Methane plumes over a marine gas hydrate system in the eastern margin of Japan Sea: A possible mechanism for the transportation of subsurface methane to shallow waters

Abstract

Large pockmarks and mud volcanoes, 50 to 500 m in diameter, have been observed on a small ridge at water depth of 900-1000 m in the Naoetsu basin of the eastern margin of Japan Sea. The ridge sediments recovered by 5.5 m long piston cores were variably fizzling and bubbling, with occasional nodular/massive gas hydrates and methane-induced nodular carbonates. Gas hydrate methane, ca. -39 PDB, was mostly derived from deep-seated thermogenic sources. Echo-sounding survey has revealed a number of magnificent methane plumes, 600 to 750 m high, reaching up to shallow waters of 200-300 m. Methane concentration of the sea waters over the area are generally high, 4 to 180 nmol/L, with sharp and significant anomalies of 18-35 nmol/L at around 200-300 m. Methane hydrate crystals would be formed at methane seep sites, then float upward until they reach the warm shallow waters, where gas hydrate would be dissociated to supply methane to shallow waters. (Author)