Sources of methane inferred from pore-water δ13C of dissolved inorganic carbon in Pockmark G11, offshore Mid-Norway

Abstract

Pockmark G11 is the most spectacular one among the pockmarks located at the southern border of Vøring Plateau and 1–2 km away from the northern flank of the Storegga Slide, mid-Norwegian continental margin. For the first time, detailed pore-water geochemical studies were conducted to address methane hydrate occurrence, methane seepage and associated geochemical processes, and methane characteristics in the pockmark. Pore-waters collected from five sediments cores inside and one sediment core outside the pockmark, were analyzed for dissolved Cl−, sulfate (SO42−), total hydrogen sulfide (ΣH2S), Ca2+, Mg2+, dissolved inorganic carbon (DIC), δ13CDIC and δ18O. Methane hydrates were recovered in all sediments below 0.75m in a core inside pockmark G11, which is in good accordance with heavy oxygen isotope (1.9 to 2.3‰SMOW) and low Cl−concentrations (84.9 to 16.1mM) in pore-waters. Pore-water profiles indicate that upward-migrating methane fluids are spatially variable in the pockmark, with methane fluxes ranging from below detection in the center and outside, to 0.30–0.54molm−2 a−1 inside. In the cores with active methane fluxes, maximum DIC concentrations (19.4 to 21.5mM) and corresponding minimum δ13CDIC values (−52.3 to −54.6 ‰ PDB) occur within sulfate-methane-transition (SMT) zones from ∼0.40 to 0.50m below seafloor (mbsf), close to the seafloor. Simple mass balance modeling and δ13CDIC measurements within the SMT zones suggest that methane in shallow sediments within pockmark G11 is microbial in origin. Pore-water geochemistry and seabed observations suggest that methane fluxes inside pockmark G11 are episodic, and derived mostly from the recycling of methane hydrate at depth during sediment burial.