Fig. 1. (A) Bathymetric map of the accretionary complex offshore Oregon. Contour interval is 100 m. Red dot shows the location of ODP Site 892, drilled during Leg 146. Box shows the location of (B). Transparent violet overlay shows where a BSR is present in seismic data. Inset shows the tectonic setting of (A). Cascade volcanoes are shown as triangles. SHR—South Hydrate Ridge; NHR—North Hydrate Ridge; SEK—Southeast Knoll; OR—Oregon; WA—Washington; JdF—Juan de Fuca plate; Pa—Pacific plate; JdFR—Juan de Fuca ridge; CSZ—Cascadia subduction zone. (B) High-resolution (15m pixel) bathymetric map [42] of the region studied during ODP Leg 204. Contour interval is 20 m. Red dots show the location of sites drilled during Leg 204. Several holes were drilled at each site. Dashed red lines show locations of vertical slices through the 3D seismic data shown in Fig. 2. Transparent color overlays show the lateral extent of zones of different gas hydrate content, estimating by averaging the data from the seafloor to the BSR (Table 1 and Table 2). The dark violet and dark green patterns outline regions in the data suggest that gas hydrate occurs in patchy zones of locally higher concentration, leading to high variability among estimates of gas hydrate content in boreholes spaced tens of meters apart. The inset shows the seafloor acoustic backscatter pattern at the summit of southern Hydrate Ridge [17] with light colors indicating high backscatter. The 800 m depth contour is shown for reference. The dark spot in the center of the region of high backscatter is the shadow of a carbonate pinnacle. Observations made with the DSV Alvin indicate that the very strong seafloor reflectivity around the pinnacle results from carbonate pavement (possibly mixed with gas hydrate) whereas the mottled reflective pattern results from gas hydrate at or near the seafloor. A larger version of this image, and its correlation with the seismic data, is found in [20].

Fig. 2. Profiles extracted from the 3D seismic data along the lines shown in Fig. 1B. Black—strong positive amplitude; gray—positive amplitude; white—strong negative amplitude. Transparent overlays indicate zones of different average gas hydrate concentration estimated as discussed in the text. Although overlay colors are the same as in Fig. 1B, values of gas hydrate content in these zones are larger than in Fig. 1B, because here the values represent averages over the gas hydrate occurrence zone rather than over the gas hydrate stability zone. Vertical red bars show sites drilled during Leg 204; tick marks are spaced 75 m apart. AC—top of the highly deformed sediments of the accretionary complex; B—seismic horizon B, which was found to be coarse-grained and gas hydrate-rich at Site 1246; A—seismic horizon A, interpreted as a stratigraphically controlled zone along which methane-rich fluids migrate from the accretionary complex to the southern summit [14 and 15].

Fig. 3. (A, left) Gamma density profiles of a HYACINTH pressure core as pressure was released. See text for further discussion. (Right) IR image of several meters of core on either side of the HYACINTH pressure core. Note that the meter of core immediately following a PCS or HYACINTH core is not recovered during normal pressure coring operations. The approximate thickness and spacing of pore water samples is also shown to illustrate the relationship between the scale length of apparent gas hydrate distribution relative to pressure core and pore water samples. (B) Two examples of IR temperature anomalies (left) and gas hydrate found at that place when the core was split (right). Both samples were from Site 1244. The upper sample (1244C-08H-1) was recovered from a depth of 64 mbsf. The lower sample (1244C-10-2) was recovered from a depth of 84 mbsf.

Fig. 4. (A) Cl⁻ concentration measured in Hole 1244C. The empirical baseline used for this study and a seawater baseline are shown. The gas hydrate content of the sediments determined assuming the empirical baseline is also shown. Uncertainties due to uncertainties in the empirical baseline are discussed in the Leg 204 Initial Report [20]. The seawater baseline would lead to a thick zone of significantly higher gas hydrate concentration in the lower portion of the GHSZ. We prefer the empirical baseline because chemical characteristics of the observed Cl⁻ concentration suggest a source of fresh water at depth [20]. (B) Temperature of cores recovered from Hole 1244C measured by scanning cores on the catwalk with an IR thermal camera. Temperature profiles were extracted from the thermal images by averaging across the central portion of the image. Cold “spikes” attributed to gas hydrate dissociation stand out from this complicated temperature structure, which depends on may factors, including the geothermal gradient, the position within a core, the ambient temperature on the catwalk, and the type of coring tool used. We note that cores obtained with the extended core barrel (XCB) cores are significantly cooler than cores obtained with the advanced piston corer (APC), probably due to the cooling effect of drilling fluid.

Fig. 5. (A) The temperature profile derived from an IR image that showed a cold spot (from Core 1245C-7H-5). ΔT and ΔL are indicated. (B) Chloride concentrations in closely spaced pore water samples along the core corresponding to the temperature profile in (A). The apparent offset in depth between the two graphs is due to compression of the core to close gas expansion voids between the time when the IR image was made and the pore water samples were taken. This can result in offsets of as much as 1 m between depths recorded on the track-mounted IR temperature scans and depths logged for core samples.

Fig. 6. Comparison of ΔT anomalies (blue or green bars) to gas hydrate content estimated from dilution of Cl⁻ concentration (red lines) and from degassing of pressure core samplers (green stars, except in Fig. 4C, where stars are red). (A) Site 1245. Expanded section shows lateral variation around the circumference of the borehole from 80 to 100 mbsf estimated from RAB data in Hole 1250B. Resistivity scans were calculated at 56 evenly spaced azimuths around the 10-in. diameter borehole. Gas hydrate content at each position was calculated [31] using average porosity from the LWD density log, grain density and pore water salinity from shipboard measurements, and the geothermal gradient from the in situ temperature tool [20]. Maximum calculated concentration is 83% for a nodule near 93 mbsf. Average concentration in this interval calculated from the RAB data is 8%, similar to that determined from IR and Cl⁻ data. (B) Site 1246. Expanded section shows the IR temperature anomaly associated with the lower of two coarse-grained layers comprising Horizon B. The picked (ΔT, ΔL) curve (red) is overlain on the temperature profile extracted from the IR images (black). A stratigraphic summary is shown on the right. The onset of the IR anomaly is associated with the top of coarse-grained layer. Unfortunately most of this layer was taken for microbiology samples and its internal structure could not be documented by Leg 204 shipboard sedimentologists. A pore water sample from this zone indicated that hydrate occupied 22% of the pore space. Integration of (ΔT, ΔL) over this zone indicates 28% gas hydrate. (C) Sites 1249 and 1250. (D) Site 1251. Expanded section shows details of the IR thermal anomaly and closely spaced Cl⁻ concentration measurements from Hole 1251D immediately above the BSR.

Fig. 7. Correlation between seafloor and subsurface reflectivity near the summit. Upper panel shows seafloor reflectivity from deep-towed sidescan sonar data [17]. Lines indicate location of two seismic sections extracted from the 3D seismic reflection volume. “x” shows the locations of ODP sites. See Fig. 1B for site labels. Numbers on the sidescan sonar and seismic images indicate collocated points. The highest seafloor reflectivity is associated with the carbonate pinnacle. Mottled reflectivity, both on the seafloor and in the subsurface, is associated with the presence of massive hydrate.

Table 1. Spacing and amplitude of IR temperature anomalies and hydrate content of sediments expressed as percent of pore space from IR and other data

(1) Sites are designated by numbers; holes at a site are designated by letters.

(2) Corrected for % recovery.

(3) Average gas hydrate content from IR data was calculated as discussed in the text. Results are shown for integration over the gas hydrate occurrence zone (GHOZ), defined as the depth range over which indicators for gas hydrate were observed, and the gas hydrate stability zone (GHSZ), defined as the region from the seafloor to the BSR. At Site 1252, the projected depth of the BSR was used. For the summit region, the GHOZ and the GHSZ are the same because indicators of gas hydrate presence extend from the BSR to the seafloor. At these sites, the gas hydrate content of the shallow zone of massive hydrate is given separately.

(4) Each number represents the average of one to three samples spaced at unequal intervals. Each sample averages gas hydrate content over 1 m. No PCS data were acquired in Holes 1245B, 124B, 1248C or 1252A. Measurement uncertainties are discussed elsewhere [2].

(5) These estimates represent averages of 15–30 samples/hole spaced at unequal intervals. Each sample averages gas hydrate content over 5–10 cm of core length. Uncertainties due to uncertainties in the baseline are estimated to be ±0.5%. b.d.: indicates that any hydrate present was below the detection level of this technique. n.b.: indicates that no baseline could be estimated because of anomalously high Cl⁻ concentration.

(6) Averages were calculated from pore water saturation estimates based on electrical resistivity data and tabulated at 15-cm intervals and include hundreds of data points. These data require no correction for core recovery. No RAB data were acquired at Site 1252. Note that all RAB data are from the “A” hole at a given site, except from Site 1244, where they are from the “B” hole. Holes characterized by RAB data are 50 m away from other holes at that site.

(7) All measurements in the GHOZ or GHSZ were included in the average, including data points indicating no gas hydrate.

(8) 88 mbsf is the depth at the bottom of this hole, which is 24 m above the BSR.

Table 2. Mean and standard deviation of gas hydrate content in the pore space in regions with different seismic reflection characteristics

The average for each hole was taken from Table 1. Only IR and RAB data are included because those estimates are based on hundreds of data points that sample most of the GHSZ. Inclusion of estimates from Cl⁻ and PCS data, weighted by the fraction of the GHSZ sampled by those measurements (<3% in for all holes), would not affect these estimates. Data for the summit region were not included in this table because of the large uncertainty in the IR data due to poor recovery and the probable errors due to the presence of free gas in RAB-based estimates at Site 1249.