Sourcing Marine Sulfate: Isotope evidence for secondary sulfide precipitation in the Marsyandi River, Nepal, Himalaya

The weathering of sulfur bearing minerals forms the largest source of sulfate to the ocean. The nature of the minerals being weathered, either sulfate minerals (e.g. gypsum), or sulfide minerals (e.g. oxidative weathering of pyrite) impacts the flux of sulfate to the ocean, the isotope composition of this flux, and is one of the long-term controls on atmospheric oxygen. Sulfur and oxygen isotopes in riverine sulfate are a powerful tool for tracing the sulfur minerals being weathered. This is because evaporite minerals are typically more enriched in the heavy sulfur and oxygen isotopes, while pyrite is much more enriched in the light sulfur isotope and has a range of oxygen isotopes depending on the oxidative pathway. We measured the sulfur and oxygen isotopes in sulfate in the Marsyandi River (Nepal), a Himalayan tributary to the Ganges River. Sulfur and oxygen isotopes in sulfate are isotopically light in the headwaters (-20 to -25% δ³⁴S and -10 to -13% δ¹⁸O)), suggesting that oxidative weathering of pyrite through iron reduction is the dominant source of sulfate to the Marsyandi. However, progressing downstream the sulfur and oxygen isotope composition in sulfate both increase, reaching +11 to +15% for δ³⁴S and +8 to +10% for δ¹⁸O), although the dominant lithology in the catchment does not change. This increase could be due to weathering of evaporites from further away or isotopic enrichment due to microbially mediated sulfate reduction. The lack of coeval changes in the calcium and chlorine concentration rule out evaporite weathering, and we conclude that the isotope shift downstream is indicative of sulfate reduction and secondary sulfide precipitation. These results have implications for the isotope composition of riverine sulfate as well as our understanding of the functioning of the deep terrestrial biosphere.