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Proterozoic seawater sulfate scarcity and the evolution of ocean–atmosphere chemistry

Nature Geosciencevolume 12pages375380 (2019) | Download Citation


Oceanic sulfate concentrations are widely thought to have reached millimolar levels during the Proterozoic Eon, 2.5 to 0.54 billion years ago. Yet the magnitude of the increase in seawater sulfate concentrations over the course of the Eon remains largely unquantified. A rise in seawater sulfate concentrations has been inferred from the increased range of marine sulfide δ34S values following the Great Oxidation Event and was induced by two processes: enhanced oxidative weathering of sulfides on land, and the onset of marine sulfur redox cycling. Here we use mass balance and diagenetic reaction-transport models to reconstruct the sulfate concentrations in Proterozoic seawater. We find that sulfate concentrations remained below 400 µM, and were possibly as low as 100 µM, throughout much of the Proterozoic. At these low sulfate concentrations, relatively large sulfate–pyrite sulfur isotope differences cannot be explained by sulfate reduction alone and are only possible through oxidative sediment sulfur cycling. This requires oxygen concentrations of at least 10 µM in shallow Proterozoic seawater, which translates to 1–10% of present atmospheric oxygen concentrations. At these oxygen and sulfate concentrations, the oceans would have been a substantial source of methane to the atmosphere (60–140 Tmol yr−1). This methane would have accumulated to high concentrations (more than 25 ppmv) and supported greenhouse warming during much of the Proterozoic Eon, with notable exceptions during the Palaeoproterozoic and Neoproterozoic eras.

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The authors declare that data supporting the findings of this study are available within this article and its Supplementary Information, and all additional data are available from the corresponding author on request.

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This work was supported in part through an Agouron Institute fellowship to S.A.C. and NSERC discovery grant no. 0487 to S.A.C. This study was also supported by the Danish National Research Foundation (grant no. DNRF53) to D.E.C.

Author information


  1. Large Lakes Observatory, University of Minnesota Duluth, Duluth, MN, USA

    • Mojtaba Fakhraee
    •  & Sergei Katsev
  2. Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada

    • Olivier Hancisse
    •  & Sean A. Crowe
  3. Department of Microbiology and Immunology and Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada

    • Olivier Hancisse
    •  & Sean A. Crowe
  4. Nordic Center for Earth Evolution and Institute of Biology, University of Southern Denmark, Odense, Denmark

    • Donald E. Canfield
  5. Department of Physics and Astronomy, University of Minnesota Duluth, Duluth, MN, USA

    • Sergei Katsev


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M.F., S.K. and S.A.C. designed the research. M.F. and S.K. developed the diagenetic model. S.A.C. and D.E.C. developed the mass balance model with input from O.H. M.F. performed mass balance and diagenetic model simulations and sensitivity analyses. M.F., S.K. and S.A.C. interpreted model results and wrote the paper, with contributions from D.E.C.

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The authors declare no competing interests

Corresponding authors

Correspondence to Mojtaba Fakhraee or Sergei Katsev.

Supplementary Information

  1. Supplementary Information

    Supplementary modelling information, Supplementary Figs. 1–14 and Supplementary Tables 1–7.

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