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

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 (δ³⁴S_SO4 and δ¹⁸O_SO4) 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 δ³⁴S_SO4, δ¹⁸O_SO4 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 δ¹⁸O_SO4 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.