Pyrite oxidation in the Brahmaputra river basin: A δ34SSO4-δ18OSO4 study

Dissolved δ³⁴S_SO4 and δ¹⁸O_SO4 data for the Brahmaputra mainstream were investigated for monsoon (September-October, 2022) and non-monsoon (February-March, 2022) periods. These data were used to evaluate the effect of sulfide oxidation­­, a dominant source of atmospheric CO₂, in this large Himalayan river basin on the global carbon cycle. Sulfate concentrations of the mainstream exhibit strong spatial variations, with relative higher values observed near the Eastern Syntaxis (326 ± 41 μM) compared to the lower reaches (184 ± 46 μM). Average SO₄ concentrations for the monsoon (227 ± 103 μM) and non-monsoon (244 ± 55 μM) periods do not show significant seasonal variations. However, the δ³⁴S_SO4 of the monsoon samples (2.1 - 5.3 ‰) are systematically lower than those for the non-monsoon samples (4.3 - 7.1 ‰), indicating enhanced sulfide oxidation during the high flow stages. Similarly, the δ¹⁸O_SO4 of the monsoon samples (-3.0 - -9.8 ‰) are more depleted than those of the non-monsoon samples (0.1 - 5.9 ‰). Enriched δ¹⁸O_SO4 values for lean-flow period may reflect seasonal changes in δ¹⁸O values of the reactive fluids, and/or relative contribution of (i) oxygen to the sulfate, and (ii) sulfate supplied through pyrite and gypsum dissolution. Source-apportionment modeling confirms that the river cations at the Guwahati supplied mainly by carbonates (~62% in monsoon; ~55% in non-monsoon) and silicates (~34% in monsoon; ~36% in non-monsoon). A Monte-Carlo modeling of the δ¹⁸O_SO4 isotopic balance equations indicates that sulfide-derived sulfate at Guwahati for monsoon (f_py ~60%) is about twice that during the non-monsoon (f_py ~30%) periods. Higher sulfide oxidation during the monsoon is likely linked to greater oxygen availability at the subsurface pyrite weathering front and increased interaction with reactive fluids. Additionally, monsoon samples were influenced by a flood event, which may have further enhanced oxidation rates in the basin.