Methane seeps on the outer shelf of the Laptev Sea: characteristic features, structural control, and benthic fauna

Two areas with cold methane seeps on the outer shelf of the Laptev Sea were studied by two interdisciplinary expeditions onboard the RV Akademik Mstislav Keldysh in August-September of 2017 and 2018. These fields lie in water between depths of 63 and 73 m, and in a region of growing interest to the international community. Characteristic features of the methane seeps were obtained, which include their distribution and appearance on the seabed based on acoustic anomalies and seafloor observations. The cold seeps are part of a domain striking in a SW-NE direction along the Laptev Sea Rift System, Khatanga-Lomonosov Fracture Zone, and the Gakkel Ridge junction, and its structure was determined by shallow faults on the outer shelf. These faults are related to subsidence of the outer shelf cutting the caprock formed by permafrost and gas hydrates. Faults serve as conduits for an intense bubble methane discharge at the seabed. Shallow-water methane seep fauna were described for the first time in the Siberian Arctic. The frenulate siboglinid tubeworm Oligobrachia haakonmosbiensis was among the dominant species of the methane seep communities. A newly discovered gastropod species Frigidalvania sp. was also found in abundance at the seeps as well as an ophiuroid Ophiocten sericeum. Significant differences were observed between benthic communities of the two seep fields and background fauna including integral community parameters and the presence/absence of certain species. Development of shallow methane seep communities in the Laptev Sea apparently is related to extremely oligotrophic conditions in this area.