Pockmarks and methanogenic carbonates above the giant Troll gas field in the Norwegian North Sea

Abstract Acoustic imaging has revealed more than 7000 pockmarks on the seafloor above the Troll East gas field in the Norwegian North Sea. We present the first comprehensive study conducted on one of the World's largest pockmark fields complementing the acoustic data with extensive sampling, geochemical and petrographical studies. Specifically, we aimed at detecting possible active seepage still present over this vast area. The pockmarks are present as isolated structures, on average ~ 35 m wide and up to 100 m in size. In addition, smaller satellite pockmarks surround some of the pockmarks. In contrast to the muddy surroundings, parts of the investigated pockmarks contain laterally extensive carbonate deposits or meter sized carbonate blocks. These blocks provide shelter to abundant colonies of benthic megafauna. The carbonate blocks are comprised of micritic Mg-calcite and calcite, micritic aragonite, and botryoidal aragonite. Framboidal pyrite is also commonly present. Carbon isotopic values of the carbonates are 13 C-depleted (δ 13 C as low as − 59.7‰) and with δ 18 O up to 4.5‰, indicating a methanogenic origin, possibly linked to gas hydrate dissociation. Pore water extracted from shallow cores from the centre and the flanks of the pockmarks show similar Cl and SO 4 profiles as the reference cores outside the pockmarks, ruling out active methane seepage. This conclusion is also supported by seafloor video observations that did not reveal any evidence of visual fluid seepage, and by the absence of microbial mats and by the fact that the carbonate blocks are exposed on the seafloor and party oxidized on the surface. We conclude that methane seepage formed this extensive gas field following to gas hydrate dissociation.