Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 109, Issue 45, Pages 18300-18305Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1120387109
Keywords
Lomagundi excursion; Great Oxidation Event; Precambrian
Categories
Funding
- National Science Foundation Graduate Research Fellowship program
- American Philosophical Society
- National Science Foundation Division of Earth Sciences
- National Aeronautics and Space Administration Exobiology Program
- Natural Sciences and Engineering Research Council of Canada
- National Research Foundation of South Africa
- Directorate For Geosciences [0951998] Funding Source: National Science Foundation
- Division Of Earth Sciences [0951998] Funding Source: National Science Foundation
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Carbonates from approximately 2.3-2.1 billion years ago show markedly positive delta C-13 values commonly reaching and sometimes exceeding +10 parts per thousand. Traditional interpretation of these positive d(13)C values favors greatly enhanced organic carbon burial on a global scale, although other researchers have invoked widespread methanogenesis within the sediments. To resolve between these competing models and, more generally, among the mechanisms behind Earth's most dramatic carbon isotope event, we obtained coupled stable isotope data for carbonate carbon and carbonate-associated sulfate (CAS). CAS from the Lomagundi interval shows a narrow range of delta S-34 values and concentrations much like those of Phanerozoic and modern marine carbonate rocks. The delta S-34 values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition. These observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone. Toward the end of the carbon isotope excursion there is an increase in the delta S-34 values of CAS. We propose that these trends in C and S isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion.
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