Origin and age of pore waters in an actively venting gas hydrate field near Sado Island,Japan Sea: Interpretation of halogen and 129I distributions

A gas hydrate field with highly active venting of methane was recently found near Sado Island in the eastern Japan Sea. Piston cores were collected from active venting sites and nearby locations in the Umitaka Spur–Joetsu Knoll area during two cruises in 2004 (UT04) and 2005 (KY05-08). We report here halogen concentrations and ¹²⁹I/I ratios in pore waters associated with gas hydrates from these expeditions. The strongly biophilic behavior of I and, to a lesser degree, of Br together with the presence of the long-lived iodine radioisotope (¹²⁹I) allow evaluation of potential source materials for methane in gas hydrate systems. Depth profiles of all three halogens, particularly the very rapid downward increases of Br and I concentrations, strongly suggest input of deep fluids enriched in Br and I, but the profiles also display the effects of gas hydrate formation and dissociation. Although the ¹²⁹I/I ratios are modified by ¹²⁹I from seawater and sediments at shallow depth, likely ratios of the deep fluids are estimated to be between 400 × 10^? 15 and 600 × 10^? 15, equivalent to a Late Oligocene to Early Miocene age. Ages in the active methane venting sites typically are closer to the old end of this range than those in the reference sites. This age range suggests that the methane associated with venting and gas hydrate formation in this area is derived from organic materials accumulated during the initial opening of the Japan Sea. The Umitaka Spur–Joetsu Knoll gas hydrate field demonstrates the movement of deep fluids associated with the release of significant amounts of methane from the seafloor, processes which might be important components of mass transfer and carbon cycle in the shallow geosphere.