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

Hanna Kuhfuß,Johanna Wiedling,Stefano M Bernasconi,Anna Lichtschlag,Patrick Meister,Wolfgang Bach,Gunter Wegener,Michael E Böttcher,Christian Deusner,Christian Lott,Xiaole Sun,Miriam Weber

doi:10.1371/journal.pone.0207305

Abstract

Seepage of methane (CH4) on land and in the sea may significantly affect Earth’s biogeochemical cycles. However processes of CH4 generation and consumption, both abiotic and microbial, are not always clear. We provide new geochemical and isotope data to evaluate if a recently discovered CH4 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 CH4) from unlithified sands was observed at seven spots in an area of 100 m2 at Pomonte (Island of Elba), with a total rate of 234 ml m-2 d-1. The measured carbon isotope values of CH4 of around -18‰ (VPDB) in combination with the measured δ2H value of -120‰ (VSMOW) and the inverse correlation of δ13C-value with carbon number of hydrocarbon gases are characteristic for sites of CH4 formation through abiogenic processes, specifically abiogenic formation of CH4 via reduction of CO2 by H2. The H2 for methanogenesis likely derives from ophiolitic host rock within the Ligurian accretionary prism. The lack of hydrothermal activity allows CH4 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 δ13C-values between -17 and -5‰ in dissolved inorganic carbon and aragonite cements suggest that the inorganic carbon is largely derived from CH4. The formation of seep carbonates is thus microbially induced via anaerobic oxidation of abiotic CH4.

Highlights

Seepage represents an important route of transport for methane (CH4) and other hydrocarbon gases from Earth’s subsurface to the oceans and the atmosphere
When CH4 comes in contact with seawater, it provides energy to drive chemosynthetic microbial activity and a zone of anaerobic oxidation of methane (AOM) coupled to sulphate reduction in marine porewater gets established
Patches of the seagrass Posidonia oceanica, as well as the remainders of dead and degrading seagrass meadow are characteristic for the area (Fig 3B)

Summary

Introduction

Seepage represents an important route of transport for methane (CH4) and other hydrocarbon gases from Earth’s subsurface to the oceans and the atmosphere. Methane is produced either by microbial methanogenesis or by thermal breakdown of larger organic molecules at higher temperatures. Both types of CH4 generation have been described in many studies from modern and fossil seep locations (e.g. microbial CH4: [3]Roberts and Aharon, 1994; [4]Birgel et al, 2011; [5]Natalicchio et al, 2012; vs thermogenic CH4: [6]Hovland and Judd, 1988; [7]Brooks et al, 1986; [8]Jessen et al, 2011). Methane can form abiotically from inorganic carbon via a Fischer-Tropsch-type reaction ([18]Lancet and Anders, 1970).

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