Implications of present‐day abiogenic methane fluxes for the early Archean atmosphere

During Earth's early history, greenhouse warming by atmospheric methane helped to maintain elevated surface temperatures. Here, we estimate the present‐day abiogenic CH4 flux generated by mineral alteration (serpentinization) at mid‐ocean ridges, volcanic emissions, and geothermal sources; in addition, we assess the impact that abiogenic methane may have had on greenhouse warming during the early prebiotic Archean. Based on estimates of the rate of seafloor spreading and the degree of serpentinization within the oceanic crust, the flux of methane generated by serpentinized lithosphere is calculated to be ∼1.35 Mt CH4 y−1, while volcanic and geothermal sources are estimated to contribute ∼0.1 and ∼0.9 Mt CH4 y−1, respectively. Furthermore, it is shown that if atmospheric CO2 partial pressures were above 0.01 bar, the present‐day level of abiogenic methane production could have been sufficient to maintain above‐freezing surface temperatures during the Archean. The very high temperatures (∼70°C) that have been suggested for the early Archean, however, would have required extremely high methane fluxes or, more likely, greatly elevated atmospheric CO2 levels.