Dual-isotopic (δ34S and δ18O) evidence for net CO2 release from the northwestern Himalayan catchments

Abstract

The plausible linkage between Himalaya weathering and Cenozoic cooling often overlooks the CO2 release during sulfide oxidation (S-oxidation), which (unlike silicate weathering) has a positive climatic effect. In this study, dissolved sulfur (δ34SSO4) and oxygen (δ18OSO4) isotopic compositions for the Indus headwater basin have been investigated to evaluate the spatial distribution and rate of S-oxidation within the basin. The δ34SSO4 of these samples varies between −11 and 5‰ (n = 50), with lower values being observed for the mainstream samples (−4 ± 2‰; n = 12). Inverse modelling of major ions and δ34SSO4 data estimates the fraction of silicate-derived cations (0.12 ± 0.09) and sulfide-derived sulfates (fpyr; 0.68 ± 0.28). The fraction of molecular O2 (fO2) involved in oxidation exhibits a negative correlation with the δ18OSO4/δ18OH2O ratio for a given fpyr value. Calculations involving δ18OSO4 and fpyr values show significant fO2 variations (<0.83), with higher values being observed for carbonate-dominated sub-basins. The CO2 uptake (1.4 × 105 mol/km2/yr) and release (4.4 × 105 mol/km2/yr) rates for the basin exceed the global riverine averages, and indicate a net CO2 release from the basin. The silicate weathering in the basin is mainly controlled by temperature, whereas intense S-oxidation is consistent with the basin lithology and extensive glacial cover. Modelling of present and compiled datasets for the entire basin confirms net CO2 release from the upper Indus basin and CO2 consumption in the floodplain regions. This underscores the dominant role of mountain erosion in driving pyrite oxidation in river basins, and hence, in the global carbon cycle.