain smectite, illite, chlorite, kaolinite and irregular illite/smectite mixed layers. Clay boron-derived paleosalinity estimates (equivalent boron content, Couch's paleosalimeter and B/Ga ratios) along with other proxies sensitive to salinity changes (e.g., Rb/K ratios and ostracod assemblages) collectively indicate an overall brackish sedimentary environment with a higher-salinity period at approximately 40.0–39.2 Ma. This higher-salinity period indicates a more arid environment and is probably related to global cooling. However, the global cooling in late Eocene cannot explain the overall stable long-term salinity pattern, implying that other factors exist. We propose that the migration of the Yiliping depression depocenter in the northern Qaidam and increased orographic rainfall induced by late Eocene tectonic activity at the northern margin of the basin might have partly offset the increase in salinity driven by global cooling. [ABSTRACT FROM AUTHOR]

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. • A clay boron-derived quantitative salinity record (~42–29 Ma) in the Qaidam Basin • Long-term lake salinization with weakened catchment weathering in ~42–29 Ma • Global cooling was the main driver regulating long-term paleosalinity variation. • The Paratethys transgression/regression cycle regulated short-term wet/dry fluctuation. [ABSTRACT FROM AUTHOR]

The relationship between tectonic uplift, climate change and continental silicate weathering has long been debated because deconstructing the tectonic and climatic impacts on continental silicate weathering is challenging. To address this issue, we present long-term climatic and silicate weathering records since 7.3 Ma on tectonically active NE Tibet retrieved from the SG-1 and SG-1b lake sediment drill cores in the western Qaidam Basin. We find distinct correlations between silicate weathering and regional climate on long-term (>106 years) and short-term (105–106 years) time scales. On long-term scales, silicate weathering intensity exhibits a consistent evolution with regional climate, both of which are dominantly controlled by global climate change. This evolution resulted in a stable weathering intensity during 7.3–~3.6 Ma and a weakening of weathering intensity since ~3.6 Ma, followed by a continuous decreasing trend after 2.6 Ma. However, on short-term scales, silicate weathering intensity displays out-of-phase changes with the regional climate at 6.9–3.7 Ma and 0.2–0.1 Ma and in-phase changes at 7.3–6.9 Ma and 3.7–0.2 Ma. We attribute this long-term consistency between the weathering intensity and climate to aridification in inland Asia, but the short-term inconsistencies at 6.9–3.7 Ma and 0.2–0.1 Ma to the change in the sediment routing system impacted by the regional climate within a tectonically active setting. Such a change in the sediment routing system along with sediment recycling may bias weathering indication on orbital time scales, and our study therefore suggests a more complex response of silicate weathering to tectonic and climatic forcings. [Display omitted] [ABSTRACT FROM AUTHOR]