SUMMARY AND CONCLUSIONS

Geochemical analyses carried out on samples collected from cores on and near the southern summit of Hydrate Ridge have advanced understanding by providing a clear contrast of the two major modes of marine gas hydrate occurrence. High concentrations (15%–40% of pore space) of gas hydrate occurring at shallow depths (0–40 mbsf) on and near the southern summit are fed by gas migrating from depths of as much as 2 km within the accretionary prism. This gas carries a characteristic minor component of C₂–C₅ thermogenic hydrocarbons that enable tracing of migration pathways and may stabilize the occurrence of some Structure II gas hydrate. A Structure II wet gas hydrate that is stable to greater depths and temperatures than Structure I methane hydrate may account for the deeper, faint second bottom simulating reflection (BSR₂) that occurs on the seaward side of the ridge. The wet gas is migrating in an ash/turbidite layer that intersects the base of gas hydrate stability on the seaward side of and directly beneath the southern summit of Hydrate Ridge. The high gas saturation (>65%) of the pore space within this layer could create a two-phase (gas + solid) system that would enable free gas to move vertically upward through the gas hydrate stability zone.

Away from the summit of the ridge there is no apparent influx of the gas seeping from depth and sediments are characterized by the normal sequence of early diagenetic processes involving anaerobic oxidation of sedimentary organic matter, initially linked to the reduction of sulfate and later continued by means of carbonate reduction leading to the formation of microbial methane. A well-defined sulfate/methane interface occurs at each of the sites cored away from the summit, but there is apparently little or no anaerobic methane oxidation occurring, as evidenced by the fact that all except one site lack any dissolved inorganic carbon more ¹²C enriched than about –24 at the sulfate/methane interface. The observed concentration gradients in sulfate, alkalinity, and methane are more a product of rapid reactions than diffusive fluxes. The concentration gradients and sedimentation rates provide estimates of the rates of microbial sulfate reduction and methane generation. Observed rates at sites away from the summit are comparable to those found in other high-productivity continental margin settings but are three orders of magnitude slower than rates of microbial processes occurring at the summit of southern Hydrate Ridge.

The microbial methane within a 100-m thickness of these sediments is generated over a period of some 200,000–300,000 yr, and the pressure conditions at Hydrate Ridge have changed dramatically during the last 100,000 yr as a result of tectonic uplift. It is probably the lowering of methane solubility requirements associated with this pressure reduction (from 30 to 8 MPa) more than particularly vigorous methane generation that brings about gas venting and widespread gas hydrate occurrence on the Cascadia and other active margins.