Hydrate Ridge is a 25-km-long and 15-km-wide ridge in the Cascadia accretionary complex, formed as the Juan de Fuca plate subducts obliquely beneath North America at a rate of ~4.5 cm/yr (Fig. F1A). Sediment on the subducting plate contains large volumes of sandy and silty turbidites. At present, most of this sediment is accreted to the continental margin either by offscraping at the deformation front or by underplating beneath the accretionary complex some 10 km east of the deformation front (MacKay et al., 1992; MacKay, 1995) (Fig. F2).
Hydrate Ridge has been the site of many geological and geophysical cruises since cold seeps were first discovered on this part of the margin nearly 20 yr ago (Kulm et al., 1986). It is characterized by a northern summit at a water depth of ~600 m and a southern summit at a water depth of ~800 m (Fig. F1B). A nearly ubiquitous bottom-simulating reflector (BSR) (Tréhu et al., 1999) suggests that gas hydrate is present throughout Hydrate Ridge.
A regional two-dimensional multichannel seismic survey was acquired in 1989 as a site survey for ODP Leg 146, a drilling leg designed primarily to study dynamics of fluid flow in accretionary complexes. The location where an upward deflection of the BSR is cut by a fault on the northern summit of Hydrate Ridge was selected for ODP Site 892 (Westbrook et al., 1994). At this site, massive H2S-rich hydrates were recovered from 2 to 19 meters below seafloor (mbsf) (Kastner et al., 1995). No hydrate was recovered near the BSR, but geochemical pore water and temperature anomalies suggested the presence of gas hydrate to 68 mbsf (Kastner et al., 1995; Hovland et al., 1995). Vertical seismic profiles (VSPs) indicated the presence of free gas for at least 20 m beneath the BSR (MacKay et al., 1994), and ocean-bottom seismometer (OBS) data suggest that free gas may extend for several 100 mbsf (Tréhu and Flueh, 2001). Methane at Site 892 is primarily of microbial origin (Kvenvolden, 1995), but C2+ hydrocarbon gases of thermogenic origin are also present (Hovland et al., 1995; Schlüter et al., 1998).
Since 1996, there have been several cruises per year to northern and southern Hydrate Ridge. These cruises have generated a comprehensive swath bathymetry and a deep-towed side-scan database as well as extensive seafloor observations and sample collections by submersibles and remotely operated vehicles (ROVs) (Suess and Bohrmann, 1997; Clague et al., 2001; Johnson et al., in press; Torres et al., 1998, 1999; Bohrmann et al., 2000; Linke et al., 2001). In addition, a high-resolution 3-D seismic survey was conducted from 19 June to 3 July 2000 as a site survey for Leg 204 (Tréhu and Bangs, 2001; Tréhu et al., 2002).
Side-scan data, seafloor camera tows, and diving with the manned submersible Alvin and various ROVs demonstrated the presence of extensive massive carbonate pavement on the northern summit of Hydrate Ridge (Carson et al., 1994; Clague et al., 2001; Sample and Kopf, 1995; Bohrmann et al., 1998; Greinert et al., 2001). Until recently, massive authigenic carbonate pavement was thought to be absent on the southern summit of Hydrate Ridge. During Alvin dives in 1999, however, a 50-m-high carbonate spire (the "Pinnacle") (Figs. F1C, F4A) was discovered ~250 m southwest of the summit (Torres et al., 1999). Deep-towed side-scan data indicate that the Pinnacle is located in the center of a buried carbonate apron with a diameter of ~250 m (Johnson et al., in press). Authigenic carbonates on the Cascadia margin form within the sediments and at the seafloor as a result of oxidation of isotopically light methane. The relative absence of carbonate on the southern summit of Hydrate Ridge was interpreted to indicate that this gas hydrate system is younger than that on the northern summit, providing a spatial proxy for temporal evolution of hydrate-bearing accretionary ridges (Tréhu et al., 1999). This interpretation is supported by U-Th dating of recovered carbonates (Teichert et al., in press), which indicates that the Pinnacle is at least 11,400 yr old, whereas the carbonate chemoherms on northern Hydrate Ridge are up to 71,000 yr old.
One especially interesting feature of southern Hydrate Ridge is the abundance of massive hydrate at the seafloor near its summit. This was first discovered in 1996, when >50 kg of massive hydrate was recovered with a television-guided grab sample (Bohrmann et al., 1998). The samples show dense interfingering of gas hydrate with soft sediment (Fig. F4B). In most cases, pure white hydrate is present in layers several millimeters to several centimeters thick. Host sediment is often present as small clasts within the pure gas hydrate matrix. On a macroscopic scale, the gas hydrate fabric varies from highly porous, with pores of up to 5 cm in diameter, to massive (Suess et al., 2001). Thin sections show a structure in which gas bubbles have been filled with hydrate. Wet bulk densities of 80 hydrate samples measured on board the Sonne range from 0.35 to 0.75 g/cm3 (Suess et al., 2002). Pore space was estimated from the change in sample volume before and after compression to ~160 kbar. The samples show high variability in pore volumes ranging from 10% to 70%, and the values are negatively correlated with sample density. From this correlation, the end-member density at zero porosity was estimated to be ~0.79 g/cm3. This value is lower than the theoretical density of pure methane hydrate (0.91 g/cm3). Field-emission scanning electron microscopy indicates that this is a result of submicrometer porosity of the massive hydrate (Suess et al., 2002). The low bulk density of the natural methane hydrates from Hydrate Ridge results in a strong positive buoyancy force, implying that the massive gas hydrate remains on the seafloor only because of the shear strength of the host sediment. The unusual seafloor topography observed on southern Hydrate Ridge during Alvin and ROPOS surveys, which is characterized by mounds and depressions with a wavelength of a few meters (Fig. F4C), may result from spontaneous breaking off of hydrate from the seafloor. This may be an important mechanism for transporting methane from the seafloor to the atmosphere (Suess et al., 2001). An important objective of Leg 204 was to determine the depth to which massive gas hydrate is present.
Vigorous streams of methane bubbles have been observed emanating from vents on the seafloor on the northern and summit regions of Hydrate Ridge (Suess and Borhmann, 1997; Suess et al., 1999; Torres et al., 1998, 1999, 2002; Heeschen et al., 2003) as well as from a similar, but smaller, reflective high in the accretionary complex known as Southeast Knoll (Figs. F1B, F3B). Because the seafloor at all three sites is well within the gas hydrate stability zone (GHSZ) (Fig. F3A), the presence of methane bubbles beneath and at the seafloor suggests rapid transport of methane to the seafloor from sediments beneath the GHSZ. Another objective of Leg 204 was to determine the mechanism whereby free gas migrates through the GHSZ to reach the water column.
Communities of tube worms, bacterial mats, clams, and other fauna are associated with seafloor hydrates and methane vents on Hydrate Ridge (Fig. F4D) and elsewhere (e.g., Kulm et al., 1986; MacDonald et al., 1989; Suess et al., 1999, 2001; Sassen et al., 2001). Chemosynthetic microorganisms, some of which are symbionts within larger fauna, are at the base of the food chain in these communities. Recent work suggests complex complementary relationships between sulfate-reducing, methanogenic, and methanotrophic microorganisms in hydrate-bearing sediments (e.g., Parkes et al., 2000; Boetius et al., 2000). These microorganisms are the kinetic controls on low-temperature methane formation and oxidation and are, therefore, a critical component of the hydrate system. Identification of these organisms and determination of their abundances, spatial variability, and activity are just beginning.
Particularly interesting are recently discovered organisms that play a critical role in anaerobic methane oxidation (AMO). Methane oxidation generates isotopically light dissolved inorganic carbon and results in the formation of authigenic carbonates. These carbonates remain in the geologic record as evidence of past fluid flow and hydrate formation and dissociation (e.g., Sample and Kopf, 1995; Bohrmann et al., 1998; Greinert et al., 2001). Very high rates of AMO have been measured in sediment overlying massive gas hydrates on southern Hydrate Ridge (Boetius et al., 2000) and attributed to structured aggregates consisting of a central cluster of methanotrophic Archaea surrounded by sulfate-reducing bacteria. That microbes oxidize methane by utilizing sulfate in the absence of oxygen was long suspected by geochemists based on interstitial sulfate and methane gradients (e.g., Claypool and Kaplan, 1974). Borowski et al. (1996) showed that steep sulfate gradients and shallow depths to the sulfate/methane interface (SMI) are a consequence of the increased influence of AMO, and Boetius et al. (2000) were the first to observe the microorganisms that presumably catalyze AMO. These microbial aggregates appear to be abundant in sediments of Hydrate Ridge and mediate AMO when enough sulfate is available.
Analysis of sulfide minerals provides a possible opportunity to reconstruct past biological activity because most of the reduced sulfide produced during microbial sulfate reduction is ultimately sequestered in various iron phases, which usually involves multiple steps terminating in the formation of sedimentary pyrite. The burial of these mineral phases contributes significantly to the oxygen level of the atmosphere, the sulfate concentration in seawater, and the pH of the oceans over geologic timescales (e.g., Garrels and Perry, 1974; Holland, 1978; Boudreau and Canfield, 1993). Anomalous intervals of high greigite content have been reported in intervals from which gas hydrates were recovered or were inferred to exist (Housen and Musgrave, 1996). Based on the rock magnetic properties at Site 889, Housen and Musgrave (1996) inferred the presence of a "fossil gas hydrate zone," which may have extended downward to 295 mbsf during the last glaciation. Data acquired during Leg 204 will be used in several shore-based studies to further understand the relationships between bacterial activity and sulfide mineralogy.