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doi:10.2204/iodp.proc.311.104.2006 Physical propertiesSite U1326 is located at the crest of the westernmost accretionary ridge on the Expedition 311 transect across the northern Cascadia margin accretionary complex. A major objective of this site was to determine the physical properties and gas hydrate distribution in this distal accretionary ridge. Physical properties were measured in cores recovered from Holes U1326B, U1326C, and U1326D. Hole U1326C extended to 86.7 mbsf, where it was abandoned for technical reasons. Coring in Hole U1326D began at 78.8 mbsf and continued to 271.4 mbsf. Figure F31 presents an overview of the physical property data obtained from these two 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 40 to 250 mbsf, and catwalk sampling was conducted based on these IR scans. Numerous IW samples were taken based on the IR images to extend the chlorinity anomaly database and to calibrate IR data as a proxy for in situ gas hydrate concentration. Two sections with thick sandy and silty zones and high gas hydrate saturation were examined in more detail. In the following sections we discuss these data and compare them to data acquired at other sites. Infrared imagesAll 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 "Site U1326 core descriptions." Temperature arrays in the format of comma-separated value files were exported from the IR camera software and then concatenated for each core. The arrays were then further concatenated for all cores available in a given hole. Figure F32 shows cold thermal anomalies in the concatenated false-color IR images for Holes U1326C and U1326D. This figure also shows the Hole U1326A LWD resistivity data and the pore water saturation derived from these data for comparison.
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 the major thermal artifacts in the images. On first look, a striking aspect of Hole U1326D is that it is consistently colder below ~150 mbsf (Fig. F33A). Comparison of the temperature profile with the ambient temperature on the catwalk indicates that this change in core liner temperature is closely correlated with a rapid change in ambient air temperature. The IR camera measures the temperature of the surface of the core liner, which is a function of the gradient between the air temperature outside the liner and the temperature of the core. In Figure F32, the cold core liner temperature makes it more difficult to see that the entire interval from 40 mbsf to the base of Hole U1326D contains many distinct cold anomalies. The Many of the IR anomalies were used to identify samples for IW chemical studies. Sample 311-U1326C-6X-4, 83–96 cm, is shown in Figure F35. This sample contained pore-filling gas hydrate in a 4–5 cm thick, medium- to coarse-grained sand layer. The gas hydrate appeared to be filling pores in the sand. Upon physical dissection, it was clear that the central part of the sand layer contained more gas hydrate than sediment, suggesting both pore-filling and grain-supporting gas hydrate in this sample. The IR image and temperature profile in Figure F35 show that the temperature of the gas hydrate at the time of core processing was as low as –2.4°C. The chlorinity anomaly corresponding to this IR anomaly was 197.7 mM, which represents very strong pore water freshening (see Table T3). A moisture and density (MAD) sample was taken from the sand layer containing the gas hydrate. This sample was notable for its relatively coarse grain size and the occurrence of buff-colored fine silt or clay coating on the sand grains after drying. MAD results were as follows: bulk density = 2.21 g/cm3, grain density = 2.84 g/cm3, and porosity = 34.7%. The porosity value is among the lowest measured and is consistent with the relatively coarse grain size of the sample. Another notable gas hydrate occurrence was at 246 mbsf in Hole U1326D. The lower part of Core 311-U1326D-18X was characterized by an unusually thick cold anomaly (Fig. F33B, F33C). The upper part of this anomaly was processed by the inorganic geochemists and the lower part (Section 311-U1326-18X-4) was opened immediately after sectioning on the catwalk. The sediment in this core (Fig. F33D) had a very unusual texture. It remained cohesive and was "foamy" rather than being soupy or mousselike (i.e., it felt like foam rubber to the touch). Sediment in this section lost its foamy texture in <1 h, collapsing to a stiff, dry sediment that occupied ~25% of the original volume in the core liner. Additional analyses will be performed postcruise to better understand the reasons for and implications of this unusual sediment character. 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 (MAD measurements; Table T10). Both estimates of density were compared to in situ densities measured by the LWD/MWD tools (Fig. F31). Significant differences are seen between MAD and LWD porosities in the uppermost 25 m of Hole U1326C, indicating significant lateral heterogeneity over the ~25 m between Holes U1326A and U1326C or, more likely, problems with the LWD data at these shallow depths. MAD porosities of 53% ± 5% in the uppermost 20 m are unusually low for near-seafloor sediments. The data, however, are consistent with relatively high shear strengths and with unusually high P-wave velocities recorded both on the Hamilton frame and by wireline logging. A possible explanation for the difference in shallow sediment characteristics between holes at this site is the presence of a fault between the holes, consistent with the seismic data and uplifted nature of Site U1326. Mass wasting that reveals previously buried and overconsolidated sediments may also be a factor at this site. Magnetic susceptibilityMagnetic susceptibility at this site was generally high and variable, except in the uppermost 10 m (Fig. F31). Causes for the variability in magnetic susceptibility will be a topic for postcruise research. It is likely related to variable input of silty and sandy turbidites, reflecting different sedimentary environments. Compressional wave velocity from the multisensor track and Hamilton frameP-wave velocities were measured using both the MST and the Hamilton frame on a limited number of sections from the top of Hole U1326C (Table T11). Velocity measurements ranged from 1480 to 1580 m/s. Velocities at Site U1326 are much higher in the shallow subsurface than at other sites (Fig. F36). This correlates well with the shear strength data and may be related either to a high carbonate content in the sediments or to mass wasting, which would have exposed previously buried sediments at the seafloor. Shear strengthShear strength measurements were made in Holes U1326C and U1326D using both the automated vane shear (AVS) system and the handheld Torvane (Tables T12, T13). Measurements were made on the working half of the split core after resistivity, velocity, and MAD samples had been taken. Efforts were made to take measurements in areas of the core where the sediment was undisturbed by gas expansion and drilling cracks. Measurements were also taken as close as possible to where MAD samples had been extracted. At the top of Hole U1326C, at least two Torvane measurements and one AVS measurement were taken per section. The number of measurements decreased deeper in the hole because of increased disturbance in the cores. Shear strength generally increases with depth (see Fig. F31 for all shear measurements and Fig. F37A for measurements from 0 to 120 mbsf). Shear strength ranges from 5 kPa in sands to 300 kPa in clay. Shear strengths are unusually high in the shallow subsurface (Fig. F37A). The ratio of shear strength to overburden pressure (Fig. F37B) is a measure of the consolidation state of the sediments. A ratio >0.25 indicates that the sediments are overconsolidated for their depth below the seafloor. Overconsolidation can be seen in the uppermost 20 m of Hole U1326C (Fig. F37C). This correlates well to the high P-wave velocity and may be caused by high carbonate content in the shallow sediments or to mass wasting, which has been documented nearby and would have exposed previously buried sediments at the seafloor. Electrical resistivityElectrical resistivity was measured in Holes U1326B, U1326C, and U1326D using both the contact and noncontact methods. Measurement interval using the contact system varied depending on the quality of the core. Measurements taken with the noncontact method were made every 2.5 cm. Gas expansion of the core caused the results to be very scattered. The high degree of scatter may also be caused by the large amount of sand at this site.
Resistivity values ranged from 0.3 to 2.5 Thermal conductivityThermal conductivity values were less scattered at this site than at other sites (Table T15; Fig. F31). Examination of split cores in the intervals in which the measurements were taken suggests that thermal conductivities <0.9 W/(m·K) are associated with core disturbance caused by gas expansion. Thermal conductivities of 0.9–1.1 W/(m·K) appear to generally be related to clay-rich intervals, whereas thermal conductivities >1.1 W/(m·K) appear to be related to coarser grained intervals. This relationship will be examined more carefully postcruise. 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 profileFour deployments of temperature tools were attempted at Site U1326 (Table T16). A third-generation advanced piston corer temperature (APCT-3) tool deployment on Core 311-U1326C-4H at 30.4 mbsf provided very good quality data (Fig. F39), although the initial frictional pulse was unusually high. The extrapolated temperature at this depth of 4.30°C is similar to the temperature of 4.44°C obtained at 33 mbsf at Site U1325. Unfortunately, the transition to XCB coring occurred immediately after this core, so that no more APCT-3 measurements were possible at this site. Given the high ship heave, a worsening weather forecast, and limited time available at this last site, the decision was made to only attempt DVTP measurements near the expected base of the GHSZ. Three attempts were made at depths of 252, 271.4, and 300 mbsf. For the last two attempts, mud was pumped into the borehole in an attempt to damp the effect of heave. All three attempts briefly penetrated the seafloor but did not remain coupled to the sediment long enough to estimate in situ temperature by modeling the decay of the frictional pulse generated by probe insertion. Instead, the data show a gradual increase in temperature that appears to approach temperatures in the range of 16°–19°C. Because of questions about the depth to the base of the GHSZ at this site, postcruise work will include an attempt to estimate the in situ temperature by modeling heating of the borehole by the surrounding rock. Figure F40 shows a compilation of all in situ temperature estimates from Expedition 311 compared to results from Site 889. The implied heat flow, assuming a constant thermal conductivity of 1.1 W/(m·K), is also shown and compared to the regional heat flow determined by Hyndman and Wang (1993). Temperatures at Site U1329 are clearly higher at a given depth. Heat flow across the lower slope (Sites U1325, U1327, and U1328) appears to be depressed compared to the regional heat flow pattern, consistent with perturbation by a high sedimentation rate and upward fluid advection (Hyndman and Davis, 1992). Postcruise analysis will focus on detailed examination of data uncertainties and on the processes that can explain intersite and intrasite variation. 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. F41). 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 U1326. 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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Ts of these cold anomalies are not affected by changes in ambient temperature because the background temperature and the cold temperature excursions are affected equally. Figure 
m (Table