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doi:10.2204/iodp.proc.311.107.2006 Physical propertiesSite U1329 is the most landward site in the Expedition 311 transect across the accretionary margin. Seismic data suggest the presence of slope basin sediments that lap onto older, uplifted accretionary complex material. Seismic indicators of gas hydrate are present but more subdued than at all of the other sites. Physical properties were measured in cores recovered from Holes U1329B, U1329C, U1329D, and U1329E. Hole U1329B missed the mudline and contained only one core. Hole U1329C extended to 180 mbsf. The maximum depth of sampling was extended to 210 mbsf in Hole U1329D. Hole U1329E was dedicated to special tools. Figure F30 presents an overview of the physical property data obtained at this site. All cores from this site were systematically scanned as soon as they arrived on the catwalk to detect IR anomalies indicative of gas hydrate dissociation during core recovery. Compared to other sites, Site U1329 is notable for the very low magnetic susceptibility of the sediments in all but the uppermost ~10 m. This very restricted zone of relatively high magnetic susceptibility was valuable for correlating the shallow cores collected from different holes. It also differs from the other sites in that no gas hydrate IR thermal anomalies were observed and because density and porosity are nearly constant over most of the depth range sampled. Infrared images
All cores were scanned on the catwalk following the track-mounted IR camera procedures described in "Physical properties" in the "Methods" chapter. Portable Document Format images of the scans of all cores are available in the IR images of the cut ends of selected sections were acquired with a handheld IR camera. Figure F33 shows a typical IR image of a section end from Hole U1329C, with a plot of the associated cross-core temperature profile. The IR images of section ends were also checked for unusually cold centers that might indicate the dissociation of gas hydrate. No such features were observed. IR images were typically made of the cut ends of sections identified for microbiological sampling and provided a measurement of the temperature profile through the cores just prior to removing them to the microbiology sampling area, which was maintained at 4°C. These images provide constraints on the maximum temperature to which the microbiology samples were exposed. For example, the core imaged in Figure F33 originated at a depth of 44.10 mbsf and a temperature of 6.5°C (see "In situ temperature profile"). The temperature had warmed to 10.1°C at the center of the core and ~12°C at the edges by the time the core reached the catwalk. These temperature estimates are based on an IR emissivity of ~0.89 for wet sediment and are corrected for ambient temperature at the time the IR images were collected. Sediment density and porosityGamma ray attenuation (GRA) densities were measured on the multisensor track (MST), and bulk density, grain density, and porosity were calculated from the measured wet and dry weights and dry volume of the sediments (moisture and density [MAD] measurements; Table T9). Both estimates of density were compared to in situ densities measured by the LWD/MWD tools (Fig. F30). These three different estimates of density were very similar, with a few notable exceptions. We note the remarkable correspondence between the MAD and GRA density measurements, particularly when outlying low GRA density data caused by the presence of cracks and voids are ignored. The MAD, GRA, and LWD/MWD densities show a gradual increase in the uppermost 10 m, below which the average density is approximately constant to at least 125 mbsf (Fig. F34A). This unusual density and porosity profile (Fig. F34B, F34C), which suggests underconsolidation of the sediments through most of the cored interval (see "Shear strength"), contrasts with porosity profiles observed at all other sites (see Fig. F37 in the "Site U1325" chapter). From 170 to 190 mbsf, porosity values determined from MAD data do not decrease as smoothly as the rapidly decreasing porosity values determined for this interval from the LWD data (Fig. F34B). This anomalous behavior is likely an artifact of drilling disruption in the MAD measurements. Although the transition from APC to XCB coring occurred at Core 311-U1329C-18X (140.68 mbsf), a distinct increase in biscuiting and intrusion of drilling mud occurred at Core 22X (198.90 mbsf), as shown in Figure F34D. We speculate that a transition from ductile to brittle behavior occurred as a result of increased lithification at 170 mbsf and that Cores 311-U1329C-21X and 22X (170–189 mbsf) shattered in response to drilling. Below 185 mbsf, density increases abruptly and LWD and MAD porosity decreases to 10% and 30%, respectively. We note that only two highly disturbed cores (Cores 311-U1329C-23P and 311-U1329E-21X) were recovered from this zone. Core 311-U1329C-23P contained numerous hard clasts that were not included in the MAD analysis; therefore, the 30% porosity obtained by analyzing the physical properties data is probably overestimated relative to the bulk formation porosity. Active migration of low-salinity fluids through this zone is suggested by IW chemistry (see "Interstitial water geochemistry"). In light of the high density and low porosity in this zone, as determined by LWD/MWD and MAD data, pervasive microcracks are unlikely here. We conclude that fluid migration occurs through this formation by macrofractures or other discrete, high-permeability structures that do not affect the index properties measured in core samples. Magnetic susceptibilityThe magnetic susceptibility at this site is relatively high in the uppermost 10 m but then decreases precipitously and remains low for the remainder of the cored interval (Fig. F30). Moreover, the pattern within the zone of high susceptibility is very consistent between holes, allowing us to determine accurate relative depths between the holes and enabling more detailed interpretation of shallow chemical and biological processes. This correlation is shown in Figure F35. We note that the relative depth shifts between holes indicated by the magnetic susceptibility data have not been applied to the tabulated data that accompany this report or to data included in the Janus database. They should, however, be taken into account for detailed comparisons between holes. Compressional wave velocity from the multisensor track and Hamilton FrameP-wave velocity was measured using the Hamilton frame and MST on shallow cores from Holes U1329B, U1329C, and U1329E (Table T10). MST velocity measurements, made at 2.5 cm intervals, from Cores 311-U1329B-1H, 311-U1329C-1H, 311-U1329E-1H, and 2H (Sections 1–3) are shown in Figure F36. The MST did not yield good results at greater depth because of pervasive gas expansion cracks in the sediment. P-wave velocity measurements were also made in all three directions using the Hamilton frame. Values were obtained for Sections 311-U1329B-1H-1 through 1H-4, Core 311-U1329C-1H through Section 2H-2, and Sections 311-U1329E-1H-1 through 1H-3. Velocities were obtained in the x-direction from slightly greater depths when compared to the y- and z-directions. P-wave velocity ranges from 1429 to 1550 m/s. Velocity measurements made with the Hamilton frame have generally higher values and more scatter than those made with the MST. The velocity versus depth profiles in the y- and z-directions are approximately parallel to the MST velocity profile; both show a general increase in P-wave velocity with depth. There are, however, intervals where velocity decreases with depth. An example can be seen in the uppermost 1 m of Core 311-U1329E-1H (Fig. F36), where MST velocity drops linearly from 1490 to 1475 m/s. There is a fairly good correlation between the density and velocity profiles, which may explain the local decreases in velocity with depth. Shear strengthShear strength measurements were made routinely throughout Holes U1329B, U1329C, and U1329E using the handheld Torvane apparatus (Table T11; Fig. F37A). Measurements were made on the working half of the split core after resistivity and velocity measurements had been completed and after MAD samples had been taken. Measurements were taken in areas of the core where the sediment was undisturbed and as close as possible to where MAD samples had been extracted. At least one shear strength measurement was taken per section and often more where there were visual changes in the sediments (i.e., color and grain size). Measurements made to 155 mbsf in Hole U1329C and to 33 mbsf in Hole U1329E varied between 20 and 190 kPa. Shear strength data from Holes U1329B, U1329C, and U1329E show good agreement. Shear strength increases linearly with depth as a result of compaction and cementation of the sediment. Values are scattered at depth because of drilling disturbance, especially in XCB cores, or gas expansion cracks. The ratio of shear strength to overburden pressure is a measure of the consolidation state of the sediments (Fig. F37B). A ratio >0.25 indicates overconsolidation (Riedel et al., 2006). No overconsolidation is apparent in these sediments. We conclude that these sediments have not been eroded, in contrast to sediments at the flanks of southern Hydrate Ridge, located in the Cascadia accretionary complex offshore Oregon (Tréhu, Bohrmann, Rack, Torres, et al., 2003; Riedel et al., 2006). The apparent underconsolidation of the sediments is consistent with rapid sedimentation and very high diatom content (see "Lithostratigraphy" and "Biostratigraphy"). Electrical resistivityElectrical resistivity was measured on core samples by both contact and noncontact methods. Although contact resistivity measurements were a part of the standard Deep Sea Drilling Project and ODP physical property measurement suite for many years, this measurement has not been routinely made during recent years. During Expedition 311, these measurements were made using a standard Wenner-type probe array provided by Randolph Enkin (Geological Survey of Canada) (Table T12). Results for both contact and noncontact resistivity are plotted in Figure F30, where they are compared with the LWD ring resistivity. Contact resistivity and noncontact resisitivity are also plotted versus depth in Figure F38A and F38D, respectively. The contact resistivity measurements show structure that is similar to the LWD ring resistivity, except that contact resistivity is shifted to lower values in the uppermost 20 m and to higher values below ~70 mbsf. Scatter in the contact resistivity data likely arises from variations in moisture content, cracking caused by gas expansion, and drift resulting from chemical reactions of the silver or gold electrodes with the sediment. The drift issues were largely solved for contact resistivity measurements at Sites U1325, U1326, U1327, and U1328. MST noncontact resistivity exhibits extensive scatter, with anomalously high values caused largely by the presence of cracks and voids. However, the low-resistivity envelope of measured values, which might be interpreted to represent the resistivity of undisturbed sediments, does not follow the same trend as the LWD ring resistivity, particularly in the uppermost 20 m and from ~140 to 190 mbsf (TD of Hole U1329C). Overall, the noncontact resistivity values are shifted to lower values compared to the LWD/MWD and contact resistivity measurements. Contact and noncontact resisitivity values can be used to calculate the formation factor (ratio of saturated sediment resistivity to pore fluid resistivity; Fig. F38B, F38E). Using Archie's relation, it is possible to estimate porosity independently of the MAD porosity, and results are plotted in Figure F38 for m = 1.7 and a = 1. Agreement is reasonably good for contact resisitivity–derived porosity except at 50–85 and 90–145 mbsf. Thermal conductivity
Thermal conductivity ranges from 0.8 to 1.2 W/(m·K), with most values above 1.0 W/(m·K) (Table
T13; Fig. F30). Values were corrected for in situ temperature and pressure using the equation of Pribnow et al. (2000). This correction is small for the depth range and geothermal gradient at this site (Fig. F39). No significant trend with depth is observed. Values <1.0 W/(m·K) are likely caused by gas expansion cracks producing poor contact between the sediment and the thermal conductivity probe. The average value of all measurements >1.0 W/(m·K) is 1.1 ± 0.06 (1 In situ temperature profileThe APCT16 and APCT-3 temperature tools were deployed three and two times, respectively, in Hole U1329C. Two APCT and two DVTP deployments were conducted in Hole U1329E (Table T14; Fig. F40). With the exception of the APCT run on Core 311-U1329C-3H, which was extracted from the sediment prematurely, the measurements are of excellent quality in Hole U1329C. Ship heave during coring in Hole U1329E led to small, secondary frictional pulses at shallower depths, although the deeper DVTP data are of excellent quality. The data for this site are relatively well calibrated because of the controlled calibration tests performed at Oregon State University (OSU) prior to the expedition (see "Physical properties" in the "Methods" chapter). During the OSU test, an absolute calibration shift of +0.03°C was determined for the APCT-3. A shift of –0.97°C was determined for the APCT16 relative to the APCT-3 by examining their responses in hot- and cold-water baths on the ship. A calibration shift of ~0.9°C relative to APCT16 (~0°C relative to APCT-3) was determined for the DVTP by deploying it immediately after the APCT16 and comparing the apparent in situ sediment temperatures. In situ temperatures determined by extrapolating the observations using TFIT (APCT and APCT-3) and CONEFIT (DVTP) are shown in Table T14. A thermal conductivity value of 1.0 W/(m·K) was assumed for this calculation; raising the thermal conductivity to 1.1 W/(m·K) increases the estimated in situ temperature by ~0.1°C. No extrapolation was needed for the DVTP deployed at 127.1 mbsf, which showed that equilibrium had been reached on both thermistors. It is important to note that this measurement was made right at the expected BSR, within the uncertainty of depth estimates for both the tool placement and the seismic data (Fig. F41). The data indicate a thermal gradient of 72 ± 4°C/km, significantly higher than the thermal gradient of 54°C/km obtained at ODP Site 889 (Westbrook, Carson, Musgrave, et al., 1994) but consistent with the regional heat flow (Hyndman and Wang, 1993). The seafloor intercept is 3.34°C ± 0.34°C, consistent with regional hydrographic data. The measurement uncertainty implied by the data misfit is 0.27°C, consistent with our estimate of the total uncertainty in the data resulting from uncertainties in measurement, calibration, and thermal conductivity. Figure F41 shows the geothermal gradient compared to the predicted gas hydrate stability boundary for pure methane hydrate and freshwater (0%), seawater (3.5%), and saline brine (10%). The depth of the base of gas hydrate stability inferred from seismic data (~126 mbsf) corresponds very well to the depth predicted from the geothermal gradient obtained from in situ temperature measurements. PaleomagnetismNote: This section was contributed by Jennifer Henderson and Katerina Petronotis (Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station TX 77845, USA). Alternating-field (AF) demagnetization of the sedimentary archive-half sections was used to determine the remanent magnetization components recorded in the recovered core (Fig. F42). The paleomagnetic data will be used postcruise to characterize the magnetic properties of the sediments and to construct a magnetostratigraphy of the sedimentary section recovered at Site U1329. The AF demagnetization applied at 10 and 20 mT should have removed most of the drill string magnetic overprint, but postcruise demagnetization at higher fields will most likely be required. Questionable data may be associated with remanence measurements over intervals disturbed or deformed by coring. Similarly, magnetic edge effects, which can be large when measurements are within ~5 cm of the edge of a section or edge of a void, can give biased results. To avoid interpreting results in these regions, we manually noted the disturbed intervals and voids in the cores. Data from these intervals will be removed prior to postcruise interpretation. |
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T for this anomaly is small at ~0.8°C. The overall downhole temperature trend is broadly consistent with increasing in situ temperature with depth and with the XCB cores typically exhibiting cooler temperatures than APC cores. Subtle cooling caused by small amounts of disseminated gas hydrate cannot be ruled out and will be assessed postcruise.
). The increase in thermal conductivity with depth proposed for this region by Davis et al. (1990) (see also