The structural and stratigraphic setting of Hydrate Ridge contrasts with that of the adjacent slope basin to the east. Beneath the slope basin, the seismic indicators of hydrate and free gas are similar to those on the Blake Ridge, with an intermittent BSR and enhancement of stratigraphic reflectivity beneath the BSR (Holbrook et al., 1996). The sedimentation rate in this basin was expected to be very high. During Leg 204, we tested the hypothesis that the distribution, texture, and chemistry of hydrate and related pore fluids beneath Hydrate Ridge differ from those on the slope basin.
The presence of massive hydrate near the seafloor is enigmatic, as most models for hydrate formation in a region of diffuse fluid flow predict a decreasing gradient in hydrate concentration above the BSR (e.g., Paull et al., 1994; Rempel and Buffett, 1998; Xu and Ruppel, 1999). Several explanations have been proposed, including formation in the past when the stability boundary was near the seafloor, formation at depth and exposure by erosion (Bohrmann et al., 1998), and transport of methane through the hydrate stability field as free gas isolated from water (Suess et al., 2001). One objective of Leg 204 was to obtain constraints on the rate of hydrate formation, the depth extent of the massive hydrate, and mechanisms for transporting free gas to the seafloor.
It has been well established that fluids play a major role in many aspects of the geologic evolution of convergent margins. Changes in the chemical and isotopic composition of interstitial fluids with depth have been shown to be powerful tracers of fluid sources and migration patterns. Important objectives of Leg 204 were to document the fluid flow regime and evaluate role of fluid flow on the formation of gas hydrates, specifically as they might provide clues to the source and flux of methane to the GHSZ. Since hydrate formation and destabilization modify the isotopic composition of the pore water, a high-resolution set of pore water samples was collected during Leg 204, with the goal of using the dissolved chloride and the isotopic composition of these waters to constrain models of formation and destabilization of gas hydrates on this margin.
Changes in the isotopic composition of the dissolved carbonate resulting from oxidation of methane enriched on 12C are thought to be incorporated into calcareous fossil tests (Wefer et al., 1994; Dickens et al., 1995, 1997; Kennett et al., 1996) and authigenic carbonate phases (e.g., Sample and Kopf, 1995; Bohrmann et al., 1998). An objective of Leg 204 is to determine the isotopic composition of the pore fluids and carbonates associated with gas hydrates to provide the framework needed to unravel the history of gas hydrate formation and destabilization recorded in benthic foraminifer and authigenic carbonate phases elsewhere.
Better calibration of regional estimates of gas hydrate and free-gas volumes based on geophysical mapping and modeling techniques is of critical importance toward estimating the global abundance of hydrate and evaluating its role in climate change and potential for economic exploitation. During Leg 204, we drilled through hydrates in a variety of settings with different seismic characteristics, measured in situ physical conditions, and conducted a series of nested seismic experiments to calibrate various techniques for remote sensing of hydrate distribution and concentration.
The possible relationship between hydrates and slope failure is presently poorly understood. On the one hand, hydrates may stabilize slopes by cementing sediment grains. On the other hand, if hydrates impede fluid flow, they may weaken the underlying sediment by trapping fluids and free gas. Several investigators have noted the possible correlation of hydrates and slope instability (e.g., Booth et al., 1994; Trehu et al., 1995; Paull et al., 1996) and have discussed how such slope instability might release massive amounts of methane into the ocean (Paull et al., 1996; Nisbet and Piper, 1998). One objective of Leg 204 was to determine the mechanical and hydrological properties of hydrate-bearing sediment to better constrain models of slope instability.
Microorganisms play an important role in both methane formation and oxidation and are, therefore, a critical component of the hydrate system. Identification of these organisms and determination of their abundance, spatial variability, and rates of activities is just beginning. Important questions addressed during Leg 204 included the following: