Methane Plumes over a Marine Gas Hydrate System in the Eastern Margin of the Sea of Japan: a Proposed Mechanism for the Transport of Significant Subsurface Methane to Shallow Waters

R&T/V Umitaka-maru sailed to a small ridge (now named O`UT04 RidgeO_L) in the eastern margin of the Sea of Japan July to August 2004 to explore the ocean floor gas hydrates and related phenomena. The Ridge is located in the southwestern extension of JapanOs most productive hydrocarbon province. Detailed bathymetric profiles and seismic profiles have revealed a number of mud volcanoes (20 to 40 m high and 300-500 m across), pockmarks (40-70 m deep and 300 500 m across), and collapse structures within 3 km x 4 km on the ridge at a water depth of 910 to 980 m. Large pockmarks develop in the NNE-SSW direction, parallel to the boundary between the Eurasian and North American Plates. Fathometer and quantitative Echo sounder have depicted 36 magnificent flares of plumes over the area, each ranging 100 to 200 m in diameter and 600 to 750 m in height, reaching up to the shallow water zone of 200 m to 350 m below sea level. Onboard geochemical analysis of the interstitial waters has identified the sulfate-methane interface (SMI) at <0.5 m to 4 m. Black silt and clay from the top 0.5 -3.5 m at active seep sites contained a number of chunks of gas hydrate and nodular carbonates. Gas hydrate is dominated by methane with minor ethane. Seawater sampling was conducted at every 100-200 m in the water column over the area. Onboard GC measurements have demonstrated characteristic methane profiles as 10 to 50 nmon/L for the bottom waters, 5 to 8 nmol/L for the intermediate waters, 20-45 nmol/L at 200 to 300 m, and again about 5 nmol/L for the surface water. CTD has shown anomalously low temperatures, 0.25^o C, for the bottom and intermediate waters, and abrupt increase at around 300 m up to about 25^o C at surface. Field observations and consideration on the stability of gas hydrates lead us to the hypothesis that methane plumes are not the columns of methane bubbles but of gas hydrate. Methane bubbles from the seeps would be converted to gas hydrate sooner or later in such a cold deep waters. Gas hydrate crystals could potentially float upward until they reach the warm waters at around 200-300 m, where gas hydrate would be dissociated, and continuously supply methane to shallow waters. This hypothesis is to be tested by submersible dives planned in 2005.