Triple oxygen isotope insight into terrestrial pyrite oxidation

Hemingway, Jordon D. and Olson, Haley and Turchyn, Alexandra V. and Tipper, Edward T. and Bickle, Mike J. and Johnston, David T. (2020) Triple oxygen isotope insight into terrestrial pyrite oxidation. Proceedings of the National Academy of Sciences, 117 (14). pp. 7650-7657. ISSN 0027-8424 DOI https://doi.org/10.1073/pnas.1917518117

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Abstract

The mass-independent minor oxygen isotope compositions (Δ′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 Δ′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 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 O2 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 pO2/pCO2 should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.

Item Type:	Article
Uncontrolled Keywords:	2020AREP; IA76
Subjects:	01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions:	01 - Climate Change and Earth-Ocean Atmosphere Systems 08 - Green Open Access
Journal or Publication Title:	Proceedings of the National Academy of Sciences
Volume:	117
Page Range:	pp. 7650-7657
Identification Number:	https://doi.org/10.1073/pnas.1917518117
Depositing User:	Sarah Humbert
Date Deposited:	15 Apr 2020 23:43
Last Modified:	25 Sep 2020 00:01
URI:	http://eprints.esc.cam.ac.uk/id/eprint/4678

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