Anaerobic methane oxidation inducing carbonate precipitation at abiogenic methane seeps in the Tuscan archipelago (Italy)

Seepage of methane (CH ₄) on land and in the sea may significantly affect Earth’s biogeochemical cycles. However processes of CH ₄ generation and consumption, both abiotic and microbial, are not always clear. We provide new geochemical and isotope data to evaluate if a recently discovered CH ₄ seepage from the shallow seafloor close to the Island of Elba (Tuscany) and two small islands nearby are derived from abiogenic or biogenic sources and whether carbonate encrusted vents are the result of microbial or abiotic processes. Emission of gas bubbles (predominantly CH ₄) from unlithified sands was observed at seven spots in an area of 100 m ² at Pomonte (Island of Elba), with a total rate of 234 ml m ^-2 d ^-1. The measured carbon isotope values of CH ₄ of around -18 (VPDB) in combination with the measured δ ²H value of -120 (VSMOW) and the inverse correlation of δ ¹³C-value with carbon number of hydrocarbon gases are characteristic for sites of CH ₄ formation through abiogenic processes, specifically abiogenic formation of CH ₄ via reduction of CO ₂ by H ₂. The H ₂ for methanogenesis likely derives from ophiolitic host rock within the Ligurian accre-tionary prism. The lack of hydrothermal activity allows CH ₄ gas to become decoupled from the stagnant aqueous phase. Hence no hyperalkaline fluid is currently released at the vent sites. Within the seep area a decrease in porewater sulphate concentrations by ca. 5 mmol/l relative to seawater and a concomitant increase in sulphide and dissolved inorganic carbon (DIC) indicate substantial activity of sulphate-dependent anaerobic oxidation of methane (AOM). In absence of any other dissimilatory pathway, the δ ¹³C-values between -17 and -5 in dissolved inorganic carbon and aragonite cements suggest that the inorganic carbon is largely derived from CH ₄. The formation of seep carbonates is thus microbially induced via anaerobic oxidation of abiotic CH ₄