High-temporal-resolution of lithium isotopes in Yangtze River headwater: Hydrological control on weathering in high-relief catchments

Abstract

How climate change regulates silicate weathering in tectonically active areas remains clear. To evaluate the roles of temperature and hydrology in continental-scale silicate weathering in high-relief catchments, we applied a high temporal resolution of lithium isotopes in the Yalong River, which drains the high-relief borders of the eastern Tibetan Plateau. The dissolved δ7Li values range from +12.2‰ to +13.7‰ in the non-monsoon season and are higher and significantly vary from +13.5‰ to +19.4‰ in the monsoon season. The negative correlation between dissolved δ7Li and the Li/Na ratio is attributed to the formation of various proportions of δ7Li-low secondary minerals during weathering. From non-monsoon to monsoon season, the weathering intensity decreases with increasing secondary minerals formation and the weathering transforms from a supply limited to a kinetically limited weathering regime, indicated by a negative correlation between dissolved δ7Li value and SWR/D ratio (SWR = silicate weathering rate and D = total denudation rate). No correlations between temperature and dissolved δ7Li values were observed, and SWR suggested that temperature is not the direct control factor of silicate weathering in high-relief areas. The dissolved δ7Li values display positive correlations with discharge, physical erosion rates (PERs), and SWR. This positive correlations was attributed to an increase in the PER which caused the formation of more secondary minerals with increasing discharge. These results indicate the rapid temporal variability of riverine Li isotopes and chemical weathering process in response to changes in hydrology rather than temperature. Combined with the compiled PER, SWR, and Li isotopes at various altitudes, we further suggest that weathering in high-altitude catchments is more sensitive to hydrological changes than weathering in low-altitude catchments. These results highlight the key role of the hydrologic cycle (runoff and discharge) and the geomorphic regime in controlling global silicate weathering.