抄録
This paper summarizes stable carbon isotopic studies of hydrocarbons, especially methane, in seafloor venting hydrothermal and cold seep fluids, and their effluent methane-enriched plume in water column. Through highly sensitive isotopic analysis using CF-IRMS, we have discussed origin and/or behavior of hydrocarbons in each system. Major results are as follows: (i) methane in cold seep fluids around Japanese islands mostly microbial origin, (ii) methane emitted from cold seep fluids mostly oxidized anaerobically within sediments just before they emitted into ocean, (iii) methane in hydrothermal plume oxidized aerobically in some extent in the water column. Besides, methane flux and heat flux can be calculated for a methane-enriched fluid venting area from δ13C distribution of methane in hydrothermal plume, by using kinetic isotope fractionation which occurs during partial destruction of methane in water column as an indicator to estimate magnitude of the partial destruction. Similar approach could be also applicable to study origin and behavior of reactive trace components in atmosphere, such as nonmethane hydrocarbons and CO. Preliminary results of such studies are also presented.
