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-resolution (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 (<about 5 m high) mounds and/or pockmarks. In the record of a 38 kHz split-beam echosounder, which was deployed for acoustic characterization of gas bubbles in the water column, there are no apparent gas flares associated with the blank zones. The recovered gas hydrates take the forms of massive layer, blocky nodule, or platy vein in mud at 0.5 to 8 m below the core tops. They were sometimes associated with abundant scattered authigenic carbonate nodules. Compositional and structural analyses of selected gas hydrate samples revealed that they consist of structure I hydrates containing more than 99% biogenic methane. Gas and porewater compositions of the core sediments also imply relatively high upward flux of biogenic methane. The results of the site survey cruise collectively suggest that the near-seafloor gas hydrates are related to cold vents where abundant gas hydrates and authigenic carbonates comprises acoustically high seafloor reflectivity. Gas seeping activity in the cold vents appears to be currently dormant.