Multi-isotopic (δ34SSO4, δ13CDIC) evidence for enhanced sulfide oxidation in the central Himalayas: A Spatio-temporal study

Sulfuric acid-mediated carbonate weathering in mountainous regions serves as a dominant CO₂ source, counterbalancing the carbon sequestration via silicate weathering. In this study, we investigated the intensity and controlling factors for sulfide oxidation for a major Himalayan River (the Yamuna River) draining the central Himalaya, using dissolved major ions, and δ³⁴S_SO4 and δ¹³C_DIC data. The water samples examined in this study include spatial collections from the mainstream and its tributaries during monsoon (2022), and biweekly samples collected at the mountain front (at Paonta Sahib, India) of the Yamuna River for a duration of one year (2022-23). The spatial δ¹³C_DIC data vary between -7.4‰ and 4.3‰, which are intermediate between carbonic acid-mediated silicate weathering (-24‰ ± 2‰) and sulfuric acid-mediated carbonate weathering (0‰ ± 2‰).At the spatial scale, the average SO₄^2- concentration (~300 µM) is about six times higher than that of the Ganga (~58 µm) and Brahmaputra (~78 µM) outflows, and about three times higher than the global rivers (~88 µM). The corresponding δ³⁴S_SO4 data vary between 2.3‰ and 25.5‰, with an average value of 13.0 ‰. The δ³⁴S_SO4 values for the mountainous samples are more depleted than those from the floodplains, hinting at intense sulfuric acid-mediated weathering in the mountainous region. The δ³⁴S_SO4 values also exhibit strong seasonal variations, with more depleted δ³⁴S_SO4 signatures (14.1‰ ± 1.0‰) during the monsoon compared to those for the non-monsoon (17.0‰ ± 1.3‰) period. The observed seasonal difference (~3‰) suggests water level and oxygen availability influence the oxidation reactions at subsurface level. Our preliminary observation indicates intense sulfide oxidation in this mountainous catchment, possibly triggered by basin lithology and oxygen availability.