Miocene 87Sr/86Sr ratios of ostracods in the northern Qaidam Basin, NE Tibetan Plateau, and links with regional provenance, weathering and eolian input

Abstract

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.