Methane-rich plumes in the Suruga Trough (Japan) and their carbon isotopic characterization

The carbon isotopic compositions (delta(13)C(CH4)) Of the methane-rich buoyant plumes, observed in the oxygenated hemipelagic sea waters of the Suruga Trough, Japan, are discussed in relation to their sources. During a survey made in May 1996, two layers of anomalous methane-rich plumes, both of which centred at the same station about a few tens of kilometres off the coast, were found in the Suruga Trough. The deeper plume (ca. 2100 m depth, with a maximum methane concentration of 13 nmol/kg) had already been detected by a previous survey in 1986 at the same station, whereas the shallower plume (ca, 1000 m depth, with a maximum methane concentration of 10 nmol/kg) was newly discovered. The estimated end-member delta(13)C(CH4) value (-59 +/- 3 parts per thousand PDB) for the deeper plume suggests a microbial origin of the methane, probably derived from some shallow (surface) layer of sediment. The plume could be supplied from a continuous cold fluid seepage on the sea floor of the Suruga Trough. On the other hand, the shallower plume is characterized by more C-13-enriched end member methane (delta(13)C(CH4) = -38 +/- 2 parts per thousand PDB), presumably produced by the thermogenic degradation of organic matter. Since thermogenic methane should originate from a deeper part (more than 1000 m) of the sedimentary layer, it is unlikely that the thermogenic methane reaches the sea water by normal transport processes. The shallower plume may be a result of some sudden, catastrophic event on the sea floor, such as earthquakes. (C) 1998 Elsevier Science B.V. All rights reserved.