Active conversion of atmospheric carbon dioxide to methane in peridotites of the Samail Ophiolite, Oman

Abstract

Methane (CH 4) is common in fluids sourced from low-temperature hydrating (serpentinizing) peridotites, but the carbon sources, rates, and mechanisms of CH 4 formation are uncertain. In CH 4 dissolved in groundwaters pumped from four wells of up to 400 m depth in the Samail Ophiolite, Oman, we observed 14 C contents ranging from radiocarbon-dead to 0.3038 ± 0.0015 fraction modern. Chemical and isotopic analyses of groundwaters and hydrocarbon gases align with microbiological data indicating that methanogens inhabiting H 2-rich (> 100μmolL−1), pH > 11 fluids produce the 14 C-rich CH 4. This “young” microbial CH 4 constitutes a portion of the light hydrocarbons dissolved in the subsurface fluids, which also contain a distinct pool of relatively 13C- and 2H-enriched CH 4 and C2+ alkanes that are likely abiotic and older. Our study of groundwaters accessed via wells complements prior studies, which have mostly found 14C-free, gas-phase CH4 from natural seeps in ophiolites and interpreted an abiotic source from unsaturated rocks. Most importantly, our radiocarbon data show that transport and localized conversion of atmospheric CO 2 to CH 4 in peridotites reacting with water at temperatures < 60 ˚C occurs at surprisingly fast rates, within the last 10,000 years.