资源环境科技发展态势分析平台

The mass-independent minor oxygen isotope compositions (Delta'O-17) of atmospheric O-2 and CO2 are primarily regulated by their relative partial pressures, pO(2)/pCO(2). Pyrite oxidation during chemical weathering on land consumes O-2 and generates sulfate that is carried to the ocean by rivers. The Delta'O-17 values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct O-2 incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation-including oxygen sources-in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Delta'O-17 measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Delta'O-17 values imply no direct tropospheric O-2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, O-17-enriched sources such as reactive oxygen species. Sulfate Delta'O-17 decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Delta'O-17 anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Delta'O-17 as a proxy for past pO(2)/pCO(2) should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.