Limited role for methane in the mid-Proterozoic greenhouse

Abstract

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 CH4early in Earth's history. Indeed, the assumption of elevated pCH4during 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 CH4levels and attendant radiative forcing. Here, we revisit the role of CH4in Earth's climate system during Proterozoic time. We use an Earth system model to quantify CH4fluxes from the marine biosphere and to examine the capacity of biogenic CH4to compensate for the faint young Sun during the "boring billion" years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4coupled to SO42∼ reduction is a highly effective obstacle to CH4accumulation in the atmosphere, possibly limiting atmospheric pCH4to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO22. If recent pO22constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3should further limit pCH4to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4for 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 CH4greenhouse.