Hydrate Ridge is the site of unusually high concentrations of gas hydrates. Pore water at Leg 204 sites shows the enrichment in iodine and bromine typical for gas hydrate sites, with the enrichment being somewhat higher than at other locations. Several of the cores display distinct maxima in iodine concentrations at the depths just above the BSR, a feature not observed in other gas hydrate sites. These maxima are most pronounced at the flank sites (e.g., Site 1245), but are absent from the slope basin sites (e.g., Site 1251). An initial set of 129I/I ratios indicates the presence of iodine in all cores derived from organic sources with ages of ~50 Ma. The maxima found at several cores contain material derived from sources younger than 10 Ma.
The ages found here suggest that contributions from sources within the sediments hosting the gas hydrates constitute only a minor part of iodine and, by association, methane in Hydrate Ridge. A large fraction of the iodine is derived from sources with early Eocene ages. Potential source formations of this age are to be found in marine sediments of Eocene age located 40 km to the east. Pliocene formations underlying Hydrate Ridge also contribute, especially at the flank sites, and are probably responsible for the local maxima in iodine concentrations observed just above the BSR. Similar to results from other gas hydrate sites, the results indicate large-scale fluid transport and derivation of a major fraction of iodine and methane from sediments of Eocene age. The transport is probably driven by compaction and/or seismic pumping and is facilitated by the presence of multiple fractures in the Cascadia margin. Further iodine isotope data are needed in order to evaluate the causes of variations observed in iodine concentrations within the gas hydrate field of Hydrate Ridge and to relate them to potential differences in the hydrologic characteristics within the area.