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

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 (>100 μ mol L⁻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 13 C- and 2 H-enriched CH ₄ and C 2+ alkanes that are likely abiotic and older. Our study of groundwaters accessed via wells complements prior studies, which have mostly found 14 C-free, gas-phase CH 4 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 100.17em000 years.