Isotope evidence for secondary sulfide precipitation along the Marsyandi River, Nepal, Himalayas

- The sulfate source to the Marsyandi River is the anoxic weathering of pyrite.
- Sulfate isotopes require sulfide precipitation downstream from the headwaters.
- Sulfide precipitation likely occurs in soils in the warmer downstream catchments.
- The isotopic composition of river sulfate may be decoupled from the minerals weathered.

We present sulfur and oxygen isotope data from 41 samples of dissolved riverine sulfate from along 65 km of the Marsyandi River in the Northern Himalayas. Coupled sulfur and oxygen isotopic composition of riverine sulfate (δ34SSO4 and δ18OSO4 respectively) yield unique constraints on the source of sulfur to the river system. The headwaters of the Marsyandi River have light δ34SSO4 and δ18OSO4, which requires that the source of sulfate to the river is through the anoxic weathering of pyrite (likely via Fe3+). The δ34SSO4 and δ18OSO4 of sulfate in tributaries to the Marsyandi increase downstream, which could result either from inputs from evaporites or bacterial sulfate reduction with subsequent sulfide precipitation in warmer and wetter catchments; either of these processes could result in heavy δ34SSO4 and δ18OSO4 of the residual river sulfate. Elemental ratios such as Sr/Ca and Ca/SO4 suggest, as previous studies have also concluded, that evaporite weathering is not important in the Marsyandi River. We conclude that the isotope data is most consistent with the onset of bacterial sulfate reduction and secondary sulfide precipitation in the soils in the warmer and wetter downstream catchments. Our results have implications for understanding the source of sulfate to the ocean as well as the redox and acidity budget within rapidly eroding catchments.