Unprecedented high-resolution acoustic data have confirmed the existence of persistent gas releases and Methane-Derived Authigenic Carbonates (MDAC) at the Aquitaine Shelf edge (Dupré et al. 2014). This has made way to the discovery of an unknown widespread active and fossil fluid system exclusively associated with microbial methane (Pierre et al. 2017; Ruffine et al. 2017). MDACs are present over an area of 375 km2 and are associated with more than 2600 gas bubble streams. The ecosystems at the Aquitaine Shelf are characterized by microbial mats including archea involved in anaerobic oxidation of methane, a scarce chemosynthetic megafauna and numerous fixed (sponges) and mobile organisms (fishes). Based on in situ flux measurements and acoustic data, and assuming steady and continuous fluxes over time, the methane entering the water column reaches 144 Mg yr-1. This discovery highlights the importance of microbial methane generation, disconnected from deep thermogenic sources and gas hydrates, at continental shelves. The shelf edge may be viewed as a focus area for methane circulation and release and related diagenesis, all having an impact on the shaping of continental shelves and potentially on the oceanic and atmospheric carbon budget.

The GAZCOGNE project is co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project.

GAZCOGNE Scientic Party : Arnaubec A1, Baltzer A2, Battani A3, Bayon G1, Bignon L1, Birot D1, Blanc-Valleron MM4, Bourillet JF1, Brandily C1, Breton C1, Caprais JC1, Croguennec C1, Demange J5, Deville E3, Donval JP1, Ehrhold A1, Eugène T1, Floodpage J6, Gautier E1, Godfroy A1, Guérin C1, Imbert P6, Lescanne M6, Lesongeur F1, Loubrieu B1, Noël P1, Marsset B1, Michel G1, Noirez S3, Ogor A1, Olu K1, Opderbecke J1, Pernet EJ1, Petit E1, Pierre C5, Pitel M1, Ruffine L1, Saout J1, Scalabrin C1, Thomas Y1, Vermesse H3

References

Dupré S et al (2014) Fluid emissions at the Aquitaine Shelf (Bay of Biscay, France): A biogenic origin or the expression of hydrocarbon leakage? Cont. Shelf Res. 88:24-33.

Pierre C et al (2017) Authigenic carbonate mounds from active methane seeps on the southern Aquitaine Shelf (Bay of Biscay, France): Evidence for anaerobic oxidation of biogenic methane and submarine groundwater discharge during formation. Cont. Shelf Res. 133:13-25.

Ruffine L et al (2017) Gas Seepage along the Edge of the Aquitaine Shelf (France): Origin and Local Fluxes. Geofluids 2017:13.