期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：14
页码：7650-7657
DOI：10.1073/pnas.1917518117
出版社：The National Academy of Sciences of the United States of America
摘要：The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O2 and C O 2 are primarily regulated by their relative partial pressures, p O 2 / p C O 2 . Pyrite oxidation during chemical weathering on land consumes O 2 and generates sulfate that is carried to the ocean by rivers. The Δ′17O 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 Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct tropospheric O 2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17O-enriched sources such as reactive oxygen species. Sulfate Δ′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 Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′17O as a proxy for past p O 2 / p C O 2 should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.
