T in Fig F13) lower than in the adjacent sediments.
In this section, we describe and discuss the primary results at each site. Relationships among sites are also discussed using figures that compare data from several sites. In general, sites fall into two primary groupings. Sites 1245, 1247, 1248, 1249, and 1250 form a north-south transect that documents the evolution of the southern Hydrate Ridge (SHR) gas hydrate system from the flank to the summit and explores the role of Horizon A. Sites 1245, 1246, 1244, 1252, and 1251 form an east-west transect that compares the west and east flanks of SHR to the adjacent slope basin. Lithology at all sites is similar, with abundant turbidites, some debris flows, and several notable ash layers (Fig. F9). These ash layers are responsible for the largest-amplitude seismic reflections and play a major role in focusing fluid flow and hydrate distribution in this system.
Site 1244 (proposed Site HR1a) is located in 890 m of water on the eastern flank of Hydrate Ridge, ~3 km northeast of the southern summit (Fig. F1). The 3-D seismic data available from a Leg 204 site survey show that the BSR is present at a depth of ~125 mbsf at this site (Fig. F5). The temperature and pressure at the seafloor are well within the GHSZ (Fig. F3), indicating that gas hydrates can exist within the entire stratigraphic section above the BSR if hydrate-forming gases are available in concentrations that exceed their in situ solubility. The 3-D seismic data also image a zone of incoherent seismic reflections that forms the core of Hydrate Ridge. At Site 1244, the top of this incoherent zone is located at a depth of ~300 mbsf. This facies has been interpreted to comprise fractured accretionary complex material. Dipping, faulted, and strongly reflective strata interpreted to be an uplifted and deformed slope basin overlie this facies.
The primary drilling objectives at this site were to (1) determine the distribution and concentrations of gas hydrate within the GHSZ; (2) determine the nature of a pair of strong reflections (referred to as B and B') that underlie much of the eastern flank of Hydrate Ridge; (3) determine the composition, structure, and fluid regime within the seismically incoherent unit underlying the stratified sediments; and (4) sample the subsurface biosphere associated with these features.
Five holes were cored at Site 1244, and an additional hole was drilled (Table T1). Hole 1244A was abandoned when the first core overshot and did not record a mudline. Hole 1244B was abandoned at 53.1 mbsf after six cores were obtained because the BHA had to be brought to the surface to retrieve a downhole instrument (Fugro piezoprobe) that had become unscrewed from the Schlumberger conductor cable. One APCT measurement was taken at 35.1 mbsf in this hole. Hole 1244C, which comprises 39 cores, began at the seafloor and continued to 334 mbsf. Special tools used in Hole 1244C included three APCT (at 63, 82, and 110 mbsf), one DVTP (at 64 mbsf), one DVTPP (at 150 mbsf), and three PCS (at 120, 131, and 142 mbsf) runs. Hole 1244C was abandoned 17 m above the target depth of 350 mbsf, when hole conditions suggested that a change from XCB to RCB coring would be appropriate. Examination of the core and the initial chemical data from this depth suggested that we had reached the deepest target (i.e., the accretionary complex). We had, thus, fulfilled the PPSP requirement that we core the primary facies we expected to encounter during LWD prior to proceeding with LWD at all sites. We returned later in the leg to drill Hole 1244D, which was dedicated to wireline and seismic work, to 380 mbsf. This was followed by a Hole 1244E, which was cored to 136 mbsf and extensively sampled for geochemistry, gas hydrates, and microbiology, and Hole 1244F, which was cored to 24 mbsf primarily for high-resolution microbiological sampling.
On the basis of visual observations, smear slides, and correlation with physical property data (especially magnetic susceptibility [MS]), the sedimentary sequence can be divided into three primary lithostratigraphic units, with three subdivisions in the second unit. Lithostratigraphic Units I (from the seafloor to 69 mbsf) and II (69–245 mbsf) are both characterized by hemipelagic clay interlayered with turbidites, with thicker, coarser turbidites common in lithostratigraphic Unit II. Individual turbidites are characterized by sand and silt layers that fine upward to bioturbated sulfide-rich silty clay and clay. The turbidites are particularly well developed in the interval from 160 to 230 mbsf. A 60-cm-thick layer at 216 mbsf that is especially rich in detrital volcanic ash shards corresponds to a strong regional seismic reflection referred to here as Reflection B' (Fig. F5).
The lithology changes to more indurated and fractured claystone interbedded with glauconite-bearing to glauconite-rich silts and sands below 245 mbsf, lithologies referred to as Unit III. The boundary between lithostratigraphic Units II and III corresponds to the top of the seismically incoherent zone that underlies the slope basins (Fig. F5) and was interpreted to represent highly deformed sediments of the accretionary complex.
Biogenic components vary downcore, with a predominance of siliceous microfossils. Biostratigraphic boundaries based on diatoms correlate fairly well with lithostratigraphic unit boundaries and with seismic stratigraphic boundaries identified in the 3-D seismic data, although there are some inconsistencies among these three data sets when comparisons are made among sites. Sediments immediately above a regional unconformity at ~240 mbsf (approximately the boundary between lithostratigraphic Units II and III) yield diatoms that indicate the age to be younger than 1.6 Ma. Sediment immediately below the uncomformity yields nannofossils that indicate the age to be older than 1.7 Ma. This unconformity is also sampled at Site 1251 at 300 mbsf and at Site 1252 at 130 mbsf (Fig. F10). Lithostratigraphic Unit III is older than 1.7 Ma.
Physical property data are generally consistent with the lithostratigraphic, biostratigraphic, and seismic stratigraphic boundaries. The boundary between lithostratigraphic Units I and II is marked by a localized decrease in wet bulk density. As mentioned above, the turbidites of lithostratigraphic Unit II are particularly well developed in the interval from 160 to 230 mbsf. This interval is characterized by high values of whole-core MS. The widest and strongest MS peak, at 168 mbsf, correlates with the seismic reflector known as Horizon B. This horizon is also coincident with an increase in wet bulk density. There is also excellent correlation between moisture and density (MAD) measurements on core samples and measurements of density and porosity obtained via LWD.
One novel aspect of Leg 204 is the regular use of both hand held and track-mounted IR cameras to image all cores. Cores from within the GHSZ were imaged several times by the physical property scientists. The hand held IR camera proved to be very effective for rapid identification of the location of hydrate specimens within the cores (Fig. F12). Gas hydrate samples were recovered as whole rounds in Cores 204-1244C-8H and 10H (samples from 63, 68, and 84 mbsf) and preserved for detailed shore-based studies. A few pieces were dissociated for chemical analysis (discussed below). In all three cases, the hydrate was present as layers or nodules several millimeters to 1.5 cm thick, aligned at an angle of 45°–60° to the core liner, suggesting formation along steeply dipping fractures.
The track-mounted IR camera imaged the cores systematically, and these records were used to confirm the presence of hydrates spotted by the hand held cameras to develop techniques for detecting more subtle signatures of disseminated hydrate and to track the temporal evolution of the thermal signature of hydrate dissociation. The IR thermal imaging of the cores on the catwalk indicated the presence of numerous nodular and/or disseminated hydrates extending from ~45 mbsf to the BSR at 124 mbsf. The presence of these are shown in Figure F13 as temperature anomalies in which local temperature along the core is 1°–7° (T in Fig F13) lower than in the adjacent sediments.
The LWD data obtained at this site are of excellent quality and provide spectacular images of electrical resistivity within the borehole (Fig. F13). High-amplitude variable resistivity from 40 to 130 mbsf (Fig. F11) suggests the presence of hydrate and correlates well with the depth range of the IR temperature anomalies and with geochemical indicators of hydrate presence discussed below. We note that this is the only site at which the LWD data were acquired after coring. At other sites, the pattern of high-amplitude variable resistivity was used as a predictor the presence of hydrate prior to coring. Sinusoidal patterns in the resistivity images of the borehole wall suggest that gas hydrate is concentrated in steeply dipping fractures as well as along bedding planes (Fig. F14). The data also show strong borehole breakouts in Unit III, which are indicative of a northeast-southwest–oriented axis of least compressive stress.
Geochemical analysis of interstitial waters has revealed that depth variations in the concentration of several different chemical species correlate with the hydrate stability zone. The most direct correlation is seen in Cl concentrations. Above the first occurrence of hydrate (from the seafloor to ~45 mbsf), Cl concentration in the pore water is similar to that in seawater (Fig. F14). Between 45 mbsf and the BSR at ~125 mbsf, there are numerous low Cl spikes that likely reflect the freshening effect of dissociated hydrate on the interstitial waters. Correlation of Cl data with the IR camera data indicates that the spatial sampling of Cl anomalies is biased, which smooths estimates of hydrate concentration based on Cl anomalies. Experiments to better quantify this bias were conducted at several other sites. Nevertheless, these data can be used to obtain a rough estimate of hydrate concentration if a "no hydrate" background concentration can be estimated. At Site 1244, Cl concentrations from the BSR to 300 mbsf decrease linearly at a rate of ~0.35 mM/m. This suggests a diffusive gradient between seawater and low Cl fluids in the accretionary complex. The reduced chloride concentration at depth may reflect dehydration of clay minerals deeper in the accretionary complex. Considering uncertainties in the background concentration of Cl, we estimate that 2%–8% of the pore space is occupied by hydrate, with locally higher and lower concentrations. The Cl concentration profile within the deepest incoherent seismic facies is approximately constant, suggesting a zone of fluid advection and mixing consistent with LWD, physical properties, and core observations, all of which suggest a pervasively fractured medium.
The methane/ethane (C1/C2) ratio also shows a clear correlation with the presence of gas hydrate (Fig. F15). Gases obtained by the headspace technique and by sampling void space in the cores show a steplike decrease in the C1/C2 ratio at the BSR. This was observed to a varying degree at all sites and will be discussed further in the site summary for Site 1251. Slightly lower C1/C2 ratios are observed in gas obtained by dissociating discrete hydrate samples (Fig. F15), suggesting some fractionation of C2 into hydrate.
After the first two cores, the cores were pervasively cracked and contained many voids, both of which are indications of degassing during recovery. The PCS offers the only opportunity to measure in situ methane concentrations directly. At Site 1244, in situ methane concentrations are below the solubility predicted for in situ conditions at depths of 24, 40, 120, and 131 mbsf and are above predicted solubility at 72 and 103 (Fig. F17). These are the first in situ gas concentrations obtained from Hydrate Ridge. They corroborate the relatively low estimates of gas hydrate concentration obtained from the chloride data.
The downhole temperature measurements (including the average of waterline temperatures) were used to define a linear temperature gradient of 0.0575°C/m (Fig. F18), very similar to the temperature gradient determined at ODP Site 892 during Leg 146 (Shipboard Scientific Party, 1994). This temperature gradient predicts that the BSR should be at a depth of 135 mbsf, based on the pure methane and seawater stability curve and seismic velocities obtained from the 2000 3-D OBS. The apparent ~10-m mismatch between the BSR depth determined from the seismic data and that calculated from the observed temperature gradient is probably within the uncertainty of the data.
Seismic data at this site include high-frequency sonic log data and lower-frequency vertical, offset, and walk-away seismic profiles. Preliminary data from automatic picks of the VSP indicate that the velocities used for initial mapping of the 3-D seismic reflection data from TWT to depth were quite close (Fig. F19). The data also indicate that positive velocity anomalies resulting from hydrate presence above the BSR and low-velocity anomalies resulting from free gas below the BSR are small and local.
The pore pressure dissipation measurement made by the DVTPP follows the expected pattern, but detailed analyses to determine whether in situ pressure departs from hydrostatic pressure awaits postcruise study.
Samples were taken to support a range of shore-based microbiological studies, but there are no results to report at this time. Measurements of sulfate concentration in the interstitial waters, which indicate that the SMI is present at 8 mbsf at this site, were used to guide high-resolution sampling for microbiological studies.
Site 1244, which is the first site that was logged and cored during Leg 204, provides strong evidence that gas hydrates are common within the hydrate stability zone on the Oregon continental margin and that they are a major factor influencing the biogeochemical evolution of the margin. It is also clear that the integration of geophysical remote sensing data such as 3-D seismic reflection surveying, LWD, and IR thermal scanning provides a reliable road map to guide further sampling and analysis.