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 n mol/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 ma ximum methane concentration of 10 nmol/kg) was newly discovered. The e stimated end-member delta(13)C(CH4) value (-59 +/- 3 parts per thousan d 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 se a floor of the Suruga Trough. On the other hand, the shallower plume i s characterized by more C-13-enriched end member methane (delta(13)C(C H4) = -38 +/- 2 parts per thousand PDB), presumably produced by the th ermogenic degradation of organic matter. Since thermogenic methane sho uld originate from a deeper part (more than 1000 m) of the sedimentary layer, it is unlikely that the thermogenic methane reaches the sea wa ter by normal transport processes. The shallower plume may be a result of some sudden, catastrophic event on the sea floor, such as earthqua kes. (C) 1998 Elsevier Science B.V. All rights reserved.