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Abstract As an important geological archive, paleosols have been widely used in reconstruction of paleoenvironmental and paleoclimatic conditions. In this study, we undertook detailed analyses of the clay mineralogy and carbon and oxygen isotopic compositions of pedogenic carbonates in a series of 13 paleosol horizons within a fluvial section of the Eocene (~52–44 Ma) Lulehe Formation in the Qaidam Basin. Pedogenic smectite and illite/smectite (I/S) mixed-layer minerals are the predominant clay minerals in the paleosols, suggesting that the early Eocene paleoclimate of the Qaidam Basin was strongly seasonal with warm-wet summers. The high levels of pedogenic smectite production and high atmospheric p CO 2 reconstructed from soil carbonate nodule δ13C (mean −7.1 ± 0.8‰) are consistent with elevated temperatures during the Early Eocene Climatic Optimum (EECO). Coeval paleosol carbonate nodules yield estimated paleo-surface water compositions (δ18O psw) of −7.9‰ to −6.0‰ (VSMOW), which are significantly higher than the δ18O value of modern summer precipitation in the Qaidam Basin. The estimated temperature difference (~5.0 °C cooling) implies uplift of the study area by nearly 1000 m to its current average elevation of ~3000 m above sea level. This paleoelevation estimate agrees with modern meteoric water isotopes of Global Network of Isotopes in Precipitation (GNIP) sites in Asia in the same elevation range. Highlights • Multiple well-exposed paleosol horizons present in lower Eocene Lulehe Formation, Qaidam Basin, northern Tibetan Plateau. • These paleosols formed at 51.3–49.7 Ma during the Early Eocene Climatic Optimum (EECO). • Abundant pedogenic smectite indicates a strongly seasonal climate with warm-wet summers during early Eocene. • Soil nodule δ13C carb values (mean − 7.1 ± 0.8‰) reflect high atmospheric p CO 2 and elevated paleo-temperatures. • High paleo-precipitation δ18O psw values reflect low paleo-elevations (<2000 m) of Qaidam Basin in the early Eocene. [ABSTRACT FROM AUTHOR]

Late Cenozoic changes in chemical weathering and the dust cycle in response to the regional aridification and global cooling in the tectonically active northern Tibetan Plateau is an intriguing issue, although it remains poorly understood. The change in the 87Sr/86Sr ratio of basin water is linked to variations in the overall input of various Sr sources with different 87Sr/86Sr ratios, and analyses of such changes can be a useful tool for revealing changes in regional lithology, weathering regimes and the dust cycle linked to tectonics and climate. In this study, we collected fossil shells of ostracods and gastropods from the Miocene Dahonggou section to reconstruct the paleolake water 87Sr/86Sr ratio from 14.8 Ma to 11.1 Ma in the northern Qaidam Basin on the northeastern Tibetan Plateau. The reconstructed paleolake water 87Sr/86Sr ratios based on fossil shells and bulk carbonates from sediments display a coincident and stable 87Sr/86Sr ratio of approximately 0.7115, which did not show any detectable change in response to the enhanced aridification since ~13 Ma as inferred by the ostracod assemblage and its stable carbon and oxygen isotopes. The results indicate that the Sr influx regime from various sources did not change significantly during the aridification processes ~13 Ma, which may have been caused by a less detectable change in proportion of silicate versus carbonate weathering, and/or an overwhelming Sr source (e.g., extrabasinal eolian dust) that occurred from 14.8 Ma to 11.1 Ma. Further, the 87Sr/86Sr ratios of modern lake water and river sand carbonates in the study region are significantly higher, approximately ≥0.001, than those of the Miocene lake water. This observation can be explained by two possible mechanisms that need to be tested in the future: 1) given a Miocene Sr input regime that is similar to today, a large contribution from some extrabasinal Sr source (e.g., eolian dust) with a much lower 87Sr/86Sr ratio is expected to have prevailed from 14.8 Ma to 11.1 Ma but is absent in the modern setting or 2) given a Miocene Sr input regime that could yield an overall lower lake water 87Sr/86Sr ratio of approximately 0.7115, a significant increase in the 87Sr/86Sr ratio of catchment-scale Sr source may have occurred after 11 Ma in response to the uplift of the Qilian Mountains, which would have enhanced hydrothermal activity, metamorphism, glaciation and landslide processes, thereby resulting in more-radiogenic Sr source. Our study suggests that reconstructions of river/lake water 87Sr/86Sr ratios can place powerful constraints on the complex interactions among tectonics, climate and landscape evolution in the northeastern Tibetan Plateau based on a new perspective. • 87Sr/86Sr ratios of paleolake water at 14.8–11.1 Ma in the northern Qaidam Basin. • Ostracod shell and bulk carbonates show stable 87Sr/86Sr ratios at ~0.7115. • Paleolake water 87Sr/86Sr ratio is lower than modern lake water value of ~0.7128. • The paleo and modern lake 87Sr/86Sr contrast links to eolian input or Qilian uplift. [ABSTRACT FROM AUTHOR]