Constraining sulfur cycling in the Eastern Tibetan Plateau: Evidence for cryptic sulfur cycling and implications for the weathering budget

Abstract

The production of sulfuric acid (H2SO4) through the oxidation of reduced sulfur removes alkalinity from the ocean–atmosphere system and increases atmospheric carbon dioxide concentration (pCO2) over geologic timescales. In practice, quantifying CO2 changes due to H2SO4-driven weathering requires deciphering the sources of sulfate (SO42−) in river water. However, river SO42− concentrations ([SO42−]) or SO42− sulfur and oxygen isotopic ratios (δ34SSO4 and δ18OSO4) can potentially be modified after the initial weathering reactions, biasing the inversion calculations that underlie quantification for the impact of chemical weathering on pCO2. Here, we identify such a non-conservative behavior with a new dataset of δ34SSO4, δ18OSO4 in the Jinsha River and Yalong River draining the Eastern Tibetan Plateau is best explained by cryptic sulfur cycling in the catchments. As a result, measurements in δ18OSO4 do not necessarily provide a simple tool for inferring SO42− sources, especially in the dry season. The partition of major dissolved ions concentrations between their different sources by inversion suggests that the discharged-weighted mean [SO42−]sulfide oxidation/[SO42−] ratio is 0.47 and 0.78, corresponding to a yield for the oxidation of sulfide of 4.55 × 104 and 6.05 × 104 mol/km2/yr, for the Jinsha River and the Yalong River, respectively. The fraction of cations from carbonate weathering and the fraction of acid from sulfide oxidation obtained from river inversion show that chemical weathering for most samples is a CO2 sink on short-term timescales but CO2 source on long-term timescales. The year-long survey shows that sulfide weathering counteracts and surpasses all atmospheric CO2 consumption by silicate weathering for the Yalong River and the Jinsha River, respectively. We attribute the enhanced role of H2SO4-driven weathering in high-elevation areas to both the erosion-induced sulfide oxidation and the limited H2CO3-driven weathering. The complex effect of mountain building on CO2 consumption and release is also likely to be strongly responsive to the occurrence of sulfide-bearing lithology. This work confirms that mountain building has an important role in sulfide weathering, which has great implications on understanding the role of orogenic weathering on atmospheric pCO2.