Leg 204 was dedicated to coring and logging the gas hydrate stability zone (GHSZ) within a 2 km x 6 km three-dimensional (3-D) seismic grid across southern Hydrate Ridge, a topographic high in the accretionary complex of the Cascadia subduction zone, ~80 km west of Newport, Oregon (USA) (Fig. F1). Hydrate Ridge is an area where gas hydrate occurs at or near the seafloor, and methane is venting into the water column (Torres et al., 2002; Heeschen et al., 2003). A regional bottom-simulating reflector (BSR) suggests that gas hydrate is widespread (Tréhu et al., 1999). Nine sites were drilled during Leg 204, four sites (1245, 1246, 1244, and 1252) along a west–east transect north of the southern summit, four sites (1247, 1248, 1249, and 1250) on a north–south transect up to the summit of the ridge, and Site 1251 in a slope basin east of the ridge. Water depths ranged from 795 to 1210 meters below sea level (mbsl) (Table T1). A logging-while-drilling hole and multiple-cored holes of varying depths were drilled at each site except at Site 1252, where a single hole was cored. Presence of gas hydrate at all sites except 1252 was confirmed by physical recovery from cores, pressure core samples, chlorinity anomalies, low core temperatures, and sonic and resistivity log responses (Tréhu et al., 2004).
Closely spaced samples of gas and pore water were collected from cores at all sites. In addition, 30 samples collected with the pressure core sampler (PCS) were retrieved intact and quantitatively degassed (Milkov et al., 2003), enabling measurement of subsurface gas contents as well as chemical and isotopic characterization. Gas and water samples were chemically analyzed onboard and isotopically analyzed postcruise. The Leg 204 Initial Reports volume (Tréhu, Bohrmann, Rack, Torres, et al., 2003) presents shipboard results, and Milkov et al. (2005) discuss isotopic compositions of gases. This report integrates the gas and pore water chemical and isotopic compositions and uses these data to infer the origins of the gas. The main issues to be addressed are the rates and quantities of gas generated by local microbial processes vs. the quantities resulting from upward flow from deeper sediments. The different gas supply processes affect the rate and magnitude of gas hydrate formation and are reflected in differences in gas hydrate occurrence on the accretionary ridge and in the slope basin. Compositionally distinctive gas migrating from depth is localized and defines the main migration pathways. In addition, gas hydrate formation imposes previously unrecognized fractionation effects on the gas geochemistry, which may be used to infer gas hydrate occurrence in other areas (Milkov et al., 2004a).