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The mass-independent minor oxygen isotope compositions (△' 17O) of atmospheric O2 and CO2 are primarily regulated by their relative partial pressures, pO2/pCO2. Pyrite oxidation during chemical weathering on land consumes O2 and generates sulfate that is carried to the ocean by rivers. The △'17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct O2 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 △'17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting △'17O values imply no direct tropospheric O2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17Oenriched sources such as reactive oxygen species. Sulfate △'17O 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 △'17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate △'17O as a proxy for past pO2/pCO2 should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling. [ABSTRACT FROM AUTHOR]