Аннотация:The Black Sea is a well-known province for methane seepage and the presence of gas hydrates. Part ofthe offshore Romania was investigated during the marine expedition GHASS2 onboard the research vesselPourquoi pas? in 2021. Acoustic water column surveys were conducted at the continental shelf area, inthe upper slope area and in deeper parts of the basin. These observations provide a larger view of thespatial distribution of gas bubbles escaping from the seafloor into the water column, compared to previoussurveys (Riboulot et al., 2017; Riedel et al., 2021), especially at the continental shelf area. Beyond theupper limit of the gas hydrate stability zone, which coincides with the 660 m isobath (Riboulot et al., 2017),two active seepage sites were investigated, a hydrate ridge (930-660 m water depth) and a mud volcano(Flora, 760-720 m water depth). Near-bottom surveys were carried out with the Nautile submersible,including two dives dedicated to exploration at the hydrate ridge. Seafloor observations, sediment, fluid(gas, water) and carbonate samplings, temperature and flow rate measurements were conducted; andassociated preliminary results will be presented. In particular, the seafloor at the hydrate ridge exhibitsa chaotic facies morphology, numerous fractures, widespread microbial mats, authigenic carbonates andsub-outcropping methane hydrates interbedded within the sediment. Low δ13C values of aragonite andhigh Mg-calcite from the sub-surface authigenic carbonates reveal a methane source for their formation.Pore-water δ18O and δD isotopic signatures indicate that part of the water originates from gas hydratedissociation.
