In:
Proceedings of the National Academy of Sciences, Proceedings of the National Academy of Sciences, Vol. 117, No. 14 ( 2020-04-07), p. 7650-7657
Abstract:
The mass-independent minor oxygen isotope compositions (Δ′ 17 O) of atmospheric O 2 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′ 17 O 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′ 17 O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′ 17 O values imply no direct tropospheric O 2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17 O-enriched sources such as reactive oxygen species. Sulfate Δ′ 17 O 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′ 17 O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′ 17 O 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.
Type of Medium:
Online Resource
ISSN:
0027-8424
,
1091-6490
DOI:
10.1073/pnas.1917518117
Language:
English
Publisher:
Proceedings of the National Academy of Sciences
Publication Date:
2020
detail.hit.zdb_id:
209104-5
detail.hit.zdb_id:
1461794-8
SSG:
11
SSG:
12
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