HR: 14:35h
AN: OS52C-04    [PDF]
TI: Sources of gas hydrates: $^{129}$I results from Blake Ridge, Nankai Trough, Peru Margin and Hydrate Ridge
AU: * Fehn, U
EM: fehn@earth.rochester.edu
AF: Univ. Rochester, Earth and Environmental Sciences, Univ. Rochester, Rochester, NY 14627 United States
AU: Snyder, G T
EM: gsnyder@rice.edu
AF: Rice Univ., Earth Sciences, MS126, Rice Univ., Houston, TX 77005 United States
AB: The presence of the cosmogenic isotope $^{129}$I (T$_{1/2}$= 15.7 Myr) allows the dating of iodine in pore waters associated with gas hydrates. Since iodine is closely associated with organic material, $^{129}$I /I ratios give information on the age of the source material of iodine and, by association, of the methane found in gas hydrate occurrences. The first application of this type was carried out at Blake Ridge (ODP 164; Fehn et al., 2000), followed by a study of the Nankai area (Fehn et al., 2003). We compare here the earlier results to new data from the Peru Margin (ODP 201) and Hydrate Ridge (ODP 204). All of the results obtained show that stable iodine in sediment pore waters associated with gas hydrates is enriched 500-fold (Nankai) to 5000-fold (Hydrate Ridge) over seawater, indicating derivation from organic sources. The high concentration of iodine in the pore waters allows preparation of targets for AMS determinations from the small quantities (3 to 10 mL) typical for individual pore water samples. All $^{129}$I /I ratios measured in the two new investigations indicate ages older than the host sediments, confirming findings from Blake Ridge and Nankai that iodine (and methane) is predominantly not derived from local sources. In contrast to Blake Ridge, however, ratios in the other three areas show considerable variation. In Nankai and Peru Margin, a clear decrease in ratio (and increase in age) with increasing depth is discernable, pointing to mixing between a deep, old and a shallow, young source. The situation is more complex at Hydrate Ridge, where in several sites maxima in iodine concentrations were found at depths between 100 and 200 m, associated with relatively high $^{129}$I /I ratios. The lowest ratios found in all four areas are consistently below 200x10$^{-15}$, corresponding to minimum ages of 45 Ma. $^{129}$I /I ratios found for iodine in pore waters associated with gas hydrates indicate that old organic sources are an important source for iodine (and probably also of methane) in gas hydrates deposits, but that addition of locally derived iodine is visible in some cases. Fehn et al., Science 289, 2332-2335, 2000; Fehn et al., Geology 31, 521-524, 2003
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting