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 CH to the atmosphere, enhancing the climatic significance of CH early in Earth's history. Indeed, the assumption of elevated pCH 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 CH levels and attendant radiative forcing. Here, we revisit the role of CH in Earth's climate system during Proterozoic time. We use an Earth system model to quantify CH fluxes from the marine biosphere and to examine the capacity of biogenic CH 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 CH coupled to SO reduction is a highly effective obstacle to CH accumulation in the atmosphere, possibly limiting atmospheric pCH to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO If recent pO constraints from Cr isotopes are correct, we predict that reduced UV shielding by O should further limit pCH to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH 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 CH greenhouse.