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

• Published in: Continental shelf research 2017-02, Vol.133, p.13-25
• Main Authors: Pierre, Catherine, Demange, Jérome, Blanc-Valleron, Marie-Madeleine, Dupré, Stéphanie
• Format: Article
• Language: English
• Subjects: Aerobic oxidation of methane; Anaerobic oxidation of methane; Aquitaine Shelf; Carbon isotopes; Geophysics; Methane-derived authigenic carbonates; Oxygen isotopes; Physics; Submarine groundwater discharge
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2016.12.003

The widespread methane emissions that were discovered in 2013 on the Aquitaine Shelf at water depth between 140 and 220m are associated with authigenic carbonate crusts that cover meter-high subcircular reliefs of 10–100m in diameter. These authigenic carbonates are primarily aragonite plus calcite and dolomite, which cement the fine- to medium-grained sandy sediment. The carbonate cement is often pierced by numerous circular cavities of 5–10µm in diameter that are considered to be moulds of gas bubbles. Conversely, micron-sized cavities in the aragonite crystals are attributed to dissolution features, in relation to the production of CO2 during the aerobic oxidation of methane. The oxygen isotopic compositions of bulk carbonate (+1.7 to +3.7‰) and aragonite cements obtained from microsampling (−0.1 to +1.4‰) indicate that these carbonates were precipitated in mixtures of seawater and freshwater, i.e., in the context of submarine groundwater discharge at the seafloor. The carbon isotopic compositions of authigenic carbonates (−51.9 to −38.1‰) and of aragonite cements (−49.9 to −29.3‰) show that the dissolved inorganic carbon of pore fluids was mostly produced by the anaerobic oxidation of biogenic methane and also partly from the groundwater system. •Authigenic carbonate crusts are related to methane seeps on the Aquitaine Shelf.•Anaerobic oxidation of methane was responsible for carbonate cementation.•Submarine groundwater discharge is mixed with seawater recharge at the shelf break.