Weathering in landslide-prone areas: Evidences from hydrochemistry and sulfur isotopic compositions in the Bailong River, northeastern Qinghai-Tibet Plateau

Abstract

Investigating weathering processes in landslide-prone areas is important for understanding the carbon budget due to weathering during mountain building. Here, we present the monthly hydrochemistry and sulfur isotopic compositions of SO42− (δ34S–SO42−) in the Bailong River which drained landslide-prone areas in the northeastern Qinghai-Tibet Plateau. Ca2+ and HCO3− were dominant in cations and anions respectively, and δ34S–SO42− values varied over a narrow range, from 5.1‰ to 6.7‰. We constrained the sources of major cations and SO42− through an inverse model and the mass balance model of δ34S–SO42−. The results showed that cations in the Bailong River were mainly from carbonate weathering, and sulfide oxidation was an important source of SO42−. Weathering in the Bailong River catchment showed an CO2 source over geologic timescales due to the reaction of H2SO4. The relationship between weathering fluxes and physical erosion rate indicated that high runoff facilitated physical erosion, and in turn accelerated chemical weathering. However, under the conditions of extreme discharge and physical erosion, SO42− fluxes from sulfide oxidation were not as high as expected, which was ascribed to the transport of old landslide deposits where most sulfides had been exhausted. Therefore, in tectonically active areas with massive landslides, the high erosion rate caused by landslides promoted chemical weathering, but sulfide oxidation did not respond sensitively to strong physical erosion under high runoff conditions. This study helped to understand the relationship among landslides, chemical weathering, and geologic carbon cycle.