I / I ratios and halogen concentrations in pore waters of the Hydrate Ridge : Relevance for the origin of gas hydrates in ODP Leg 204

We report iodine and bromine concentrations in a total of 256 samples of pore waters collected from all nine sites of ODP Leg 204, Hydrate Ridge. In a subset of these samples, we have also determined iodine ages in the fluids using the cosmogenic isotope I (T1/2 = 15.7 Ma). The presence of this cosmogenic isotope combined with the strong association of iodine with methane allows the identification of the organic source material responsible for iodine and methane in gas hydrates. In all cores, iodine concentrations were found to increase strongly with depth from values close to that of seawater (0.0004 mM) to concentrations in excess of 0.5 mM. Several of the cores taken at the NW Flank of the Southern Summit show a pronounced maximum in iodine concentrations at depths between 100 and 150 mbsf in the layer just above the BSR. This maximum is especially visible in Site 1245, where concentrations reach values as high as 2.3 mM, but maxima are absent in the cores taken at the Slope Basin Sites (Site 1251 and 1252). Bromine concentrations follow similar trends, but enrichment factors for Br are only between 4 and 8 relative to seawater, i.e. considerably lower than those for iodine. Iodine concentrations were sufficient to allow AMS determinations for individual pore water samples collected on shipboard (~ 5 mL). We report I/I in a few samples from each core and a more complete profile for one flank site (Site 1245). All I/I ratios are below the marine input ratio (Ri = 1500x10). The lowest values found in most sites are between 150 and 250x10, which correspond to minimum ages between 40 and 55 Ma. These ages rule out derivation of most of the iodine (and, by association, of methane) from the sediments hosting the gas hydrates or from currently subducting sediments. The iodine maximum in Site 1245 is accompanied by an increase in I/I ratios, suggesting the presence of an additional source with an age younger than 10 Ma; there is indication that younger sources also contribute in other sites, but data coverage is not sufficient yet to allow a definitive identification of sources there. Likely sources for the older component are formations of Early Eocene age close to the backstop in the overriding wedge, while the younger sources might be found in recent sediments underlying the current locations of the gas hydrates. Submitted to Proceedings of ODP, Scientific Results 204 January 2005 * Corresponding author: [email protected]