​​Discovery of a bathyal methane seep field at the north-western Vestnesa Ridge, Fram Strait

The Vestnesa Ridge at 79°N in the Arctic Fram Strait is known for sub-surface methane hydrate reservoirs and numerous gas emitting pockmarks, with associated microbial and faunal communities on its eastern flank. The expedition MSM95 in 2020 found the first evidence of active methane seepage on the north-western flank of the Vestnesa Ridge in water depths of 1200 to 1375 m. Subsequent investigations during the PS136, MSM108 and PS143/1 expeditions with towed camera, Remotely Operated Vehicle and trawl sampling confirmed the MSM95 early indications. Here we report the first faunal assemblage description of the discovered active methane seep field, named Hœnir seep field. In-situ image analyses of the methane seep field and its active and inactive areas showed a regional community comprising 47 megafaunal taxa. The presence of obligate seep-associated faunal taxa, such as the tube-building siboglinid polychaetes and oligochaetes were interpreted as indicative of active methane seepage in the area. The most frequent taxon observed, ophiuroids in the genus Ophiocten Lütken, 1855, occurred in high densities with up to 80 ind. per m2 in inactive areas but was absent from the seafloor closer than a few meters from bacterial mat or tubeworm forests. The fauna seen in the Hœnir seep field shares most faunal elements with seep fields previously studied on the eastern flank of the Vestnesa Ridge and on the nearby Svyatagor Ridge. For taxonomic identifications of fauna associated with the methane seepage, 95 specimens of 43 selected taxa, mostly ophiuroids, polychaetes and amphipods, were COI barcoded.