SOURCE AND ACCUMULATION OF METHANE IN GAS HYDRATE DEPOSIT IN KUMANO BASIN, NANKAI TROUGH, JAPAN: INTERPRETATIONS FROM 129I DISTRIBUTION IN PORE WATER

Because iodine has a strong association with methane in marine environments, it is often enriched in methane-rich system including hydrate deposits. The presence of a long-lived radioisotope of iodine (¹²⁹I), therefore, allows us to investigate the source formation of methane and its migration history. In this study, we present the ¹²⁹I distribution together with the concentrations of halogens dissolved in sedimentary pore waters collected intensively from thick sandy methane hydrate interval between 200 and 400 mbsf in the Kumano forearc basin of the Nankai Trough, Japan, to examine the loci of source formations and processes to deliver and accumulate methane in the present methane hydrate stability.

Concentration of iodine rapidly increases downward to the value of >500 µM, >1000 times higher than seawater, at the top of methane hydrate interval at 200 mbsf, decreases to ~200 µM at the bottom of the interval at 400 mbsf, and then become stable down to ~1000 mbsf. The ¹²⁹I/¹²⁷I ratio, on the other hand, decreases in the first 200 m and shows the lowest value of ~200x10^-15 at the top of methane hydrate interval which indicates that methane-iodine is preferentially derived through the top layer of methane hydrate accumulated interval and such methane-iodine is oldest in the hydrate interval. Methane and iodine could have been derived from the landward older organic-rich sediments through the sandy aquifers to the present seaward methane hydrate zone.