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

Abstract

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 $$0.3038\pm 0.0015$$ fraction modern. Chemical and isotopic analyses of groundwaters and hydrocarbon gases align with microbiological data indicating that methanogens inhabiting H $${}_{2}$$ -rich $$(>100 \mu \text{mol }{\text{L}}^{-1}$$ ), $${\text{pH}}>11$$ 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 $$10\hspace{0.17em}000$$ years.