Hydrate Ridge is a topographic high in the accretionary complex of the Cascadia subduction zone, ~80 km west of Newport, Oregon (Fig. F1). BSRs consistent with pressure and temperature conditions of methane hydrate decomposition occur throughout the region (Hyndman and Spence, 1992; Tréhu et al., 1999). Hydrate Ridge features two topographic summits, northern and southern, separated by a saddle. Site locations, water depths, BSR depths, and sediment accumulation rates are listed in Table T1. Previous coring (Ocean Drilling Program [ODP] Leg 146 Site 892) on northern Hydrate Ridge recovered gas hydrate at 2–19 meters below seafloor (mbsf), apparently related to a fault conduit containing migrated gas with a thermogenic component (Kastner et al., 1998; Hovland et al., 1995). Previous studies of Hydrate Ridge have documented the presence near both summits of seafloor gas vents, outcrops of massive gas hydrate, authigenic carbonates, and chemosynthetic communities (Suess et al., 1999; Torres et al., 2002; Boetius and Suess, 2004). Leg 204 objectives were to investigate the distribution and concentration of gas hydrates on southern Hydrate Ridge and in the adjacent slope basin and to evaluate the mechanisms that generate and transport methane and other gases into the GHSZ.
Subduction on the Oregon margin caused by convergence between the Juan de Fuca and North American plates occurs at a rate of ~400 km/m.y. (MacKay et al., 1992). Hydrate Ridge is the second accretionary ridge, ~20 km landward of the frontal thrust that marks the seaward edge of the accretionary prism. Allowing for some shortening associated with folding and faulting, this distance and convergence rate suggest that ~100 k.y. ago, sediments under Hydrate Ridge were being deposited on the outer edge of the Astoria Fan in water depths of 3 km. Since that time, Hydrate Ridge sediments have been tectonically uplifted to water depths of ~0.8 km, while accumulating only ~10–60 m of additional sediment (at biostratigraphic sedimentation rates of 100–600 m/m.y.) (Tréhu, Bohrmann, Rack, Torres, et al., 2003). This change in water depth has lowered hydrostatic pressure at the seafloor from 30 to 8 MPa and decreased the methane concentration requirement for hydrate stabilization at the base of hydrate stability by half, from ~180 to 90 mM dissolved CH4. Superimposed on the large pressure decrease associated with tectonic uplift is a smaller pressure change (±0.12 MPa) associated with glacial–interglacial sea level changes of 120 m during the last 100 k.y.
Sediments at the outer edge of the Astoria Fan were cored at Site 174 during Leg 18 of the Deep Sea Drilling Project (DSDP) and are known to be gas-charged (Claypool and Kaplan, 1974), although the inferred methane contents at Site 174 are probably below solubility limits required for gas hydrate stabilization. Similar sediments originally deposited in deep water that have undergone pressure reduction associated with subduction thrust faulting and tectonic uplift could have liberated a gas phase that would be free to find or create permeable channels and undergo buoyant migration toward the seafloor. Gas in excess of solubility would be available to form gas hydrates within the GHSZ, which would have thinned from ~400 to 120 m thick during uplift.