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doi:10.2204/iodp.proc.311.103.2006

Physical properties

Site U1325 is located in a small basin on the lower slope between the first and second accretionary ridges and provides a contrast to the accretionary ridges sampled at Sites U1326 and U1327.

Physical properties were measured in cores recovered from Holes U1325B and U1325C. Hole U1325B extended to 210 mbsf, where it was abandoned because of a stuck PCS tool. Coring in Hole U1325C began at 188 mbsf and continued to ~305 mbsf. Figure F34 presents an overview of the physical property data obtained for these holes. All cores from this site were systematically scanned upon arrival on the catwalk to detect IR anomalies indicative of gas hydrate dissociation during core recovery. Cold temperature anomalies were observed at a wide range of depths from ~70 to 250 mbsf, and catwalk sampling was conducted based on these scans. A total of 86 IW samples were taken based on the IR images to extend the chlorinity anomaly database available for calibrating IR data as a proxy for gas hydrate saturation. In the following sections we discuss these data and compare them to data acquired at other sites.

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 "Site U1325 core descriptions." Temperature arrays in comma-separated value file format were exported from the IR camera software and concatenated for each core. The arrays were then further concatenated for all cores available in a given hole. Figure F35 shows cold thermal anomalies in the concatenated false-color IR images for Holes U1325B and U1325C. The Hole U1325A LWD/MWD resistivity images and pore water saturation derived from these data are shown on the left for comparison. Core recovery and core handling times (i.e., the time from the "core on deck" call to IR imaging) are shown on the right. Core recovery decreased and coring time increased in zones where large amounts of gas hydrate are indicated.

Although the upper two cores at this site were cold (7°–8°C), this was likely caused by their low in situ temperature (3°–4°C), which was a result of the cold bottom water temperature at this depth (~1.9°C). The bottom water temperature is lower than the seafloor intercept calculated from linear regression of subsurface temperatures (see "In situ temperature profile"), suggesting a relatively recent change in bottom water temperature. The first gas hydrate–related IR anomaly occurred at ~70 mbsf and was confirmed by a low-chlorinity anomaly. Anomalies continued to be observed sporadically to ~250 mbsf, with a cluster of IR anomalies at ~140–160 mbsf. Coverage from 80 to 230 mbsf was adversely affected by poor recovery (Fig. F35) and one instance of IR track failure (Core 311-U1325C-24X). We note that the maximum depth of observed IR anomalies is deeper than the anticipated BSR depth of 230 mbsf. Gas hydrate was present to at least 250 mbsf, as inferred from low-chlorinity anomalies in IW samples chosen on the basis of IR anomalies. This deeper depth for the base of the GHSZ is compatible with downhole temperature measurements.

As a precursor to quantitative studies of the temperature anomalies, downhole temperatures were averaged for each pixel row in the IR temperature array, excluding pixels ~1 cm from the edge of the image and 2 cm along the midline of the image to eliminate major thermal artifacts in the images. Figure F36A shows the complete temperature profile in this hole. Figure F36B and F36C provides a detailed look at one particular temperature anomaly at 209.5 mbsf with a T of –2.5°C. This anomaly corresponds very closely to a high-resistivity anomaly in the Hole U1325A LWD resistivity data (Fig. F36D). Figure F37 shows a sample that was taken based on a similar IR anomaly. A coincident photograph and IR image of the sample are shown as well as a temperature profile through the anomaly. This anomaly was associated with a ~3 cm thick dipping sand-rich layer.

Sediment density and porosity

Gamma 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 are compared to in situ densities measured by the LWD/MWD tools (Fig. F34). The MAD data from 55 to 85 mbsf are compared to LWD/MWD data and to contact resistivity, magnetic susceptibility, and thermal conductivity in Figure F38. The different data sets show coincident decreases in density, resistivity, magnetic susceptibility, and thermal conductivity and an increase in porosity that corresponds to Core 311-U1325B-10X, where sand layers are conspicuously absent compared to the overlying sediments. The first IR anomaly occurs near the boundary between this clay-rich interval and the overlying sand-rich sediments. This IR anomaly is coincident with a low-chlorinity IW analysis.

Figure F39 shows a comparison between the data at this site and at other sites drilled during Expedition 311. It shows the increasing amount of sand, indicated by low porosity "outliers," toward the outer part of the accretionary complex. It also shows that the sediments cored at Sites U1328 and U1329 have generally higher porosity than sediments at Sites U1327 and U1325, especially at depths >150 mbsf. The difference between Sites U1327 and U1328 is consistent with differences in the velocity structure at these sites, as indicated by VSP results.

Magnetic susceptibility

Magnetic susceptibility at this site was generally high and variable, similar to that observed at Site U1328. Causes for the intersite variability in magnetic susceptibility will be a topic for postcruise research. The variability is likely related to changes in the abundance of silty and sandy turbidites, reflecting different sedimentary environments. The spatial correlation between low magnetic susceptibility and a high-porosity, clay-rich layer was discussed above. This contrasts with the correlation between a moderate-porosity, very stiff clay-rich interval and the low magnetic susceptibility observed at Site U1327.

Compressional wave velocity from the multisensor track and Hamilton frame

Velocity measurements using both the MST and the Hamilton frame were made only on the single core from Hole U1325D. Velocity measurements were obtained from 0.1 to 4.2 mbsf and ranged from 1465 to 1530 m/s. The MST produced results that were consistently lower than the results from the Hamilton frame. The Hamilton frame results generally had a greater scatter than the results from the MST; however, both data sets follow the same trend. Velocity tends to decrease with depth (Fig. F40), contrary to typical observations. Ordinarily, velocity increases with depth in near-seafloor sediments. Velocity also appears to decrease with increasing density (Tables T9, T10). Again, this contrasts with typical data trends, and we have no obvious explanation for these observations.

Shear strength

Shear strength measurements were made throughout Holes U1325B and U1325C using both the handheld Torvane and the automated vane shear (AVS) system to compare the two techniques over a broad range of shear strengths (Tables T11, T12). Measurements were made on the working half of the split core after resistivity and velocity measurements had been completed (for the AVS) and before or after MAD samples had been taken (for the Torvane). Measurements were taken in areas of the core where the sediment was minimally disturbed and as close to the location of MAD samples as possible. At least two Torvane shear strength measurements were taken per section and often more where there were visual changes in the sediment (i.e., color and grain size).

Shear strength measurements obtained with the AVS are similar to those obtained with the handheld Torvane (Fig. F41A). Shear strength values generally increase with depth and range from 5 kPa at the top of the hole to 205 kPa at the bottom. The ratio of shear strength to overburden pressure is a measure of the consolidation state of the sediments (Fig. F41C). A ratio >0.25 indicates overconsolidation and a ratio <0.25 indicates underconsolidation (e.g., Riedel et al., 2006). Except for the uppermost 5 m of the hole, the sediments are underconsolidated. Overconsolidation at the top of the hole may be caused by erosion or high carbonate content in this area.

Electrical resistivity

Electrical resistivity was measured on core samples by both contact and noncontact methods. The contact resistivity measurement interval varied depending on the quality of the core. Noncontact resistivity (NCR) measurements were made using the MST at 2.5 cm intervals on all cores. The values obtained from the NCR sensor were scattered because of gas expansion cracks in the cores. Contact resistivity measurements were made to 260 mbsf and range from 0.3 to 3.0 m (Fig. F42A; Table T13). Higher resistivities were recorded, but these are almost certainly caused by cracks in the sediment or roughness of the split core surface, which occurs in stiff sediments. Resistivity generally increases with depth and becomes more scattered; however, there are areas where there is a marked decrease in resistivity with depth (i.e., at 60–70 mbsf).

Pore water resistivities were calculated from IW salinities using equations developed by Fofonoff (1985) and corrected to 20°C (Fig. F42A). These were used to calculate the formation factor (ratio of saturated sediment resistivity to pore fluid resistivity) of the contact resistivities (Fig. F42B). Archie's parameters were then determined by fitting Archie's equation to the formation factor and MAD porosity data. At this site, sands and clays were separated based on notes made when samples were taken for MAD analysis. Distinctly different Archie's parameters were obtained in the two cases (Fig. F42D). Porosities obtained using Archie's equation for clays were compared to MAD porosities (Fig. F42C); both techniques produced similar values and trends.

Thermal conductivity

Thermal conductivity was measured on whole-round cores after completion of the MST measurements. Thermal conductivity values ranged from 0.6 to 1.3 W/(m·K) (Table T14; Fig. F34). Low thermal conductivities below 100 mbsf may be caused by the presence of gas expansion cracks. Above 100 mbsf, thermal conductivies >1.1 W/(m·K) were consistently obtained in sand-rich zones; lower thermal conductivities appear to be associated with clay-rich zones. This apparent correlation with lithology will be further investigated postcruise by identifying the measurement points on core photos to better evaluate the impact of core disturbance on the measurements. Because thermal conductivity is measured on whole-round cores, determination of sediment type for each thermal conductivity measurement cannot be assigned at the time the measurement is made. The highest thermal conductivity values generally follow the regional trend defined by Davis et al. (1990). For determination of in situ temperature and heat flow, a constant thermal conductivity value of 1.1 W/(m·K) was assumed.

In situ temperature profile

Six deployments of temperature tools were attempted at Site U1325, and all deployments provided useable data (Table T15; Fig. F43). The three APCT-3 tool deployments in Hole U1325B occurred during times of relatively low heave and yielded excellent quality data. Although heave increased during coring of Hole U1325C and the deployment at 218.6 mbsf was clearly affected, enough time elapsed that the data could be reliably extrapolated to obtain an estimate of in situ temperature. It should be noted, however, that the DVTP recorded unusually high temperatures on the upper thermistor (black line in Fig. F43B). We suspect that the probe rubbed against an asperity in the hole near the upper thermistor. Low temperatures recorded on the upper thermistor at 275.4 mbsf suggest incomplete insertion of the probe into the very stiff sediments near the bottom of Hole U1325C.

Temperatures extrapolated from the data are plotted on Figure F44. The size of the symbols corresponds to data uncertainties of ±0.35°C and ±2.5 m, generous estimates for these high-quality, well-calibrated 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. This is similar to the thermal gradient observed at Sites U1327 and U1328 and lower than the regional gradient expected at this distance from the deformation front. The results are consistent with perturbation of the geothermal gradient because of rapid sedimentation and upward fluid advection (Hyndman and Davis, 1992) and should help to better constrain models for these processes.

The temperature data indicate that the base of the GHSZ for hydrostatic pressure and seawater salinity is at 275 ± 25 mbsf. This is considerably deeper than the depth to the BSR of 230 mbsf determined by assuming an average velocity of 1636 m/s for sediments above the BSR, which is located 0.282 s TWT beneath the seafloor. Several explanations can be considered for this apparent discrepancy. The apparent mismatch may be because of uncertainty in the seismic velocity; an average P-wave velocity of 1920 m/s would place the BSR at 270 mbsf but appears unreasonably high. Alternative explanations that have been proposed to explain mismatches observed elsewhere between the seismically determined BSR depth and the depth predicted by in situ temperature measurements include inhibition of gas hydrate formation in clay-rich sediments and disequilibrium caused by climate change (e.g., Ruppel, 2000). A GHSZ limit of 275 mbsf at Site U1325 is consistent with the occurrence of IR anomalies at this site, assuming Structure I gas hydrate, which are observed to at least 245 mbsf (with possible candidates to 265 mbsf) at this site.

Paleomagnetism

Note: 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 cores (Fig. F45). 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 U1325. 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 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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