Figure F1. A. Tectonic setting of Leg 204 in the accretionary complex of the Cascadia subduction zone. The box shows region of Figure F1B. B. Bathymetric map in the region of Hydrate Ridge. The box shows location of Figure F1C. Locations of ODP Site 892, seismic Line L2-89 (shown in Fig. F2), north Hydrate Ridge (NHR), south Hydrate Ridge (SHR), and Southeast Knoll (SEK) are also shown. C. Detailed bathymetric map of the region of Leg 204. Leg 204 sites are shown along with their site numbers (e.g., Site 1244) and precruise designation (e.g., HR1a). Bathymetry from Clague et al. (2001).
Figure F2. A. Schematic line drawing of the crustal structure across Hydrate Ridge based on depth-converted migrated seismic reflection data (from Westbrook et al., 1994). Interpretation is based on Line 9 from the 1989 ODP site survey (MacKay et al., 1992), along which ODP Site 892 was located. On the scale shown here, primary structural features are the same as those along Line 2, which is shown in B. B. Line 2 from the 1989 ODP site survey showing primary structural features of the deformation front and the location of data shown in Figure F5.
Figure F3. A. Temperature profiles from conductivity/temperature/depth recorder over southern Hydrate Ridge. The stability boundaries for (A) pure methane hydrate and (B) a mixture of 97.5% methane and 2.5% hydrogen sulfide are shown as are the water depths of Leg 204 sites. B. Echo sounder records (12 kHz) from the region of Hydrate Ridge showing bubbles in the water column.
Figure F4. A. A carbonate sample from the top of the Pinnacle showing the porous nature of carbonates from this environment. B. A sample of sediment from near the seafloor at southern Hydrate Ridge showing hydrate lenses parallel to bedding connected to hydrate veins perpendicular to bedding. C. Landscape at southern Hydrate Ridge showing mounds covered by bacterial mats. A clam colony is seen in the right edge of the picture taken during an Alvin dive. D. Illustration of the complex biogeochemical relationships expected near the southern summit of Hydrate Ridge.
Figure F5. East-west vertical slice through the 3-D seismic data showing the stratigraphic and structural setting of Sites 1244, 1245, 1246, and 1252. Seismic reflections A, B, B', Y, and Y' are anomalously bright stratigraphic events and are discussed further in the text. Seismic reflection AC is the top of the seismically incoherent core of Hydrate Ridge, interpreted to represent older highly deformed rocks of the accretionary complex. The depth scale in meters is shown on the left, assuming a velocity of 1550 m/s above 150 mbsf and 1650 m/s below 150 mbsf. The length of the lines representing sites indicates maximum depth of penetration at that site, and horizontal ticks are located 75 m apart.
Figure F6. Panels above, left show north-southtrending vertical slices from the 3-D seismic data volume that extend from the western flank to the summit (see map above, right). Sites drilled during Leg 204 are shown. From left, lower panels are maps of the depth of Horizon A beneath the BSR and the seafloor and of the amplitude of Horizon A. These maps show that changes in the amplitude of Horizon A are correlated with the depth of Horizon A beneath the sea surface (pressure) rather than depth beneath the seafloor (primarily temperature), suggesting that the onset of very strong reflectivity may indicate the onset of gas exsolution within Horizon A. Other labels as in Figure F5. The length of the lines representing sites indicates maximum depth of penetration at that site, and horizontal ticks are located 75 m apart.
Figure F7. Seismic details near the summit of southern Hydrate Ridge in the vicinity of Sites 1248, 1249, and 1250. Seafloor reflectivity is also shown (from Johnson and Goldfinger, pers. comm., 2002).
Figure F8. East-westtrending vertical slices through the seismic data around Sites 1251 and 1252. The slice for Site 1252 overlaps and extends through the slice shown in Figure F5. The slice for Site 1251 is offset to the south. Sites 1251 and 1252 are on the east and west flanks of anticline B, respectively. This anticline appears to have been active throughout the depositional history of the slope basin on the east flank of Hydrate Ridge, which included an unconformity (U) and two apparent massive debris flows (DF1 and DF2), which can be traced throughout the slope basin. Reflectionary accretionary complex is an unconformity that marks the top of older (>1.6 Ma) indurated and fractured sediments of the accretionary complex.
Figure F9. Examples of some of the characteristic lithologic features observed in cores from Leg 204. A. Turbidites. B. Debris flows. C. Volcanic ash and glass horizons.
Figure F10. Summary of biostratigraphic and lithologic observations. Seismic correlation of Horizons A, B, B', and Y and of unconformities (U) and the accretionary complex are also shown (see Figs. F5, F6, F8). See text for more detailed descriptions of the characteristics of each lithologic unit. Note that Unit III along the north-south transect is not correlative with Unit III on the east-west transect. Correlation between these two transects is tentative. A comparison between the age constraints shown here and the geometric relationships shown in Figure F5 suggests that Unit III at Site 1245 was deposited very rapidly at the base of the continental slope (i.e., in the trench) and, thus, represents a recent addition to the accretionary complex. Unit II was deposited in a very active tectonic environment on the lower slope and varies laterally in both thickness and lithology. Thorough integration of the seismic, biostratigraphic, and lithologic data is an ongoing postcruise project.
Figure F11. Correlation between 3-D seismic data, density, and resistivity measured downhole by LWD (gamma density and magnetic susceptibility measured by MST on cores) and lithology at Sites 1244 and 1246. Seismic data were converted to depth assuming a velocity of 1550 m/s above the BSR and 1700 m/s below the BSR, as suggested by 3-D velocity tomography (Arsenault et al., 2001). VSPs confirmed that the velocities from the tomography study are quite close to in situ velocities (Fig. F19). Seismic Horizon B corresponds to a double-peaked, high-density, high-magnetic susceptibility zone. Lithologic observations indicate that these anomalies results from ash-rich layers that contain a relatively high concentration of gas hydrate.
Figure F12. Comparison of IR images and hydrate samples extracted from the core liner. Outer core liner diameter in the IR images is 71.5 mm. Inner core diameter in the photographs is 66 mm.
Figure F13. Relative borehole resistivity as imaged by LWD (golden/brown columns) compared to the presence of low temperature anomalies in recovered cores as imaged by IR camera scans (blue spikes). The resistivity data show a clockwise scan from north of the borehole wall. Light shades represent higher resistivity. Horizontal bands are parallel to strata, whereas S-shaped bands represent steeply dipping structures. IR anomalies represent the difference between local and background temperature. Background temperature was determined by eye and can be due to many factors, including air temperature on the catwalk and coring method. The base of the GHSZ (BGHSZ) is defined by the BSR, Horizon A, and the top of accretionary complex as defined by increased fracturing and variable density in the LWD data are also shown.
Figure F14. Comparison of the resistivity structure between Sites 1244 and 1248, showing the difference between a site where hydrate fill is primarily along steeply dipping fractures and a site where hydrate fill is primarily along bedding planes.
Figure F15. Chloride concentration values measured at all sites. In most cases, samples are spaced at ~5-m intervals (two per core). A. Sites on the western flank of Hydrate Ridge. B. Sites on the eastern flank of Hydrate Ridge. C. Sites near the summit of Hydrate Ridge. BSR depth at each site is shown as a color-coded dashed line.
Figure F16. C1/C2 values measured at all sites from vacutainer samples. The same patterns are shown in headspace samples, although absolute values of C1/C2 are somewhat smaller. Samples are spaced at ~5-m intervals (two per core). A. Sites on the western flank of Hydrate Ridge. B. Sites on the eastern flank of Hydrate Ridge. C. Sites near the summit of Hydrate Ridge. BSR and Horizon A depth at each site is shown as a color-coded dashed line.
Figure F17. Methane concentrations determined from PCS measurements. Approximate phase boundaries are shown between fields where dissolved gas, gas hydrates, and free gas are predicted to be present in the subsurface. Uncertainties (~30%) in the position of these boundaries are a result of a number of factors, including variations in subsurface temperature gradient, gas composition, and pore fluid salinity. Although these boundaries give a first order view of where gas hydrate should be present, data points that fall near boundaries should be interpreted with caution. More detailed analysis of the predicted stability fields for conditions measured during Leg 204 is the subject of postcruise research. Near the summit, all PCS measurements from within the GHSZ indicate the presence of gas hydrate, with the highest concentration near the seafloor. On the flanks and in the basin, hydrate presence within the stability zone is intermittent and the highest concentration is just above the BSR (no PCS measurements are available from the thin hydrate-bearing layer just above the BSR at Site 1251) (see Figs. F14, F18).
Figure F18. Downhole temperature measurements at all sites. Although temperature gradients are probably known to be better than 0.005°C/m, postcruise calibration of downhole temperature tools is needed to determine absolute temperature. Scatter in the data is greater at sites near the summit, perhaps because of the effect of gas hydrate on in situ thermal conductivity and/or to dissociation of gas hydrate as a result of insertion of the probe.
Figure F19. Traveltime picks from vertical seismic profiles. The shallowest point in all profiles is the calculated traveltime to the seafloor. These data confirm the generally very low velocities throughout the GHSZ. Small deflections of the curves that imply a decrease in velocity coincide with the BSR and Horizon A.
Figure F20. A. Sulfate and methane concentration at Site 1245 showing the sulfate-methane interface (SMI). B. Summary of sulfate measurements showing the SMI at all sites, except for Sites 1248, 1249, and 1250 where no sulfate was detected in the uppermost sample.
Figure F21. Density and RAB recorded through seismic Horizon A at Sites 1245 and 1247. These examples show the remarkable consistency in the signature of this horizon over distances of several kilometers. RAB images are static normalized.
Figure F22. A. IR thermal anomalies measured in two different holes at Site 1251. Note that a major thermal anomaly in the 10 m above the BSR observed in Hole 1251D was not observed in Hole 1251B because that interval was not recovered from Hole 1251B (perhaps because of high hydrate concentration in situ). At this site, IR thermal anomalies are generally small and rare compared at most other sites drilled during Leg 204. B. Comparison between chloride anomalies measured from whole rounds and thermal anomalies in Hole 1251D. The IR data suggest two layers of high hydrate concentration immediately above the BSR but do not provide a good constraint on the concentration of hydrate within these layers. Chloride anomalies suggest a hydrate concentration of ~20% of pore space but do not provide a good constraint on the thickness of hydrate-bearing layers because of the small number of samples. Moreover, at this site probable thin hydrate-bearing layers (based on IR and resistivity anomalies) were missed completely geochemical sampling. By calibrating the densely spaced geophysical data using the sparser geochemical data, improved estimates of hydrate distribution and concentration will be obtained.Table of Contents