U-Li Isotopes highlight the dominant role of glacier coverage in weathering processes across Northeast Tibetan Plateau glacial watersheds

Abstract

This study investigates uranium (U) and lithium (Li) isotope fractionation in suspended particulate matter (SPM) from glacial basins in the northeastern Tibetan Plateau, aiming to explore the coupling between physical and chemical weathering processes under glacial influence. The (234U/238U) activity ratio is correlated with parameters related to glacial activity, such as the elevation of the basin and glacier area. Samples from glacial watersheds are enriched in 234U (mean 1.28 ± 0.11), indicating the efficient comminution of rocks. Conversely, non-glacial areas show lower (234U/238U) ratios due to weakened physical weathering (mean 1.09 ± 0.04). The Li isotopes are sensitive to chemical weathering, providing complimentary information to U isotopes. In glacial watersheds, the variation of δ7LiSP is minimal and compatible with the signature of unaltered bedrock (mean 1.58 ± 0.87 ‰). Samples from lower basins show a stronger fractionation (mean − 2.61 ± 1.26 ‰), pointing to a stronger chemical weathering. As for U, Li fractionation is also related to topographic variables influencing the degree of glacial activity. Where glaciers are active, physical weathering fractionates U isotopes but no Li isotopes. Where glaciers are not present, physical weathering is less important, while chemical alteration is more relevant, leading to a low fractionation of U isotopes and a strong fractionation of Li isotopes. The combined use of U and Li isotopes offers an effective tool for tracing weathering regimes shaped by geomorphic and climatic factors, highlighting the role of glaciers in silicate weathering and landscape evolution, and demonstrating the proxies' potential for reconstructing recent glacial weathering