Limited role for methane in the mid-Proterozoic greenhouse

Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH₄ to the atmosphere, enhancing the climatic significance of CH₄ early in Earth’s history. Indeed, the assumption of elevated pCH₄ during the Proterozoic underlies most models for both anomalou...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2016-10, Vol.113 (41), p.11447-11452
Main Authors: Olson, Stephanie L., Reinhard, Christopher T., Lyons, Timothy W.
Format: Article
Language:English
Subjects:
Anoxia
Atmosphere
Biosphere
Climate
Climate change
Climate system
Earth
Geological time
Glaciation
Global warming
Greenhouse gases
Greenhouses
Methane
Physical Sciences
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Summary:Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH₄ to the atmosphere, enhancing the climatic significance of CH₄ early in Earth’s history. Indeed, the assumption of elevated pCH₄ during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH₄ levels and attendant radiative forcing. Here, we revisit the role of CH₄ in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH₄ fluxes from the marine biosphere and to examine the capacity of biogenic CH₄ to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH₄ coupled to SO₄2− reduction is a highly effective obstacle to CH₄ accumulation in the atmosphere, possibly limiting atmospheric pCH₄ to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO₂. If recent pO₂ constraints from Cr isotopes are correct, we predict that reduced UV shielding by O₃ should further limit pCH₄ to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH₄ for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH₄ greenhouse.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1608549113