Enhanced sulfide oxidation by monsoon rainfall in a small typical karstic catchment of Southwest China

Abstract

CO2 is released to the atmosphere via sulfuric acid weathering of carbonate rocks, affecting the flux of CO2 drawdown via silicate weathering. Precisely quantifying the proportion of oxidative weathering of sulfide minerals to riverine sulfate is critical to improving the estimation of the global carbon budge. Sulfur and oxygen isotopic compositions of sulfate and major ion concentrations in river water samples collected monthly at 12 sampling sites from a small typical karstic catchment of Southwest China were analyzed, in order to quantify the relative contribution of sulfide oxidation to riverine sulfate and elucidate the factors affecting its changes. Results show that the sulfate concentration and δ34Ssulfate and δ18Osulfate values at most sampling sites have obvious seasonal changes, with low values in the monsoon season and high values in the non-monsoon season, suggesting that the relative contributions of different sulfur sources to riverine sulfate are seasonally variable throughout of the year. Results from the Beyesian isotope mixing model show that overall, the fraction of sulfide-derived sulfate to riverine sulfate ranges from 44.3 ± 13.7% to 53.3 ± 10.5%, although it has a wide range for different sampling sites. In addition, the relative contribution of sulfide weathering displays a seasonal variation and is higher in the monsoon season, which is mainly attributed to the influence of high rainfall in wet season. This study indicates that an increase in rainfall intensity under climate change may lead to a raised sulfide oxidation rate in the karstic catchment, which is important for assessment of global carbonate and silicate weathering in future.