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doi:10.2204/iodp.proc.311.101.2006 ResultsSite U1325Site U1325 (proposed Site CAS-02C) is located near the southwestern end of the margin-perpendicular transect established during Expedition 311 and is within a major slope basin that developed eastward of the deformation front behind a steep ridge of accreted sediments (Figs. F3, F4). Bathymetry data show that the seafloor in the western part of this slope basin is relatively flat. Around Site U1325, the seafloor (~1200 m) becomes gradually shallower before it rises rapidly to the east where it forms the plateau of the second main accreted ridge, on which Sites U1327, U1328, and 889 are located. A BSR is clearly visible in the eastern part of the slope basin (Fig. F7), but it fades to the west (common depth points [CDPs] 1180–1280 along MCS Line 89-08). At Site U1325, the BSR is weaker than it is at the core of a buried ridge of accreted sediments located ~700 m west of this site. The BSR appears to be at ~230 ± 5 mbsf based on seismic traveltime-depth conversion, using an average P-wave velocity of 1636 m/s. This average velocity was determined at Site 889 to match observed traveltime depths of the BSR and VSP data (MacKay et al., 1994). We acknowledge that this velocity may not be appropriate in this setting. However, lack of good control in seismic interval velocities from MCS semblance analyses of Lines 89-08 and PGC9902_ODP_7 prohibited accurate depth migration. The BSR also shows the typical frequency-dependent reflection strength pattern (Chapman et al., 2002) with reduced reflector strength in higher-frequency seismic data (see Fig. F4 in the "Site U1325" chapter). The primary research objectives for this site are linked to the transect-concept of the expedition. The objectives include
The slope basin is expected to show a different geochemical regime and related geophysical properties than the uplifted ridges of accreted sediments. A summary of major logging and core data is shown in comparison with two key seismic data sets (coincident but of different seismic frequency) in Figures F8 and F9. Four holes were occupied at Site U1325. Hole U1325A was dedicated to the LWD/MWD program to a total depth (TD) of 350 mbsf (Table T1). Hole U1325B was spudded with the advanced piston corer (APC) system, but problems arose because of thick sand accumulation, causing a switch to the extended core barrel (XCB) for one core and a switch back to APC. The hole was advanced by combination of APC, XCB, and pressure coring to 206.5 mbsf. Interspersed with the XCB cores was a Davis-Villinger Temperature Probe (DVTP) run at 150.2 mbsf, which yielded high-quality temperature data. Five pressure cores were deployed in Hole U1325B, but all attempts failed to retrieve sediments under pressure. The last run with the PCS resulted in a stuck tool, which led to abandonment of the hole. Hole U1325C was drilled to 188.8 mbsf, the depth where core recovery in Hole U1325B began to deteriorate. Coring operations resumed with the XCB system, interspersed with two pressure cores (one failed FPC run and one successful PCS run) and two DVTP deployments, deepening the hole to a TD of 304.3 mbsf. Because of degrading weather conditions, we decided that two separate logging runs would be attempted without calipers to reduce the potential risk of damage to the tool strings. The first run included the phasor Dual Induction Tool (DIT) and the Hostile Environment Gamma Ray Sonde (HNGS). The second run included the Dipole Sonic Imager (DSI), the Scintillation Gamma Ray Tool (SGT), and the Temperature/Acceleration/Pressure (TAP) tool. We were only able to lower the first tool string to 259.8 mbsf (44.5 m shallower than total depth for coring). The hole was successfully logged, followed by some difficulty reentering the drill pipe. The second tool string (sonic without the Formation MicroScanner [FMS]) was deployed but was only able to reach 185.8 mbsf. The available hole was logged successfully. The repeat pass of the sonic tool string reached 183.0 mbsf. To ensure that a mudline core was obtained at this site, Hole U1325D was established with an APC core (4.69 m recovery), which indicated a seafloor depth of 2193.2 meters below sea level (mbsl; 2204.8 meters below rig floor [mbrf]). Analyses of the pore water showed that the first core in Hole U1325C missed the mudline by ~3 m. Examination of diatoms from Holes U1325B and U1325C indicated that the sediments cored at Site U1325 are mostly Quaternary in age. The 304.3 m thick sedimentary section at Site U1325 was divided into four lithostratigraphic units based on visual inspection of the recovered cores and mineralogy data from X-ray diffraction (XRD) analyses of smear slides (Fig. F7). Lithostratigraphic Subunit IA (0–24.00 mbsf in Hole U1325B and 0–4.69 mbsf in Hole U1325D) is characterized by very abundant, thick, coarse-grained sand layers within fine-grained (clay and silty clay) detrital interlayers. The thickness of the sand layers suggests the depositional environment of a distributary channel within the slope basin with mass transport parallel to the uplifted bounding ridges. The sedimentation rate for the upper 129 m of sediments (lithostratigraphic Units I, II, and upper part of Unit III) is estimated at 43 cm/k.y., the highest sedimentation rate calculated for any Expedition 311 site. Lithostratigraphic Subunit IB (24.00–52.23 mbsf in Hole U1325B) is composed of fine-grained (clay to silty clay) detrital sediments with few thin silty/sandy interlayers from turbidites. The occurrence of only marine diatoms suggests that terrigenious sediment flux was somehow restricted. In addition, the occurrence of abundant bioturbation and sulfide mottles suggests more hemipelagic-dominated sedimentation. Mousselike texture related to the presence of gas hydrate is seen at depths as shallow as 42.63 mbsf. The boundary between lithostratigraphic Subunits IA and IB is an unconformity marked by a seismic horizon (unconformity) that can be traced for ~1.5 km around Site U1325 (Fig. F7). It separates undeformed seafloor-parallel slope sediments from underlying deformed and slightly faulted sediments. Lithostratigraphic Unit II (52.23–102.30 mbsf in Hole U1325B) is characterized by fine-grained (clay to silty clay) detrital sediments with intervals of silty/sandy interlayers. We interpret the coarser interlayers as turbidite deposits. Their frequent occurrence might indicate times of active tectonism. The boundary between lithostratigraphic Units I and II is marked by the absence of diatoms, bioturbation, less intense sulfide mottles, and a sharp decrease in the occurrence of sponge spicules. Mousselike texture related to the presence of gas hydrate was also observed in lithostratigraphic Unit II. Lithostratigraphic Unit III (102.30–197.40 mbsf in Hole U1325B and 188.80–198.40 mbsf in Hole U1325C) is characterized by fine-grained (clay to silty clay) detrital sediments with abundant sponge spicules and diatoms. We interpret this interval as mixed hemipelagic-turbidite sediments. The presence of unlithified authigenic carbonate cement suggests that diagenetic processes are active in lithostratigraphic Unit III. Soupy and mousselike sediment textures related to the presence of gas hydrate are present in lithostratigraphic Unit III within the depth interval from ~121 to ~145 mbsf. The boundary between lithostratigraphic Units III and IV is marked by the sudden decrease in diatoms. Lithostratigraphic Unit IV (197.40–206.56 mbsf in Hole U1325B and 198.40–299.17 mbsf in Hole U1325C) is characterized by fine-grained (clay to silty clay) detrital sediments with few silty/sandy interlayers from turbiditic deposits. The poor recovery in this unit limits the observation of sedimentary structures; however, the appearance of several coarser-grained layers indicates low-energy turbidity currents. Mousselike sediment textures were observed to ~242 mbsf. The stratigraphic section logged at Site U1325 was divided into three logging units based on obvious changes in the LWD/MWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements. There is no obvious correlation between the logging units and lithostratigraphic units.
Logging Unit 1 (0–122 mbsf) is characterized by a well-defined gradual increase in density with depth and a corresponding decrease in porosity. This increase in density is matched by a corresponding increase of resistivity with depth, from ~1
Logging Unit 2 (122–250 mbsf) is characterized by a uniform density that averages ~1.9 g/cm3. In contrast, the resistivity logs in Unit 2 show alternating thin intervals of high and low resistivity, spanning the range 1–15
Logging Unit 3 (250–350 mbsf) displays uniform background resistivity (~1 LWD/MWD-derived resistivity-at-the-bit (RAB) images from Hole U1325A suggest that gas hydrate was concentrated in thin sand layers within the interval between 173 and 240 mbsf (logging Unit 2). The LWD/MWD porosity and resistivity logs from Hole U1325A further show that it is a very heterogeneous gas hydrate–bearing section composed of alternating layers of gas hydrate–saturated sands and clay-rich layers with little to no gas hydrate. This interpretation is in general agreement with the marked freshening of the interstitial waters observed in sampled sand layers. There appears to be little correlation between the core-derived physical property data and defined lithostratigraphic or logging units. Sediment density and porosity determined from moisture and density (MAD) analyses show a typical sediment compaction trend with depth. Slight deviations in the density and porosity profiles with depth reflect small-scale lithologic variability and generally match the downhole measured LWD/MWD data. Infrared (IR) imaging of the recovered cores was routinely carried out on the catwalk to detect and characterize the nature of gas hydrate in the cores. A large number of IR-imaged cold spots were detected in the cores from Holes U1325B and U1325C and were partially subsampled for focused interstitial water analyses and microbiology studies. In many cases the IR-imaged cold temperature anomalies correlated with layers of high resistivity and low interstitial water salinities and chloride concentrations, which reflect gas hydrate dissociation during core recovery. Six deployments of temperature tools were attempted at Site U1325, and all six provided usable data. Three APC3 and one DVTP deployment in Hole U1325B, as well as two additional DVTP deployments in Hole U1325C, occurred during times of relatively low heave and yielded high-quality data. A linear fit to the points indicates a thermal gradient of 0.060° ± 0.003°C/m and a seafloor intercept of 3.03° ± 0.55°C. If a hydrostatic pore pressure gradient and seawater salinity are assumed, the temperature data indicate that the base of the GHSZ for this site is 275 ± 25 mbsf. Pressure coring tools were deployed seven times at Site U1325, including two PCS cores, two HRC cores, and one FPC core in Hole U1325B. Hole U1325C included one FPC core from above the projected depth of the BSR and one PCS core from below the predicted depth of the BSR. Pressure coring proved to be extremely difficult at Site U1325. Only the deepest PCS deployment (Core 311-U1325C-10P; 256.5 mbsf) successfully recovered core under pressure, which was investigated by a controlled shipboard degassing experiment. All other attempts to deploy pressure cores failed for various reasons (e.g., the presence of unconsolidated fine sand and the potential effect of adverse ship heave conditions). Degassing of Core 311-U1325C-10P yielded 2.07 L of methane gas and may have contained small amounts of gas hydrate (0.4%) or free gas (0.3%), depending on where the base of gas hydrate stability (BGHS) is situated (see uncertainty in temperature-derived BGHS above). A total of 92 interstitial water samples were processed from two holes cored at Site U1325. The salinity and chlorinity profiles at this site appear to indicate an advective transport system with the occurrence of higher than seawater salinity and chlorinity (~36 and 600 mM, respectively), which likely comes from a deeper source. The chlorinity of this fluid is 7.3% above the modern seawater value, more than twice the interglacial salinity increase (3.5%). The elevated solute concentration may be caused by low-temperature diagenetic hydration reactions in the deeper parts of the basin. The most plausible candidate for such a reaction is alteration of volcanic ash to clay minerals and/or zeolites. In the zone extending from ~70 to 240 mbsf (the BSR is at ~230 ± 5 mbsf), salinity and chlorinity data show discrete excursions to fresher values, indicating that gas hydrate was present in the cores and dissociated prior to processing the samples. Intense microbial activity at Site U1325 results in sulfate depletion, phosphate and alkalinity production, and significant Ca and Mg depletion in the interstitial waters of the upper 3 mbsf. At this site the sulfate/methane interface (SMI) was between 4 and 5 mbsf, based on samples recovered from Hole U1325D (see Fig. F32). Organic geochemical studies at Site U1325 included analysis of the composition of volatile hydrocarbons (C1–C5) and nonhydrocarbon gases (i.e., O2 and N2) from headspace gas samples, void gas samples, and gas samples recovered during the PCS degassing experiment. With the exception of several samples that contained a large percentage of air, the void gas was almost entirely methane with a small percentage of carbon dioxide (~0.1%–0.5%). Trace quantities of C2+ hydrocarbons (<5 ppmv) were present above the predicted BSR depth (230 mbsf). With greater depth, the concentrations of ethane, propane, and isobutane increased but did not exceed 15 ppmv. The molecular ratios of methane to ethane (C1/C2) were the highest observed during Expedition 311. C1/C2 values from where gas voids were first observed (9.1 mbsf) to the predicted BSR depth ranged from ~61,000 to ~173,000 and showed no apparent trend with depth. Decreasing C1/C2 values below the BSR indicate a slight thermogenic or diagenic contribution of ethane. Microbiological subsampling was routinely conducted on cores recovered from Holes U1325B and U1325C. On each core run, perfluorocarbon tracer (PFT) was continuously metered into the drilling fluid and fluorescent microspheres were deployed to investigate potential drilling fluid contamination of the core. These analyses confirmed that the center of each APC core and XCB biscuit remain undisturbed and suitable for microbiological analyses. Additional IR images were taken on the cut ends of each microbiological core section to document the thermal warming process of the core before subsampling. Seismic data (Fig. F7) suggest that the sediments cored at Site U1325 are similar to those seen in a seafloor-parallel package of sediments on the southwest side of the buried accreted ridge, but they have been deformed and tilted with small offset faults associated with the uplift of the underlying ridge. Enhanced advective fluid flow from the buried accreted basin resulted in gas hydrate formation within the overlying deformed section, preferentially in the coarser-grained turbidite sequences. In contrast, the southwest part of the basin has relatively little deformation. The postulated reduced fluid flow and therefore limited gas hydrate accumulation also explains the lack of a strong BSR in that part of the basin. Although the BSR is not very prominent near Site U1325, the stratigraphic section beneath the projected BSR generally shows high-amplitude reflectors and updip amplitude truncations at the BSR, typical of free gas trapped beneath the BSR. Data from downhole logging and coring also reveal lateral homogeneity between the holes at this site, which is a testament to the orientation of the holes along structural strike in a northwest-southeast direction, avoiding the steep southwest-northeast–dipping slope seen in the seismic data. Except for difficulties with pressure coring and temperature deployments, all objectives for Site U1325 were fulfilled. Site U1326Site U1326 (proposed Site CAS-03C) is located on an uplifted ridge of accreted sediments at the southwest end of the multisite transect established during Expedition 311 (Fig. F4). Recently acquired bathymetry data reveal a collapse structure near the originally proposed primary Site CAS-03B, which may have complicated the recent geologic history of this site (Fig. F5). Site CAS-03C was designated as the primary site to avoid coring the slump feature. The headwall of the slump feature near proposed Site CAS-03B is ~250 m high and the slump has eroded a ~2.5 km long section into the ridge. Further examination of the bathymetry data from this area also reveals the occurrence of continuous linear features crossing the ridge in an almost east–west direction. These linear features are clearly associated with faults as seen on Figure F10. The outcrop of these faults at the seafloor generate a surface displacement of as much as 25 m and can be seismically traced from the surface through the sedimentary section to depths below the BSR. The occurrence of these deep throughgoing faults is generally limited to the area of the slump scar. The seismic character of the ridge also changes from the southwest to the northeast across the ridge, with the southwest-facing part of the ridge characterized by strong semicontinuous reflectivity (Figs. F11, F12), whereas the seismic reflectivity disappears underneath the northeast-facing flank of the ridge. The differences in the seismic character across the ridge cannot be easily explained, and it is one of the research focuses of the site. The BSR is present underneath most of the ridge (Fig. F10), but it is almost absent underneath the slump feature in association with the most heavily faulted sediments. The objectives of coring and logging at this site are tied to our goal of understanding processes along the transect of scientific research holes drilled across the northern Cascadia margin. Site U1326 is the closest location to the deformation front. It probably represents the tectonically youngest occurrence of gas hydrate on the northern Cascadia margin and as such, our primary research objectives include
The operational plan to achieve these objectives was based on a general three-hole concept, which included an LWD/MWD logging hole, a continuous core hole to characterize geochemical and microbiological processes, including generation of proxies for the occurrence gas hydrate, and an additional "tool" hole for specialized pressure coring and selected spot-coring using conventional systems. The special tools hole was also intended to be logged with the conventional IODP wireline logging tool strings. After transiting from Site U1325, the LWD/MWD bottom-hole assembly (BHA) was tripped to the seafloor. Hole U1326A was spudded at 1445 h on 22 September 2005 at an estimated seafloor tag depth of 1828.1 mbsl (1839.0 mbrf). To avoid blowing out the top of the hole, we adopted a controlled-spud program that featured reduced drilling fluid circulation rates of only 100 gpm and a bit rotation rate of 10–15 rpm. As the hole was advanced, both the circulation rate and bit rotation rate were increased to maintain a 50 m/h rate of penetration. The LWD/MWD safety protocol was followed without incident and required no corrective action. After drilling the hole to a TD of 300 mbsf, the drill string was pulled clear of the seafloor at 0600 h on 23 September. Hole U1326B was spudded 15 m northeast of Hole U1326A at 1205 h on 23 October. The first core failed to establish the depth of the mudline, so a new hole (U1326C) was spudded without offsetting from the location of Hole U1326B at 1245 h on 23 October. The first core established a seafloor depth of 1828.0 mbsl (1839.6 mbrf). On the fourth APC core (~30 mbsf), we unexpectedly hit APC refusal and switched to XCB coring. Hole U1326C was advanced by XCB coring to 82.7 mbsf and followed by three consecutive pressure core deployments within a high–electrical resistivity zone identified on LWD/MWD downhole logs. In this case, the FPC and HRC pressure core runs were added to the traditional continuous core hole. The FPC was the first pressure core system deployed at this site and recovered a partial core (15 cm) under pressure. The second system deployed was the PCS, which also recovered a partial core under pressure. The third pressure core system deployed was the HRC, which was "packed off" with sand. Consequently, the cutting shoe and the lower part of the autoclave were unscrewed and left in the hole, resulting in the termination of Hole U1326C at a TD of 86.7 mbsf (Table T1). After tripping the BHA back to the seafloor, the ship was moved ~30 m southwest (15 m from Hole U1326A). Hole U1326D was spudded at 1130 h on 24 October and drilled to 78.8 mbsf in preparation for continued coring to a target depth of 300 mbsf. Because of problems associated with the sea state and schedule limitations, all pressure coring operations were suspended. XCB coring deepened the hole to 271.4 mbsf. With the forecast of deteriorating weather conditions on the morning of 26 October, we decided to stop coring and drill the hole to 300 mbsf. Despite the marginal conditions, we attempted three deployments with the DVTP (252.2, 271.4, and 300 mbsf), which yielded degraded data. After completing coring and drilling operations, we decided to conduct a single downhole logging run with a nonstandard IODP tool string, which included the SGT, DIT, and the DSI. At 2315 h on 26 October, the logging tool string was lowered to 298.4 mbsf and two successful logging passes were made, with the tool string back on deck at 0345 h. The drill string was pulled clear of the seafloor at 0530 h on 27 October, ending operations in Hole U1326D. The 271.40 m thick Quaternary sedimentary section cored at Site U1326 was divided into three lithostratigraphic units (Fig. F13). Lithostratigraphic Unit I (0–1.50 mbsf in Hole U1326B and 0–24.13 mbsf in Hole U1326C) is characterized by fine-grained (clay to silty clay) detrital sediments with thin silty/sandy interlayered turbidite sequences. Foraminifers, bivalve shell fragments, sponge spicule remains, mottling, bioturbation, and a high marine versus nonmarine diatom ratio indicate hemipelagic sedimentation mixed with turbiditic inputs (coarse-grained facies). Unlithified authigenic carbonate cement is also present in lithostratigraphic Unit I. Lithostratigraphic Unit II (24.13–82.70 mbsf in Hole U1326C and 88.40–146.30 mbsf in Hole U1326D) is also dominated by fine-grained (clay to silty clay) detrital sediments with intervals of turbidite deposits (silty/sandy interlayers), but diatoms are absent and other biogenic components are rare. The frequent occurrence of turbidites might indicate times of active tectonism. Below 30.4 mbsf, abundant soft-sediment deformation and dipping strata show that tectonism is more active at the westernmost end of the transect drilled during this expedition. Unlithified authigenic carbonate cement and lithified carbonate are also present in lithostratigraphic Unit II. Soupy and mousselike textures indicative of gas hydrate occurrence were observed in this unit starting at ~52 mbsf. The lithostratigraphic Unit II/III boundary is marked by a low abundance of diatoms and the beginning of coring-related biscuiting. Lithostratigraphic Unit III (146.30–271.40 mbsf in Hole U1326D) is characterized by fine-grained (clay to silty clay) detrital sediments with few thin silty/sandy turbidite interlayers. We interpret this interval as mixed hemipelagic-turbiditic deposition. Soupy sediment textures related to the presence of gas hydrate are also present in lithostratigraphic Unit III to ~246 mbsf. Precoring LWD/MWD logging was conducted at Site U1326 to direct special tool deployments, such as collecting PCS, HRC, and FPC pressure cores. The stratigraphic section logged at Site U1326 was divided into three "logging units" based on obvious changes in LWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements. The three identified logging units do not correspond to the defined lithostratigraphic units.
Logging Unit 1 (0–72 mbsf in Hole U1326A) is characterized by a resistivity trend that steadily increases from ~1
Logging Unit 2 (72–240 mbsf in Hole U1326A) is characterized by uniform density with depth and includes a prominent high-resistivity section from 72 to 107 mbsf in Hole U1326A, with peaks >40
The top of logging Unit 3 (240–300 mbsf in Hole U1326A) is marked by an increase in density, which remains constant to 300 mbsf. The background resistivity also increases slightly and displays a clear peak, reaching 5 LWD/MWD-derived RAB images from Hole U1326A also indicate the presence of gas hydrate in the high-resistivity interval observed in logging Unit 2. Gas hydrate–rich layers alternate with low-resistivity layers that likely contain little or no gas hydrate (Fig. F13). Closer examination of the RAB images also shows apparent stratigraphic layers in the interval from 50 to 300 mbsf, uniformly dipping to the north-northeast with dips between 45° and 85°. This is in agreement with the west-northwest–east-southeast strike of the uplifted ridge penetrated by Hole U1326A. The Archie calculation of water saturations from electrical resistivity logging data yield evidence for relatively shallow gas hydrate occurrences at this site between ~50 and 100 mbsf, with inferred gas hydrate concentrations as high as 80% of the pore space (Fig. F13). This interpretation is in general agreement with the freshening of pore waters observed in sand layers at Site U1326. The IR core images taken on the catwalk revealed cold anomalies caused by gas hydrate dissociation, which give an independent map of gas hydrate occurrence and concentrations. In some cases, IR-imaged cold temperature anomalies correlate with layers of high resistivity. In other cases, however, cold intervals in the IR images are harder to correlate to high-resistivity peaks in the wireline or LWD/MWD logs because of incomplete core recovery and intrasite geologic variability. Cold temperature anomalies were observed from 70 to 250 mbsf, and catwalk sampling was conducted based on these scans. Many interstitial water samples were taken from the cold temperature anomalies to extend the chlorinity anomaly database available for calibrating IR data as a proxy for gas hydrate saturation. We note that the maximum depth of observed IR anomalies is deeper than the anticipated BSR depth of 234 mbsf. It is also suggested that the chlorinity anomaly associated with gas hydrate may actually extend to a depth as great as 270 mbsf at this site. This apparent conflict between the predicted depth of the BSR and the observed occurrence of gas hydrate can be further explored with closer examination of gas hydrate stability conditions at this site. Because of difficult weather conditions during coring operations at this site, we deployed only four temperature tools. For the most part, data from the temperature tool deployments were degraded. At this time, we have extrapolated the temperature data from Site U1325 to estimate downhole temperatures at Site U1326. A predicted GHSZ depth of 275 ± 25 mbsf is estimated and would explain most of the deeper IR and chlorinity inferred gas hydrate occurrences. At Site U1326, only one PCS core (Core 311-U1326C-12P; 83.7 mbsf) was recovered under pressure and investigated by controlled shipboard degassing experiments. The degassing of this single PCS core yielded 19.1 L of methane, equivalent to a pore space gas hydrate saturation of 40%, which is in close agreement with the gas hydrate saturations estimated from the LWD/MWD Archie resistivity calculations in the that interval. Microbiological subsampling was routinely conducted on cores recovered from Holes U1326C and U1326D. On each core run, perfluorocarbon tracer (PFT) was continuously metered into the drilling fluid and fluorescent microspheres were deployed to investigate potential drilling fluid contamination of the core. These analyses confirmed that the center of each APC core and XCB biscuit remains undisturbed and suitable for microbiological analyses. Additional IR images were taken on the cut ends of each microbiological core section to document the thermal warming process of the core before subsampling. Considering its location within the westernmost portion of the deformation front, we selected this site to further characterize what could be the tectonically youngest gas hydrate occurrence on the northern Cascadia margin. Hydrocarbon headspace gas measurements from Holes U1326C and U1326D show that methane is the dominant hydrocarbon gas within the cored interval at this site. Although the C2+ hydrocarbon void gas concentrations were <125 ppmv for all samples, their relative abundances and distribution were valuable for describing the gas hydrate system at Site U1326. Low C1/C2 ratios within the interval from ~35 to ~72 mbsf were associated with two recovered gas hydrate samples. With greater depth, ethane concentrations returned to the near-surface concentrations close to the predicted depth of the BSR. Isobutane concentrations were also elevated within the same interval, which may indicate the presence of Structure II gas hydrate. The occurrence of more complex hydrocarbon gases generally indicates contribution of a deeper gas source. The salinity and chlorinity profiles at this site also point to a deeper fluid source with a chloride concentration higher than seawater, indicative of low-temperature diagenetic reactions in the deeper parts of the site. Except for the difficulties experienced with pressure coring and special temperature tool deployments, all of the objectives set for Site U1326 were fulfilled. Site U1327Site U1327 (proposed Site CAS-01B) is located near Sites 889/890, approximately at the midslope of the accretionary prism over a clearly defined BSR estimated at 223 mbsf (Figs. F14, F15). The primary research objectives for this site are linked to the transect concept of this expedition. Sites 889/890 provided critical baseline data for the development of the objectives of this expedition. The concentration of gas hydrate in sediments from Sites 889/890 (ODP Leg 146) have been estimated by using (1) deviations in interstitial water chlorinities, (2) electrical resistivity measurements by applying Archie's relation, and (3) both seismic- and downhole logging–derived P- and S-wave velocities. Many questions were left unanswered (Riedel et al., 2005), such as:
Answers to these questions are needed to calibrate remote sensing techniques, such as reflection seismic and CSEM surveys, which can be obtained through research coring and downhole logging. Five holes were occupied at Site U1327 (Table T1). Hole U1327A (TD = 300 mbsf) was dedicated to LWD/MWD measurements. Initially we planned to drill to 350 mbsf, but the short time allocated for LWD/MWD operations made it necessary to reduce the depth of the planned penetration at this site. The first APC core in Hole U1327B missed the mudline with a full core. The core was curated and a new hole was spudded without offsetting the ship. Hole U1327C was cored (10 APC cores, 22 XCB cores, and 3 PCS cores; recovery = 88.3%) to 300.0 mbsf. The PCS was deployed three times in Hole U1327C, and four advanced piston corer temperature (APCT) tool measurements were made (Cores 311-U1327C-3H, 5H, 7H, and 9H). Three additional deployments using the DVTP and Davis-Villinger Temperature-Pressure Probe (DVTPP) were attempted, but data quality is marginal because of strong heave and instrument failures. In Hole U1327D, which was drilled and cored as a special tools hole, two APC cores were taken from the surface to 16.4 mbsf for a high-resolution microbiological and geochemical study of the SMI. In this hole, the PCS was deployed three times, the HCR was deployed four times, and the FPC system was deployed twice. The last pressure core was taken at 246.0 mbsf before advancing the hole to a TD of 300 mbsf for the logging program. The first tool string deployed was the triple combination (triple combo), which reached 295.4 mbsf. The hole was logged to ~70 mbsf without incident, except for a damaged caliper arm. The VSP logging program in Hole U1327D was successfully carried out with 16 clamping stations covering the depth range from 181 to 276 mbsf before the arm on the Well Seismic Tool (WST) broke. The WST was returned with some difficulty to the ship. Because of the critical nature of the acoustic logging data to the expedition, we decided to drill a new hole and acquire additional pressure cores and a wireline acoustic log. Hole U1327E was advanced to 3 mbsf, and a single 9.5 m long APC core was taken for high-resolution microbiological and geochemical sampling of the SMI, which had been missed during an earlier attempt. Two additional PCS cores and one HRC core were taken, but only the second PCS deployment at 80.0 mbsf recovered sediment under pressure. Excessive heave (>3.5 m) forced the termination of pressure coring operations, and the hole was advanced by drilling to a TD of 300 mbsf for the second wireline logging run. Because of excessive heave conditions, we decided to run a nonstandard tool string devoid of any protruding arm (HNGS-DIT-DSI), which allowed measurement of natural gamma radiation, resistivity, and acoustic data (VP and VS). Two successful runs of this tool string recovered high-quality downhole logging data. Site U1327 is located near two prominent topographic highs composed almost entirely of accreted sediments rising >200 m above the surrounding seafloor (Fig. F14). The ridges of accreted sediment lack any coherent seismic reflections and are associated with underlying thrust faults that contributed to the overall uplift of the area between the two ridges (Westbrook, Carson, Musgrave, et al., 1994; Riedel, 2001). Site U1327 is near the northwest flank of the easternmost ridge and is characterized by a ~90 m thick cover of slope-basin sediments underlain by a thick section of accreted sediments (Fig. F15). The stratigraphy at Site U1327 was divided into three lithostratigraphic units (Figs. F15, F16). Lithostratigraphic Unit I (0–90.1 mbsf in Hole U1327C; age = Holocene(?)–Pleistocene; <0.3 Ma) is composed of dark greenish gray and dark gray clay and silty clay, often interbedded with silt, clayey silt, sandy silt, sand, and gravel layers. Unit I is characterized by fine-grained detrital sediments (clay and silty clay) with abundant coarse-grained layers as much as 6 cm thick that indicate turbidite deposits and a sedimentation rate of 22 cm/k.y. Unlithified authigenic carbonate cement is present in Unit I, but no evidence for dolomite precipitation was found, which is inferred from interstitial water analyses. A unique characteristic of Unit I is the anomalous occurrence of lithified carbonates and rocks of various lithologies (e.g., granite), which probably are dropstones from rafting icebergs. The lithostratigraphic boundary between Units I and II is marked by a sharp decrease in sand and silt layers and the onset of diatom-rich sediments. It is also the seismically inferred boundary between slope-basin type and accreted sediments (Fig. F15). Lithostratigraphic Unit II (90.1–170.4 mbsf in Hole U1327C; age = Pleistocene; >0.3–1.0 Ma) is composed of dark greenish gray and dark gray clay, clay with diatoms, and silty clay, silty clay with diatoms, and diatom silty clay locally interbedded with sandy silt and sand layers and lenses. Very few unlithified carbonate cements are present in Unit II, but lithified carbonates and exotic rocks (some as much as 4 cm in diameter) similar to those described in Unit I are very abundant in some cores, especially between 142 and 148 mbsf in Hole U1327C and 151 and 157 mbsf in Hole U1327D. Within Unit II, soupy and mousselike sediment textures related to the presence of observed gas hydrate can be correlated to IR-imaged cold spot anomalies detected on the catwalk. The sedimentation rate of Unit II is estimated at ~16 cm/k.y., and the intervals with a high ratio of nonmarine versus marine diatoms indicate an increased contribution of terrigeneous detrital sediments from land sources by turbidites. The great abundance of marine diatoms along with resting spores within lithostratigraphic Unit II suggest deposition beneath a coastal upwelling shelf environment followed by reworking by turbidity currents. Lithostratigraphic Unit III (170.4–TD in Hole U1327C; age = Pleistocene; >1 Ma) is composed of dark greenish gray and dark gray silty clay in the upper part of the unit, whereas the lower part (below 248 mbsf) is dark greenish gray silty clay with diatoms. Unit III is distinguished from Unit II by the sudden absence of diatoms, as well as the degree of induration of the sediments. Diatoms reappear at the bottom of Unit III. Few unlithified carbonate cements are present and lithified carbonates and exotic rock clasts were found, especially within Core 311-U1327C-24P (197.3 mbsf). The depositional environment for Unit III was interpreted to have been dominated by low-energy turbidity currents in an abyssal plain. The biostratigraphically determined sedimentation rate in this section is low (~12 cm/k.y.) and is consistent with a basin-plain setting. The downhole logged section at Site U1327 was divided into three logging units based on obvious changes in the LWD/MWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements. There is no apparent correlation between the logging units and lithostratigraphic units at this site.
Logging Unit 1 (0–120 mbsf) is characterized by a resistivity trend that steadily increases from ~1
Logging Unit 2 (120–240 mbsf) is characterized by a constant background resistivity value of ~2
The top of logging Unit 3 (240–300 mbsf) is defined by a sharp decrease in VP and VS. Below 240 mbsf, VP decreases to low values near fluid velocities (~1500 m/s), which suggests the presence of small amounts of free gas. Unit 3 also displays a small drop in resistivity compared to Unit 2. Although resistivity tends to be just above 2 Variations in the core-derived physical properties do not correlate well with the identified lithostratigraphic and logging units, with the exception of the boundary between lithostratigraphic Units I and II, which is marked by a sharp increase in magnetic susceptibility. Sediment density and porosity determined from MAD analyses show a typical trend with depth, suggesting normal compaction. Slight deviations from this trend reflect small-scale lithologic variability and generally match the porosity and density trend determined from the LWD/MWD data. IR imaging of the recovered cores was routinely carried out on the catwalk to detect and characterize the nature of gas hydrate in the cores. A large number of IR-identified cold spots were detected in the cores from Holes U1327C and U1327D. Mismatches between the IR-inferred gas hydrate occurrences in Holes U1327C and U1327D and the logging resistivity–inferred gas hydrate occurrences in Holes U1327A, U1327D, and U1327E document the lack of lateral continuity at this site. In situ temperature measurements with the APCT, DVTP, and DVTPP3 (Table T2) suggest a geothermal gradient of ~59°C/km, which is not significantly different from that estimated for Site 889 (~54°C/km). The PCS was deployed eight times at Site U1327 (three times in Hole U1327C, three times in Hole U1327D, and twice in Hole U1327E) (Table T3). Five deployments recovered sediment under pressure. In addition to the PCS deployment, the HRC was used four times (three cores recovered under pressure) and the FPC was used twice, but only one was recovered under pressure. Degassing of the five PCS cores recovered under pressure showed variable gas concentrations with depth. The deepest PCS core was recovered from 246.0 mbsf, which is ~25 m below the anticipated depth of the BSR. This pressure core (Core 311-U1327D-17P) had a pore space methane concentration of 209 mM, which is equivalent to a free gas concentration in the sediment pore space of 1.0%. The other four PCS cores were taken from within the predicted depth interval of the GHSZ. Core 311-U1327E-3P, the shallowest (80.0 mbsf) PCS core taken from this site, yielded 0.91 L of methane (83 mM) or, equivalently, 0.2% gas hydrate concentration. The remaining three PCS cores were taken at 121.8, 155.7, and 197.3 mbsf (Core 311-U1327C-15P yielded 1.1 L [96 mM] of methane; Core 311-U1327D-10P yielded 9.4 L [674 mM] of methane; and Core 311-U1327C-24P yielded 4.1 L [253 mM] of methane). Core 311-U1327D-10P yielded enough gas to infer an average gas hydrate pore space concentration of ~8%. The other two PCS cores yielded gas hydrate pore space concentrations ranging from 0.2% to 1.8%. Three HRC cores (Cores 311-U1327D-4E at 125.3 mbsf, 12E at 170.5 mbsf, and 14E at 217.7 mbsf) and one FPC core (13Y at 203.6 mbsf) were successfully recovered and transferred under pressure to storage chambers for shore-based analyses. These cores were imaged with X-ray and logged for P-wave velocity and density within their storage vessels. High P-wave velocities and anomalous low density readings indicate the recovered pressure cores contained gas hydrate. High-resolution interstitial water sampling was carried out to characterize the SMI, which was determined to be located between 9 and 10 mbsf (see Fig. F32). An overall smoothly decreasing trend in pore water chlorinity and salinity is observed, similar to results from Sites 889/890 (Westbrook, Carson, Musgrave, et al., 1994). From 128 mbsf to the depth of the BSR the chlorinity and salinity profiles exhibit distinct anomalies that likely indicate freshening as a result of the dissociation of gas hydrate during the core recovery process (Fig. F16). Chloride values as low as 70 mM (salinity = 3.7) from two samples correlated to sand layers (as much as 3 cm thick) that also showed IR cold spot anomalies. Such a low pore water chlorinity is equivalent to a pore space gas hydrate concentration of ~80%. No chlorinity anomalies occur below ~225 mbsf, which is close to predicted depth of the BSR. The strongly decreasing chlorinity profile from the surface to below the BSR at Site U1327 suggests mixing between in situ seawater and modified deep-sourced pore water, which is relatively fresh fluid. Interstitial water chlorinity and salinity remain almost constant beneath the BSR. Organic geochemical studies at Site U1327 included analysis of the composition of volatile hydrocarbons (C1–C5) and nonhydrocarbon (i.e., O2 and N2) gases from headspace gas samples, void gas samples, and gas samples recovered during PCS degassing experiments. The predominant hydrocarbon gas found in the cores from Site U1327 was methane; however, we did see an increase in ethane concentrations in the void and headspace gases collected from the stratigraphic section overlying the projected depth of the BSR. The increase in ethane concentrations near the BSR is reflected in the C1/C2 void gas ratios (Fig. F16), which decrease with depth toward the BSR. Other studies have shown that dissociation of ethane enriched gas hydrate can depress void gas C1/C2 ratios. In general, the C1/C2 ratios were high (>1000), indicating mostly a microbial origin for the observed methane. Microbiological subsampling was routinely conducted on cores recovered from Hole U1327C. On each core run, PFT was continuously metered into the drilling fluid, and fluorescent microspheres were deployed on selected cores to investigate potential drilling fluid contamination. Additional IR images were taken on the cut ends of each microbiological core section to document the thermal warming process of the core before subsampling. The primary research objectives of this site are linked to the results from Sites 889/890 of Leg 146 to delineate critical geochemical, geophysical, and microbiological data for gas hydrate proxy analyses. Site U1327, along with Sites 889/890, is the center of a research study area that has been the focus of many interdisciplinary gas hydrate studies since 1992. Geochemical analyses of recovered gas samples and interstitial fluids documented a relatively complex fluid regime for this site, similar to what was observed at Sites 889/890 and U1329. The rapid chlorinity concentration decrease with depth strongly suggests fluid communication with a deep-seated, fresher fluid source. Earlier interpretations of the chlorinity profile measured at Sites 889/890 attributed ~60% of the pore water freshening to gas hydrate dissociation upon core recovery (Hyndman et al., 1999). However, the new interstitial water data collected at Site U1327, in combination with other gas hydrate proxy data (such as IR images and downhole logging measurements), strongly support the development of a new undisturbed baseline for interstitial water chlorinity along the northern Cascadia margin that decreases with depth in the manner suggested by Ussler and Paull (2001). Based on IR and chlorinity fluctuations, gas hydrate–related pore water freshening at Site U1327 is interpreted to be limited to the interval from ~110 to ~225 mbsf. The hydrocarbon gas data suggest the presence of gas hydrate above the BSR and a deep source of thermogenic gases, which supports similar conclusions drawn from the interstitial water data. Identifying gas hydrate and constraining its relative abundance in combination with providing physical models for the BSR were the critical objectives for this site. A suite of geophysical experiments was conducted including precoring LWD/MWD logging, conventional wireline logging, and VSP to characterize the physical properties of gas hydrate–bearing sediments. The presence of gas hydrate is generally characterized by increases in measured electrical resistivity and acoustic velocity. The LWD/MWD data from Hole U1327A showed an 18 m thick zone of increased resistivity and acoustic velocity values combined with a decrease in density, which was interpreted to contain large amount of gas hydrate with concentrations >50% of the pore volume. However, the same interval was penetrated in the adjacent core holes, Holes U1327C and U1327D, at much greater depths and with lower estimated gas hydrate concentrations inferred from the wireline electrical logs. No evidence of this interval was found in the wireline log in Hole U1327E. This shows large intrasite variability in gas hydrate content that is probably controlled by lithostratigraphic changes or structural complexities. This observation has dramatic implications on the calibration of geophysical surveying techniques such as seismic methods or CSEM surveys that are commonly used to detect and quantify gas hydrate. The combined use of IR imaging, interstitial pore water analyses, and void/headspace gas sampling revealed that many IR cold spots identified on the catwalk occur in relatively thin but coarser-grained intervals of sand and/or silty sand (interpreted to represent turbidite deposits) with gas hydrate concentrations in several cases exceeding 80% of the pore space. The gas hydrate preferentially forms in these coarser-grained sections. Therefore, it is likely that the 18 m thick LWD/MWD-measured high-resistivity and borehole fluid velocity interval (120–138 mbsf) in Hole U1327A is a localized feature made up of discontinuous thinly bedded gas hydrate–saturated turbidite sand lenses that appears as a massive single unit when surveyed with a lower vertical resolution logging tool. The results of the VSP seismic experiment taken in combination with the downhole acoustic logging from Hole U1327E also show a relatively thick zone below the depth of the BSR that probably contains a considerable amount of free gas. The acoustic velocity decrease at the BSR is not abrupt; it is more transitional across a ~25 m thick interval from >1800 m/s in the GHSZ to a velocity as low as 1280 m/s in the underlying section. This confirms earlier interpretations of the frequency-dependent reflection behavior of the BSR from vertical incidence seismic data where the BSR was thought to be a gradient zone in which velocity decreases over several meters (Chapman et al., 2002). This site was intended to characterize the gas hydrate occurrence on the northern Cascadia margin to establish an understanding of the geologic, geophysical, and geochemical controls on the occurrence and abundance of gas hydrate along the five-site transect established during this expedition. All of the objectives set for Site U1327 were fulfilled. Site U1328Site U1328 (proposed Site CAS-06A) is located within a seafloor cold vent field (2 km x 4 km) consisting of at least four vents associated with near-surface faults. The cold vents are characterized by near-vertical seismic blank (or wipeout) zones that are between 80 and several 100 m wide and show a clear east–west trend as identified from 3-D seismic imaging. The most prominent vent in the field, referred to as Bullseye vent, was the target of this site and has been the subject of intensive geophysical and geochemical studies since 1999 (e.g., Riedel et al., 2006). Site U1328 is different from all of the other sites visited during this expedition in that it represents an area of active, focused fluid flow. A number of seismic surveys cross this site, including two single-channel seismic (SCS) 3-D surveys and a regional MCS data set. In Figure F17 we show MCS Line XL07 and the main lithostratigraphic units, hole locations, and penetration depths at this site. Several mechanisms have been proposed to explain the seismic blanking and nature of fluid venting in the Bullseye vent area (Riedel et al., 2002, 2006; Zühlsdorff and Spiess, 2004; Wood et al., 2002). The objectives of coring and logging at this location are to test the different models for the cold vent structure and associated causes of seismic blanking, the rate of methane advection, and potential loss of methane into the water column. It is important to obtain a high-resolution temperature profile for this area to assess any evidence of active fluid flow. Pressure coring using the PCS system will help assess the occurrence of free gas within the GHSZ below Bullseye vent and possibly ascertain the cause of the observed seismic blanking. Additional pressure coring using the HRC and FPC systems was used to recover high-quality gas hydrate–bearing core for shore-based analyses. Five holes were occupied at Site U1328. Hole U1328A was dedicated to LWD/MWD measurements to a TD of 300 mbsf. Hole U1328B was continuously cored (six APC, two XCB, and two PCS cores; recovery = 72.8%) to 56.6 mbsf before strong winds and severe ship heave conditions required us to pull out of the hole. Two APC temperature measurements were made (APCT and APC3). As a calibration experiment, the DVTPP was deployed at the bottom of Hole U1328B. After waiting on weather for 16 h, improved conditions allowed drilling and coring operations to continue. Hole U1328C was drilled from the seafloor to the maximum depth of Hole U1328B (56.5 mbsf). Hole U1328C was continuously cored (4 APC cores, 22 XCB cores, and 1 PCS core; recovery = 80.7%) to 300 mbsf. In addition, one APCT, two APC3, and two DVTPP temperature measurements were made. Hole U1328C was wireline logged with the triple combo and FMS-sonic tool strings. The triple combo logged from a TD of 294 mbsf, and two uphole passes of the FMS-sonic tool string logged the same depth interval. The wireline logging program in Hole U1328C included a VSP survey, with the deepest clamping position at 286 mbsf. The VSP included 35 clamping positions, with the uppermost station at 106 mbsf. Hole U1328D was cored as a special high-resolution combined microbiology and geochemistry research hole. Two XCB cores and a single FPC core were taken at the bottom of the hole. Hole U1328E was a special tools hole, which also included the deployment of six XCB cores within the upper 46.0 mbsf to recover additional samples of gas hydrate. Seven pressure cores (three PCS, two HRC, and two FPC) were separated by XCB cores and drilled intervals. Hole U1328E also included two DVTP deployments for temperature data. In Figures F18 and F19, data from LWD/MWD logging (RAB images, porosity, and Archie-derived pore water saturations), core-derived physical property data (porosity), interstitial water chlorinity, and C1/C2 ratios from void gas samples are shown in comparison to the two key crossing seismic lines from this site. At Site U1328, a 300 m thick sequence of Quaternary (0 to <1.6 Ma) slope and slope basin sediments was recovered (Fig. F17). The biostratigraphy determined for Site U1328 is based on the examination of diatoms from Holes U1328B and U1328C. The stratigraphic section cored and logged at Site U1328 is divided into three lithostratigraphic units. Lithostratigraphic Unit I (0–56.98 mbsf in Hole U1328B, 56.98–132.60 mbsf in Hole U1328C, 0–15.00 mbsf in Hole U1328D, and 0–92.26 mbsf in Hole U1328E) is characterized by fine-grained detrital sediments (dark greenish gray clay and silty clay) with abundant coarse-grained layers as much as 6 cm thick indicative of turbidite deposits. The large impact of turbidite deposits is reflected in high sedimentation rates (37.5 cm/k.y.). Lithified and/or partly lithified carbonates were often found associated with soupy and mousseline sediment textures. The presence of argonite (besides high-Mg calcite) seems to indicate carbonate formation related to the presence of gas hydrate. The boundary between lithostratigraphic Units I and II is marked by a sharp decrease of sand and silt layers and by the onset of diatom-bearing sediments. Lithostratigraphic Unit II (132.60–197.10 mbsf in Hole U1328C and 197.00–198.00 mbsf in Hole U1328E) is characterized by fine-grained (clay to silty clay) detrital sediments with some silty interlayers from turbiditic deposits and siliceous fossils (sedimentation rate = 24.6 cm/k.y.). Abundant marine diatoms along with resting spores of Chaetoceros in lithostratigraphic Unit II suggest the diatoms bloomed in a shallow-water shelf environment associated with coastal upwelling and were then reworked. The boundary between lithostratigraphic Units II and III is distinguished by the sudden absence of diatoms. Lithostratigraphic Unit III (197.10–300.00 mbsf in Hole U1328C) is characterized by fine-grained (clay to silty clay) detrital sediments with very few silty interlayers from turbidite deposits, likely deposited in an abyssal plain environment. The presence of unlithified authigenic carbonate cements show that diagenetic processes are active in Unit III. At Site U1328, gas hydrate was sampled and evidence of gas hydrate was found in the recovered cores in the form of soupy and mousselike textures. At this site, soupy texture is only present within Unit I and mousselike textures are present occasionally within Units I to III. The LWD/MWD and wireline logged section in Holes U1328A and U1328C was divided into three logging units based on changes in the downhole measured gamma ray, density, electrical resistivity, and acoustic measurements. For the most part, these logging units correspond closely to lithostratigraphic Units I, II, and III.
Logging Unit 1 (0–128 mbsf in Hole U1328A) is characterized by a well-defined increase in density with depth. The most striking feature in this unit is seen in the LWD/MWD logs between the seafloor and 46 mbsf, where high resistivities (>25
Logging Unit 2 (128–200 mbsf in Hole U1328A) is marked at the top by a clear decrease in density log values. Unit 2 is also characterized by a constant and relatively low background resistivity value just above 1 Logging Unit 3 (200–300 mbsf in Hole U1328A) is characterized by small increases in background resistivity and notably more variability in the resistivity and P-wave velocity log values. Physical properties were measured in cores recovered from four of the holes at this site. Core-derived MAD data compare favorably to both the LWD/MWD and downhole wireline data. Numerous low-porosity outliers, however, were interpreted to represent sand-rich samples. It has been shown that RAB images derived from the LWD/MWD GeoVISION tool can be used to identify gas hydrate distribution in sediment (Tréhu, Bohrmann, Rack, Torres, et al., 2003). High concentrations of gas hydrate are evident in the interval 0–46 mbsf of Hole U1328A, where resistivities are high. The RAB images from Hole U1328A also reveal numerous high-resistivity sinusoidal features in the upper 46 mbsf in Hole U1328A and again at 90–100 mbsf, which can be interpreted as dipping fractures containing gas hydrate or possibly free gas. These near-vertical fractures may act as gas migration conduits that feed the gas hydrate accumulation observed near the seafloor. The GeoVISION-derived resistivity logs were used as input to the Archie relation to estimate the amount of gas hydrate at Site U1328. These calculations show highly variable gas hydrate saturations in the upper 46 mbsf, with values as high as 95% in several intervals up to 10 m thick. These layers of high gas hydrate concentrations alternate with layers of much higher water saturations. The combined analysis of acoustic velocities and waveform amplitudes can also help identify the occurrence of gas hydrate and/or free gas. In the interval between 210 and 220 mbsf, near the predicted depth of the BSR at ~219 mbsf, the P-wave velocity drops slightly while the S-wave velocity increases. This could be due to the coexistence of free gas and gas hydrate in the vicinity of the BSR, which has been observed on Blake Ridge and Hydrate Ridge. Additional evidence for the occurrence of free gas below the BSR was noted in the analysis of the LWD/MWD acoustic coherence data, borehole fluid pressure response during drilling, and the wireline P-wave, resistivity, density, and neutron logs. The PCS degassing experiments of a single core in Hole U1328E also indicate the presence of a free gas phase below the BSR at this site (see below). However, it is important to note that the VSP survey obtained in Hole U1328C yielded a very uniform velocity profile with depth and no clear velocity contrast around the expected depth of the BSR. We deployed the APCT, APC3, DVTP, and DVTPP temperature tools a total of ten times, but recorded only three reliable data points because of severe ship heave conditions (Table T2). The in situ temperatures determined for Site U1328 are generally similar to those at Site U1327 and slightly higher than those determined at similar depths during Leg 146. For Site U1328 we obtained a seafloor temperature of ~3.5°C and a geothermal gradient of ~54°C/km, which corresponds to a predicted depth to the base of the GHSZ ranging from 222 to 247 mbsf given the uncertainty in the data. All cores from this site were systematically scanned on the catwalk to detect IR anomalies indicative of gas hydrate dissociation during core recovery. Strong cold anomalies were detected in the shallowest cores from this site. Our recovery of a large amount of gas hydrate in the upper 30 mbsf was anticipated based on LWD/MWD resistivity measurements and previous piston coring at this site (Riedel et al., 2002, 2006; Solem et al., 2002). However, the LWD/MWD high resistivity–inferred gas hydrate occurrence at 90–100 mbsf in Hole U1328A was not seen on either the IR images or the wireline logs in Hole U1328C. The IR data may have missed this interval due to poor core recovery, but the lack of a high-resistivity layer in the wireline log points to horizontal inhomogeneity. This is not unexpected, in that the high-resistivity layer in the LWD/MWD image contains a steeply dipping fracture that is unlikely to be intersected at the same depth even in two closely spaced holes. The strongest anomalies were detected just above the predicted BSR depth (210–222 mbsf), and a few small but distinct cold anomalies were observed below it, one of which exhibited pore water freshening indicative of the presence of gas hydrate (Core 311-U1328C-22X; 247 mbsf). The possibility of gas hydrate beneath the BSR may indicate the presence of Structure II gas hydrate, which is stable below the base of the pure methane Structure I GHSZ. However, it should be noted that substantial uncertainty remains in the exact depth of the base of the gas hydrate stability field at this site and the depth of the BSR. All available data suggest that the GHSZ could be deeper than the initially inferred depth of the BSR and result in shifting the observed IR anomalies to depths well within the gas hydrate stability field of Structure I gas hydrate. At Site U1328 we attempted 11 deployments of the 3 different pressure coring tools. The PCS was deployed six times at Site U1328 (twice in Hole U1328B, once in Hole U1328C, and three times in Hole U1328E), five of which recovered sediment under pressure. The HRC was deployed two times and the FPC was deployed three times, but none were recovered under pressure. Degassing of the five successful PCS cores from this site showed variable gas concentrations with depth. Two PCS cores taken within the near-surface gas hydrate–bearing section from 0 to 46 mbsf (Core 311-U1328B-4P at 14.5 mbsf and 7P at 26.0 mbsf) yielded 21.5 and 2.5 L of methane, respectively, which is equivalent to pore volume gas hydrate saturations of ~15% and ~2%. X-ray images of Core 311-U1328B-4P under pressure show 2–6 mm thick low-density structures that disappeared after degassing and are interpreted as gas hydrate veins. This core showed large amounts of gas expansion and sediment extrusion during depressurization, as did Cores 311-U1328B-7P and 311-U1328E-10P to a lesser extent. The two PCS cores taken in Holes U1328C and U1328E at 92.0 mbsf (Cores 311-U1328C-5P and 311-U1328E-10P) yielded different amounts of methane gas equating to the occurrence of 38% (Core 311-U1328E-10P) and 0.7% (Core 311-U1328C-5P) of gas hydrate. One PCS core (Core 311-U1328E-13P) was taken at 233.0 mbsf, very close to the inferred depth of the BSR. This core yielded 59.9 L of methane, the largest recovery of gas during this expedition. This amount of methane would fill 58% of the pore space if it was present as free gas or 50% if it was present as gas hydrate. However, gamma ray density scans during degassing and X-ray scans after depressurization showed almost no evidence of gas expansion cracks or large voids in the core. We speculate that most of the gas released during degassing of this core came from the space between the inner and outer core barrel, where a gas bubble was observed during X-raying. Volumetric analysis suggests the presence of a 540 mL headspace in the outer core barrel, which corresponds well with the 410 mL volume that would have been occupied by the released amount of gas at in situ conditions. The apparent extra gas in the PCS might have swabbed into the core barrel during the coring process. A total of 90 interstitial water geochemistry samples were processed from four of the holes cored at Site U1328. The composite chlorinity profile for this site shows four distinct zones. In the upper ~60 mbsf, we see a striking increase in chlorinity, with maximum values in the interval between 5 and 20 mbsf exceeding 850 mM. The observed excess solutes are interpreted to result from salt exclusion from the water lattice structure during in situ gas hydrate formation that has not been removed by advective or diffusive processes. The second zone from ~60 to ~150 mbsf is characterized by linearly decreasing chlorinity values ranging from 538 to 570 mM. A third zone, extending from ~150 to 250 mbsf, across the BSR, shows discrete excursions to fresher chlorinity values (as low as 348 mM), suggesting that gas hydrate was present in the cores and dissociated prior to processing the samples. The chlorinity anomalies are consistent with observations of distinct negative thermal excursions in IR scans. Our observations further indicate that most of the gas hydrate occupies relatively thin sand layers. In the deepest zone, below 250 mbsf, the chlorinity remains nearly constant at 493 ± 3 mM, suggesting communication with a fluid at greater depth that is notably different in composition from the deep-seated fluid sampled at Sites U1327 and 889/890. To form and maintain shallow gas hydrate deposits near the seafloor, methane has to be supplied continuously. Furthermore, the shallow elevated chlorinity values would dissipate rapidly if they were not continuously maintained by ongoing gas hydrate formation. However, the chlorinity profile below 30 mbsf at Site U1328 indicates a diffusion-controlled system. These observations indicate that the methane needed to sustain the shallow gas hydrate formation is likely supplied along faults or fracture zones, probably the ones imaged by the RAB tool within the gas hydrate–bearing section immediately below the seafloor. The shipboard organic geochemistry program for Site U1328 included analysis of hydrocarbons and nonhydrocarbon gases from headspace gas samples, void gas samples, gas samples recovered during PCS degassing experiments, and gases from gas hydrate samples. Methane was the most prominent hydrocarbon gas in all of the samples analyzed; however, ethane was also present in almost all of the headspace samples. Most of the gas hydrate samples and gas hydrate–bearing sediments collected within the upper 46 mbsf exhibited elevated ethane concentrations. These near-surface samples also contained slightly more air contamination and elevated H2S concentrations. It is also notable that the concentration of ethane and other gas hydrate–forming gases, including propane and isobutane, increase within the gas samples collected from the cores crossing the predicted depth of the BSR, where the chlorinity and IR temperature anomalies both indicate evidence for gas hydrate. The occurrence of propane and elevated isobutane to n-butane (i-C4/n-C4) ratio support the suggestion that the gas hydrate near the depth of the BSR contains Structure II gas hydrate. It is also interesting that the ethane concentrations remain elevated below the depth of the BSR. Structure II gas hydrate is stable to greater depths with increasing temperature than Structure I gas hydrate; however, the observed IR and chlorinity anomalies are not necessarily below the base of the Structure I gas hydrate stability field and therefore not automatically related to the suggested Structure II gas hydrate given the uncertainty in the temperature measurements. However, the gas geochemistry does suggest the presence of some Structure II gas hydrate near the predicted depth of the BSR. Microbiological subsampling was routinely conducted on cores recovered from Holes U1328B and U1328C. On each core run PFT was continuously metered into the drilling fluid, and fluorescent microspheres were deployed on all cores in the continuously cored holes to investigate potential drilling fluid contamination. These analyses confirmed that the center of each APC core and XCB biscuit remain undisturbed and suitable for microbiological analyses. Additional IR images were taken on the cut ends of each microbiological core section to document the thermal warming process of the core before subsampling. The main objectives at this site were to
However, only the very shallowest (<30 mbsf) portion of the sedimentary section cored at Site U1328 is dominated by fluid advection. It appears that diffusion transport controls the chemical depth profiles below 30 mbsf, except perhaps in the deepest 50 m (250–300 mbsf) where Cl and salinity data suggest control by advection. This apparent contradiction can be explained with the vent model brought forward by Riedel et al. (2006). The model, based on various geophysical and geochemical observations, predicted that the shallow gas hydrate accumulations are the result of isolated feeder channels or fractures. Along these fractures the bulk of the methane gas is transported upward and results in the near-seafloor concentrated gas hydrate accumulation. Evidence of the existence of these fractures comes from the LWD/MWD downhole logging data showing at several depths steeply dipping resistivity anomalies typical of fractures filled with gas hydrate or possibly free gas. These potential gas migration conduits may be connected to the seafloor chemosynthetic cold vent communities observed by bottom video surveys (Riedel et al., 2002, 2006). The growth of gas hydrate near the seafloor has resulted in solute exclusion and development of high pore water salinities. This brine cannot be maintained in situ over a long time period, unless gas hydrate is constantly formed at relatively high rates. The cold vent must be supplied with fluids from greater depth. This is also indicated by the detection of a methane plume in the water column just above an active vent outlet. However, venting is most likely episodic since the methane plume is not always detectable (Riedel et al., 2006). There is further evidence for the presence of Structure II gas hydrate at this site as indicated by the presence of propane and isobutane. Various geophysical data support the presence of a BSR at this site, as proposed by Riedel et al. (2006); however, the BSR is difficult to identify with the available seismic data. The lack of good 3-D seismic velocity control could potentially result in misinterpretation of the existing two-dimensional (2-D) seismic data, in that the observed BSR may actually be a side diffraction and may not be exactly located beneath the active part of the cold vent and seismic blanking. In the LWD/MWD data a sharp decrease in electrical resistivity was observed at ~220 mbsf, although it is coincident with a high-density interval potentially related to the presence of carbonates. Acoustic wireline logging, however, shows a sharp discontinuity in P-wave and S-wave velocity at a depth of ~215 mbsf, close to the predicted BSR depth of 219 ± 2.5 mbsf. The downhole temperature measurements from this site yielded a temperature gradient of ~54°C/km, which predicts the base of gas hydrate stability at ~234.5 ± 12.5 mbsf for the observed pore water and gas geochemistries. There is evidence for the presence of free gas below the predicted BSR depth, as demonstrated from pressure coring, LWD/MWD pressure monitoring, and elevated LWD/MWD electrical resistivity values. There are also seismic indicators for the occurrence of free gas below the BSR in the form of strong amplitude anomalies. However, the VSP does not show any evidence of a velocity change over the entire logged interval from 105 to 285 mbsf, which will be further examined postcruise. Except for difficulties with pressure coring and temperature deployments, all objectives for Site U1328 were fulfilled. Site U1329Site U1329 (proposed Site CAS-05D) is at the eastern end of the southwest-northeast–trending margin-perpendicular transect of sites occupied during this expedition and is located closest to shore (65 km) at a water depth of ~946 mbsf. The location of this site is believed to be at the eastern limit of gas hydrate occurrence on the northern Cascadia margin. The objectives of coring and downhole logging at this site are tied to completing the transect of scientific drill sites across the northern Cascadia margin to further constrain models for the formation of marine gas hydrate in a subduction zone accretionary prism. The depth to the BSR rapidly becomes shallower around this site and is only about half the depth (~126 mbsf) of that at Site U1327 (~225 mbsf) (Figs. F20, F21). At this eastern end-member site of gas hydrate evolution in the accretionary prism, the objectives include
Five holes were occupied at Site U1329 (Table T1). Hole U1329A was dedicated to LWD/MWD measurements to a TD of 220 mbsf. Hole U1329B consisted of one missed mudline APC core to 9.5 mbsf. Hole U1329C was continuously cored (17 APC, 5 XCB, and 3 PCS cores; recovery = 99.3%) to 189.5 mbsf and was terminated before the TD of 220 mbsf when the PCS cutting shoe broke off in the bottom of the hole. The PCS was deployed three times in Hole U1329C, and four APC temperature measurements were made (APCT and APC3). Swell from a passing low-pressure system resulted in ship heave that was too high for wireline logging or coring operations, and we waited 4 h for the weather to improve. With a forecast for improving weather the following day, we decided to abandon Hole U1329C to drill a dedicated logging hole. Hole U1329D was drilled from the seafloor to 201.0 mbsf, which included another 1.5 h of suspended operations due to excessive ship heave, and a single XCB core was taken to 210.5 mbsf. Hole U1329D was wireline logged with the triple combo and FMS-sonic tool strings. The triple combo and the first pass of the FMS-sonic were logged to 209 mbsf. The second pass of the FMS-sonic only reached 171 mbsf. Hole U1329E was a special tool hole to 127.1 mbsf, where five APC cores were taken for high-resolution microbiological and geochemical studies, with two additional APC temperature measurements. Two DVTP deployments were also conducted. Five pressure cores were taken (three PCS, one HRC, and one FPC) separated by drilled intervals. In Figures F22 and F23, data from LWD/MWD logging (RAB images, porosity, and Archie-derived pore water saturations), core-derived physical property data (porosity), and interstitial water chlorinity, as well as C1/C2 ratios from void gas samples are shown in comparison to the two key crossing seismic lines from this site. Site U1329 is located near the foot of a relatively steep slope, and sedimentation at this site is dominated by slope processes. The stratigraphy at Site U1329 was divided into three lithostratigraphic units. Lithostratigraphic Unit I (0–37.18 mbsf in Hole U1329C and 0–33.31 mbsf in Hole U1329E) is characterized by fine-grained detrital sediments (clay and silty clay), locally interbedded with coarse-grained sediments. Authigenic carbonates are abundant in some cores from Unit I. Dolomite was observed in Holes U1329C and U1329E at depths that coincide with changes in pore water Ca and Mg profiles, suggesting dolomite formation is an ongoing process. The sedimentation rate in Unit I is relatively low at ~10 cm/k.y. The boundary between lithostratigraphic Units I and II is defined by the first occurrence of diatom ooze. Lithostratigraphic Unit II (37.18–135.60 mbsf in Hole U1329C and 33.31–125.95 mbsf in Hole U1329E) is characterized by a high abundance of biogenic silica (mainly diatoms). The sedimentation rate within Unit II appears to increase from ~4 to 10 cm/k.y. The boundary between Units II and III is marked by a conglomerate composed of partly lithified to lithified rounded clasts supported by a silty clay matrix. The conglomerate at the base of Unit II corresponds to an unconformity between upper Miocene and Pleistocene sediments (no sediments preserved from 2–6.7 Ma) that can be traced seismically for several kilometers along Line 89-08 (Fig. F20). The unconformity is not clearly seen in the higher-frequency seismic data Line SCS CAS05C-01-04 (Fig. F21). Lithostratigraphic Unit III (135.60 mbsf–TD) is characterized by fine-grained (clay to silty clay) detrital sediments with only a few coarser interlayers from turbidite deposits. The input of detrital sediments from turbidites, however, is more obvious deeper in the section with the presence of nonmarine diatoms. In the lowermost part of Unit III, a conglomerate deposit marks a major change in deposition, possibly representing a debris flow. The sediments in Unit III were deposited at a low sedimentation rate of ~0.8–2.8 cm/k.y. The biostratigraphy at this site was established by the analysis of diatoms. These analyses showed an interval of Quaternary sediments from the surface to ~35 mbsf that corresponds to lithostratigraphic Unit I. The age of lithostratigraphic Unit II was shown to range from 0.3 to 2.0 Ma. This interval showed an abundance of diatoms and resting spores of Chaetoceros, suggesting a depositional environment typical of a coastal shelf area with upwelling. The sediments of this interval may also have been transported to this site from the shelf by turbidites. Sediments deeper than 137 mbsf, near the contact between lithostratigraphic Units II and III, contain diatoms of only late Miocene age (>6.7 Ma); thus, the Pliocene at this site is completely absent, representing ~4 m.y. of missing sediment, which was likely eroded. The downhole logged section at Site U1329 was divided into three logging units based on changes in the LWD/MWD and wireline gamma ray, density, electrical resistivity, and acoustic measurements. Logging Unit 1 (0–130 mbsf), which corresponds to lithostratigraphic Units I and II, is characterized by low electrical resistivities and densities. Logging Unit 2 (130–183 mbsf) is characterized by a small increase in resistivity and density compared to logging Unit 1. The top of Unit 2 corresponds to the bottom of lithostratigraphic Unit II, which was identified as an unconformity separating Pleistocene from upper Miocene sediments. Logging Unit 3 (183 mbsf–TD) is characterized by an abrupt increase in electrical resistivity, density, and P-wave velocity, indicating an increasingly consolidated formation with lower log-derived porosities. Logging Units 2 and 3 correspond to lithostratigraphic Unit III. Variations in physical properties do not correlate well with the lithostratigraphic and logging units. MAD analyses of porosity and bulk density as well as gamma ray attenuation density data from the multisensor track show anomalously uniform trends throughout most of the section cored from 25 to 180 mbsf. A slightly different trend is observed in the upper 25 mbsf where porosity rapidly decreases to values as low as 55%, coincident with the depth where rapid changes in the core-derived shear strength and electrical resistivity occur. Except for the uppermost 15 mbsf, the magnetic susceptibility record shows extremely low and uniform values of <50 x 10–7 SI. Grain densities generally decrease downhole, crossing all lithostratigraphic boundaries, starting from values of ~2.8 g/cm3 near the seafloor to ~2.6 g/cm3 at 175 mbsf, but this trend reverses below this depth and again reaches values as high as ~2.8 g/cm3. This deep increase in grain density and associated increase in bulk density occurs near the top of logging Unit 3, which is characterized by a sudden increase in electrical resistivity and drop in porosity. Analysis of the LWD/MWD density logs from this site yielded sediment porosities ranging from ~65% near the seafloor to ~23% at 220 mbsf. IR imaging of the recovered core assisted in immediate gas hydrate detection on the catwalk. At this site, core IR temperatures did not show any significant cold spot anomalies that could be related to gas hydrate dissociation in the recovered core. In situ temperature measurements with the APC3, APCT, and DVTP tools indicate a temperature gradient of 72° ± 4°C/km and a seafloor intercept of 3.34° ± 0.34°C. For measured thermal conductivities of 1.0–1.1 W/(m·K), this gradient results in a heat flow of 68–84 mW/m2. This is consistent with the regional heat flow across the northern Cascadia margin (predicted to be ~72 mW/m2 here) but higher than the heat flow measured at Site 889 during Leg 146, indicating that heat flow at this site is not affected by sedimentation and fluid expulsion. In total, six PCS deployments were made, three in Hole U1329C and three in Hole U1329E. Of the six deployments, four retrieved sediments under pressure. All PCS cores retrieved under pressure were investigated by shipboard degassing experiments. In addition to the PCS, one HRC and one FPC were deployed in Hole U1329E. A full core at near in situ pressure was recovered by the HRC that contained 10 cm thick high-velocity zones and had a pore space methane concentration of 207 mM, equal to 1%–2% gas hydrate in total pore volume or 5%–10% gas hydrate if the gas hydrate were confined to the high-velocity zones. The FPC deployment recovered a core without pressure. Degassing of the four PCS cores from this site that were recovered under pressure showed variable gas concentrations with depth. Cores 311-U1329C-7P (55.6 mbsf) and 311-U1329E-7P (73.5 mbsf) from within the predicted depth of the GHSZ had total pore space methane concentrations of 69 and 76 mM, respectively, which would equate to a gas hydrate pore volume concentrations of <0.1% for both core samples. Core 311-U1329E-10P (125.0 mbsf), from near the base of the predicted GHSZ for this site, had a methane concentration of 92 mM, which falls below methane saturation at this depth, with no gas hydrate or free gas. The deepest PCS core (311-U1329C-23P; 188.5 mbsf) was taken at a considerable depth below the base of the GHSZ. This deep PCS core had a total pore space methane concentration of 236 mM, which suggests a potential pore volume free gas saturation of 2.5%. A potential gas release during the coring operation may have caused this to be an overestimate, yet it still supports the presence of free gas below the BSR at this site. Interstitial water analyses were carried out on samples from Hole U1329C with additional high-resolution sampling in the uppermost 15 mbsf of Hole U1329E to capture the SMI. Interstitial water salinities decrease from ~34 to ~30 at a depth of ~50 mbsf but remain almost constant at values of ~30.5 until near the bottom of the cored section in Hole U1329D. The deepest water samples from this site (>180 mbsf) show a sharp drop in salinities, from 28 to 22. A distinct chlorinity minimum (~15 mM decrease relative to the rest of the trend) is observed at the predicted depth of the BSR. Assuming that this chlorinity minimum is the result of gas hydrate dissociation during core recovery and handling, the observed chlorinity minimum would equate to ~2% in situ pore space filling of gas hydrate. The strong decrease in chlorinity and salinity below 180 mbsf are coincident with the pronounced increase in electrical resistivity in the LWD/MWD downhole logging data in logging Unit 3 and suggests communication with a deeper-sourced fresher fluid. The Ca and Mg depth profiles reflect carbonate and dolomite formation in the shallow section (SMI–60 mbsf) and also suggest mixing with a deeper-sourced fluid over a depth range from 60 mbsf to the BSR. Organic geochemical studies at Site U1329 included analysis of the composition of volatile hydrocarbons (C1–C5) and nonhydrocarbon (i.e., O2 and N2) gases from headspace gas samples, void gas samples, and gas samples recovered during PCS degassing experiments. The predominant hydrocarbon gas found in the cores from Site U1329 was methane; however, we did see an increase in ethane concentrations in the void gases collected from the stratigraphic section overlying the projected depth of the BSR. The increase in ethane concentrations near the BSR is reflected in the plot as low C1/C2 void gas ratios (Figs. F22, F23). Other studies have shown that preferential selection of ethane into the gas hydrate structure can depress the C1/C2 ratio of the gas hydrate relative to the source gas. Dissociation of the gas hydrate may, therefore, alter the void gas composition in recovered cores. However, diagenic production of ethane and mixing with a deep thermogenic source can produce the same effect. The C1/C2 ratios were generally high, suggesting a microbial origin for the observed methane. However, samples collected from deeper than 180 mbsf at this site exhibited significant increases in C2–C5 concentrations, which suggest an influence from a thermogenic hydrocarbon source. Detailed stable isotope data (13C/12C and D/H) are required to differentiate the relative contributions of microbial and thermal gases. Microbiological subsampling was routinely carried out on cores recovered from Hole U1329C. On each core run, PFT was continuously metered into the drilling fluid, and fluorescent microspheres were deployed on selected cores to investigate potential drilling fluid contamination of the core. These analyses confirmed that the center of each APC core and XCB biscuit remains undisturbed and suitable for microbiological analyses. Additional IR images were taken on the cut ends of each microbiological core section to document the thermal warming process of the core before subsampling. When considering the primary research objectives of this site, one of the most important features is its location on the relatively steep slope and the observation that sedimentation at this site is dominated by both slope turbidite deposition and erosional processes. The presence of thick turbidite sequences, including the presence of nonmarine diatoms in the stratigraphic section, is testament to the history of slope-dominated processes. The occurrence of a significant depositional hiatus or erosional event that juxtaposes Miocene with Pleistocene sediments at this site further documents the complex depositional and tectonic histories of this margin. The deepest core and downhole logging penetrations at this site also revealed a complex geologic history with unique high-resistivity, low-porosity conglomerate deposits that may represent thick debris flows. Geochemical analyses of recovered gas samples and interstitial fluids also document a complex fluid regime for this site. The relatively low interstitial fluid chlorinities observed near the seafloor may reflect paleoceanographic changes in seawater salinity. However, the Cl concentration decrease with depth strongly suggests communication with a deep-seated, fresher fluid, especially below 140 mbsf. For the most part, geochemical analyses of gases from this site reveal a similar story as the analyses of interstitial fluids, with most of the cored section dominated by a simple methane gas-rich system. But near the bottom of the cored section there are indications for the influence of a deeper hydrocarbon source. Identifying gas hydrate and supportive physical evidence of the BSR at this site remains an elusive goal. As seen in Figures F20 and F21, the subtle BSR is located only ~10 m above a prominent unconformity, which results in a complicated reflectivity pattern. Precoring LWD/MWD and conventional wireline logging data were used to identify and further characterize potential gas hydrate and related free gas occurrences at each site occupied during this expedition. The presence of gas hydrate is generally characterized by increases in measured electrical resistivities and acoustic velocities. The relative lack of resistivity or acoustic downhole log anomalies at this site supports a general inference of very low to no gas hydrate occurrence at this site. The lack of visual observations of gas hydrate and the occurrence of IR-detected thermal anomalies along the recovered cores also supports the assumption that this site contains little to no gas hydrate. However, geochemical analyses of interstitial fluids from near the depth of the predicted BSR may suggest the presence of a small amount of gas hydrate with minimum dissolved chloride concentration at ~125 mbsf, coincident with the predicted BSR depth. The HRC core taken from 114.6 mbsf had high-velocity layers that contained 5%–10% gas hydrate. The relative increase in ethane concentrations immediately above the depth of the BSR may indicate the presence of gas hydrate. Degassing experiments of three PCS cores that were collected from within the zone of predicted gas hydrate stability contained little gas hydrate, if any. However, a PCS core from 188.5 mbsf in Hole U1329C does suggest the occurrence of a free gas phase. The presence of free gas below ~160 mbsf may also be supported by a distinct change in seismic reflection character in the low-frequency seismic data of Line 89-08 (Figs. F21, F23). In contrast to the shallower section of the seismic line above 1.5 s two-way traveltime (TWT), no clear reflection can be identified and seismic frequency is strongly reduced below. This site was intended to characterize the eastern limit of gas hydrate occurrence on the northern Cascadia margin and establish an understanding of the geologic controls on the occurrence of gas hydrate along the transect of sites established during this expedition. All of the objectives set for Site U1329 are fulfilled. |
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m near the seafloor to ~1.5