Halogen and ^{129}I Distributions in Pore Waters From Actively Venting Gas Hydrate Fields Near Sado Island, Eastern Japan Sea

Large amounts of massive gas hydrates were recovered by piston coring from the Umitaka Spur-Joetsu Knoll area near Sado Island in the eastern margin of the Japan Sea during UT04, KY05-08, and UT06 cruises. Enormous methane seepages were observed in the echogram, reflecting active and large methane- storage/discharge systems in the continental margin. We report here halogen concentrations and ^{129}I/I ratios in pore waters collected from piston cores taken during cruises in 2004 to 2006. Depth profiles of these halogen concentrations are used to investigate geochemical characteristics of fluids associated with methane delivery. In addition, the presence of the long-lived radioisotope ^{129}I (half life; 15.7 Myr), provides information on potential source materials for iodine and methane in gas hydrates. The Cl concentrations in the Sado fields increase to 800 mM or decrease to 400 mM at the bottom of the cores from seawater values. These variations reflect the input of brines during gas hydrate formation and the pore water freshening during dissociation of gas hydrates below the cored depths, respectively. In contrast to the Cl profiles, I and Br gradually increase with depth in all profiles, reaching values of 170 μM and 1400 μM, respectively, which represent strong enrichment of deep fluids in I and Br compared to seawater (I; 0.4 μM, Br; 840 μM). The ^{129}I/^{127}I ratios rapidly decrease with depth from the seawater value of 1500×10^{-15} to values between 400×10^{-15} and 600×10^{-15}, which are equivalent to ages between 20 and 30 Ma. These ages of potential source formations of iodine (and, by association, of methane) correspond well with the time when the opening of the Japan Sea was initiated at ~28 Ma, followed by the rotation of NE and SW geological parts of Japan forming the Fossa Magna sedimentary basin at ~18 Ma. Methane in the Sado gas hydrate fields is thus principally associated with the tectonic processes forming the Japan Sea and derived from deep sources accumulated during the early stages of the formation of the Japan Sea.