Halogen and iodine-129 systematics in gas hydrate fields: Implications for the transport of iodine and methane in active margins

Halogen systematics and iodine-129 dating were applied to fluids collected in marine gas hydrate fields at Pacific active continental margins, in order to identify the organic source formations responsible for the release of methane and iodine in the forearc region. Main areas of investigation include central and northern Cascadia, Nankai Trough and Costa Rican margin. At all of these forearcs, iodine ages in the pore water associated with gas hydrates very often fall in a range of 30-50 Ma which is significantly older than the age of host (local) sediments (< 5 Ma) and the sediments on the subducting plate (< 25 Ma), indicating that the organic source has to be in the upper plate. Formations with mid-early Tertiary ages were found deep in the accretionary wedge landward to the hydrate fields. They were added to the upper plate by off-scraping sediments from the oceanic plate during the early stage of margin formation and are currently responsible for the release of old iodine. Sources younger than 10 Ma have also been found commonly at the forearcs, as indicated by the mixing behavior of iodine isotopes. Both the subducting sediments and young formations in the upper plate close to prism toe can potentially be sources of the relatively young iodine.Since the Eocene source formations are typically located at long distances landward from hydrate fields, the transportation of methane and iodine relies on the fluid conduits, like decollement, faults and fractures. The presence of diagenetically altered fluids, such as low chlorinity water from clay dehydration, together with old iodine supports the scenario of long distance migration of fluids from deep sources. Although the source region for clay dehydration and iodine release might not be the same, the complex flow network in the highly deformed forearc wedges very likely mixed these fluids on their pathway.A numerical model is developed to further test the contributions of in-situ release of young iodine due to biogradation of organic matter vs. transportation of old iodine by advection. The modeling results indicate that only a very limited amount of pore water iodine is produced by the local sediments, which is consistent with the iodine ages.All the iodine ages determined are also used to derive a statistical trend in the age distribution which shows that the iodine and methane currently returning into the surface reservoirs were buried, on average, at 30 Ma in the active margins. Based on the same observation at all of these margins that the hydrates are derived from old carbon sources in the upper plate, we suggest that the forearcs might play an important role in recycling iodine and associated organic carbon at tectonic time scales.