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

U. Tsunogai, J. Ishibashi, H. Wakita, T. Gamo

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The carbon isotopic compositions (δ13CCH4) 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 δ13CCH4 value (-59±3‰ 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 13C-enriched end-member methane (δ13CCH4= -38±2‰ 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.

Original languageEnglish
Pages (from-to)97-105
Number of pages9
JournalEarth and Planetary Science Letters
Issue number1-2
Publication statusPublished - Jul 1998
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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