Pyrite oxidation flux underestimated due to landscape-mediated microbial sulfate reduction

Oxidative weathering of pyrite consumes O ₂ , generates dissolved sulfate (SO ₄ ²⁻ ) and acidifies carbonates to release CO ₂ to the ocean-atmosphere system. Mountain belts are hotspots of chemical weathering and geological carbon cycling, but understanding the net role of orogenesis in modulating the evolution of atmospheric CO ₂ and O ₂ levels requires robust constraints on weathering pathways such as pyrite oxidation. Sulfur and water-normalized oxygen isotopic compositions of riverine SO ₄ ²⁻ (δ ³⁴ S _{SO4 ²⁻} and Δ ¹⁸ O) are potentially powerful tracers to quantify pyrite oxidation, but they may be overprinted by microbial sulfate reduction (MSR)—a process often overlooked in terrestrial environments. To address this, we generated paired geomorphological, geochemical, and microbiological data from the headwaters of the Ganga River in the Himalayas. Our results indicate that MSR overprinting is maximized when low erosion rates are coupled to relatively low precipitation rates, revealing a landscape-mediated hydrological control on microbial terrestrial sulfur cycling. By applying this insight to global compilations of riverine δ ³⁴ S _{SO4 ²⁻} , Δ ¹⁸ O and major ion chemistry, we show that MSR overprinting leads to systematic shift in riverine sulfate isotopic signals, implying that global CO ₂ emissions and O ₂ consumption fluxes from pyrite weathering may be up to double previous isotope-based estimates.