Occurrence of near-seafloor gas hydrates and associated cold vents in the Ulleung Basin, East Sea

During the site survey for drill sites of the Ulleung Basin Gas Hydrate Expedition 1, near-seafloor gas hydrates were discovered in core sediments from basin plain (2010-2130 m water depth) and southern slope (820-1160 m). The gas hydrates were exclusively retrieved from the sites of high backscatter intensity anomalies of 13 kHz multi-beam echosounding, implying high seafloor reflectivity. In high-resoltition (2-7 kHz) subbottom profiles, the coring sites are also characterized by narrow (250-850 m wide) acoustic blank zones reaching seafloor, where they have surface expressions of low-relief (