Abiogenic methanogenesis during experimental komatiite serpentinization: Implications for the evolution of the early Precambrian atmosphere

Hydrothermal alteration of oceanic ultramafic rocks can produce significant abiotic methane as a byproduct; however, the abundance of peridotites is limited in the modem basaltic oceanic crust. Abiogenic methanogenesis was likely more productive in the early Precambrian when komatiitic volcanism delivered a much larger proportion of ultramafic material to the crust. It is therefore possible that hydrothermal alteration in the Hadean and Archean could have led to a higher methane flux to the atmosphere than today, potentially influencing the average surface temperature of the early Earth. This hypothesis was tested in a series of experiments in which natural and synthetic komatiite powders were reacted with an aqueous solution containing isotopically labeled formic acid at 300 degrees C and 350 bars. Over 2 months, experiments containing the natural powder generated labeled methane ((CH4)-C-13) at a rate 5-8 x faster than concurrent experiments containing the synthetic komatiite. A similar difference was also observed between the natural komatiite and previous experiments on olivine serpentinization at the same conditions. Faster rates of methanogenesis in the natural komatiite experiments could be due to catalysis by magnesiochromite, which was present only in the natural komatiites. However, this was ruled out because parallel experiments containing additional magnesiochromite failed to increase methane yields. The most likely catalysts of the remaining candidate minerals were deduced to be Ni-sulfides, common accessory minerals in komatiites that have previously been shown to accelerate organic reactions. Because Ni-sulfides were particularly abundant in Archean komatiites, the experiments imply that methane yields during seafloor alteration were higher on the early Earth than today. This inference, in combination with the assumptions of faster crustal production rates and greater volumetric proportions of ultramafic lithologies, leads to the conclusion that methane flux from serpentinization of an Archean komatiite-bearing oceanic crust was likely higher than that from metamorphism of the modern oceanic crust. Incorporation of this result into existing geodynamic and atmospheric models suggests that alteration of the Archean seafloor may have contributed to the regulation of temperature of the paleoatmosphere. However, additional sources of greenhouse gases are required to solve the Faint Young Sun paradox for the early Archean. (C) 2012 Elsevier B.V. All rights reserved.