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

Methane (CH [Formula: see text]) is common in fluids sourced from low-temperature hydrating (serpentinizing) peridotites, but the carbon sources, rates, and mechanisms of CH [Formula: see text] formation are uncertain. In CH [Formula: see text] dissolved in groundwaters pumped from four wells of up to 400 m depth in the Samail Ophiolite, Oman, we observed [Formula: see text] C contents ranging from radiocarbon-dead to [Formula: see text] fraction modern. Chemical and isotopic analyses of groundwaters and hydrocarbon gases align with microbiological data indicating that methanogens inhabiting H [Formula: see text]-rich [Formula: see text]), [Formula: see text] fluids produce the [Formula: see text] C-rich CH [Formula: see text]. This "young" microbial CH [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] to CH [Formula: see text] in peridotites reacting with water at temperatures < 60 ˚C occurs at surprisingly fast rates, within the last [Formula: see text] years.