Elucidation of source and flux of methane in marine environments

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This study aimed at elucidating the behavior of methane in the oceanic environment, especially in oceanic waters and pore fluids in sedimentary layers. Concentration and stable carbon isotope ratios (^<13>C/^<12>C) of methane were precisely measured by using an isotope-ratio-monitoring gas chromatography/mass spectrometry system newly constructed in this study. Submarine hydrothermally active areas at spreading centers and arc volcanoes (Central Indian Ridge, Gulf of Aden, Mariana Trough, Okinawa Trough, Suiyo Seamount etc.) and cold seepage areas at plate subduction zones (Nankai Trough, Eastern Japan Sea etc.) were investigated in detail using research vessels and submersibles, in order to take fluid samples for methane analysis from these areas. The carbon isotope ratio of methane has enabled us to distinguish the following three different types of methane: magmatic methane, methane from thermocatalytic decomposition of organic matter, and biogenic methane. In addition, oxidative decomposition of methane was recognized in pore fluids as well as in seawater, by detecting the enrichment of ^<13>C of methane to occur during microbiological processes. Gas hydrate below seafloor was also discussed as an important source of methane from the sediment to seawater.