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doi:10.2204/iodp.proc.311.105.2006 Physical propertiesSite U1327 is located near Leg 146 Site 889. Seismic data suggest the presence of slope basin sediments deposited on older, accretionary complex material. Seismic indicators of gas hydrate are present, and LWD/MWD data show a resistivity increase and density decrease at a depth of ~120 to 140 mbsf, indicative of significant gas hydrate accumulation. IR and in situ temperature data indicate that the GHSZ extends to 220–230 mbsf, consistent with the BSR. Gas hydrate distribution within the GHSZ is clearly heterogeneous on the scale of the distance between holes (~25 m). Physical properties were measured in cores recovered from Holes U1327B through U1327E. Hole U1327B missed the mudline and contained only one core. Hole U1329C reached a depth of 300 mbsf. Selected cores from gas hydrate–bearing zones were obtained from Hole U1327D, which was dedicated to special tools. Figure F34 presents an overview of the physical property data obtained at this site. 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. Because IR anomalies indicate gas hydrate dissociation during core recovery, catwalk sampling was conducted based on these scans. Portable Document Format images of the scans of all cores are available in the "Site U1327 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 F35 shows cold thermal anomalies in the concatenated false-color IR images for Holes U1327C and U1327D. This figure also shows the Hole 1327A LWD/MWD resistivity and pore water saturation derived from these data for comparison. Cold anomalies were detected in the cores from this site, consistent with the occurrence of significant amounts of gas hydrate between ~120 and 220 mbsf based on LWD resistivity measurements (see "Downhole logging") and chlorinity analyses (see "Interstitial water geochemistry"). 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. The resulting temperature profiles are shown in Figure F36. This processing enables us to measure the average amplitude of the cold anomalies and separate warm anomalies caused by voids from the background temperature field ("Physical properties" in the "Methods" chapter).
In Hole U1327C, cold thermal anomalies start at ~109 mbsf and continue to 225 mbsf. The highest number of anomalies and the largest negative difference from the background temperature ( It should be noted that recovery was poor in this depth range, especially for Cores 311-U1327C-14X, 17X, and 18X (see right side of Fig. F35), which is a common occurrence in gas hydrate–bearing sediments. Nonetheless, Core 311-U1327C-16X, which was located well within the depth range of the gas hydrate–bearing zone inferred from the LWD resistivity log in Hole U1327A, did not show any strong IR anomalies. Selected XCB cores obtained from Hole U1327D also show a different IR structure relative to Hole U1327C and relative to the Hole U1327A LWD resistivity results. Further discussion of the intrasite differences in resistivity and IR thermal anomalies is included in "Downhole logging" (see Fig. F66). The overall conclusion from this analysis is that there is significant lateral and vertical heterogeneity in gas hydrate distribution on a scale of ~25 m at Site U1327. Catwalk IR anomalies were systematically sampled for IW study (see "Interstitial water geochemistry"). The handheld IR camera was used to image the thermal structure of whole-round IW Sample 311-U1327D-15X-3, 49–59 cm (221.96–222.04 mbsf), after it was removed from the core liner and before it was dissected. The resulting IR images show that the cold zone in the IR image is coincident with a sand layer, indicating that gas hydrate was present in the sand but not in the surrounding fine-grained sediment. The chlorinity data from the sand portion of this IW sample show strong freshening (74.1 mM Cl), indicative of gas hydrate dissociation (see Table T3). IR images of the cut ends of core sections were also acquired with the handheld IR camera. Figure F37 shows a series of IR images of the section ends of Core 311-U1327D-2H, with a plot of associated cross-core temperature profiles. These images were collected sequentially as the core was processed for microbiology and IW studies and illustrate the temperature changes in the core over space and time. In addition to the IR images, core centerline temperatures were also taken and permit a direct estimate for emissivity (~0.89) of wet hemipelagic sediments near the seafloor at Site U1327. The deepest section shown in Figure F37 originated at 16.44 mbsf and 3.5°C (see "In situ temperature profile"). By the time catwalk sampling of the core was complete (1 h, 18 min) the core had warmed to 13°C in the center and 13.4°C at the edges. 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 T10). Both estimates of density were compared to in situ densities measured by the LWD/MWD tools (Fig. F34). In general, the agreement among these three data sets is good. Outlying low density points in the GRA data reflect the presence of cracks and voids. MAD and GRA data diverge significantly from the LWD density at specific depths. For example, we attribute differences among these data sets at 145 mbsf to the occurrence of high-density dropstones detected in the GRA measurements and visually in the cores. Dropstones were intentionally not sampled for MAD but would be detected and averaged out in the LWD/MWD measurements. Generally, MAD densities increase and porosities decrease with depth, although the rate of increase is less with increasing depth (Table T11). This density profile differs from that at Site U1329, which shows no increase in density from ~10 to 60 mbsf. The increase in density with depth, however, is slower than that predicted for normal consolidation (see "Shear strength") and consistent with rapid sedimentation based on biostratigraphic analysis (see "Biostratigraphy"). In detail, however, a few anomalous zones can be noted. Figure F38 shows porosity, bulk density, resistivity, and magnetic susceptibility from 100 to 170 mbsf. LWD/MWD porosity (Hole U1327A) increases from ~45% to 60% in the interval from 123 to 141 mbsf. This porosity increase, which is coincident with increased resistivity, has been interpreted to indicate the presence of gas hydrate. MAD porosity and bulk density are essentially unchanged over this depth interval. Contact resistivity also shows no change in this interval, with the exception of two data points that match the in situ resistivities. The MAD data suggest that density and porosity in this interval in Hole U1327C may differ from those in Hole U1327A. Although not definitive, this indication is consistent with the apparent lack of abundant gas hydrate in the upper part of this section based on the IR scans. Other significant mismatches between densities measured on core samples and in situ measurements occur at 100–110 and 150–160 mbsf. In both of these intervals, the MST and MAD bulk densities are lower and porosities are higher than in the LWD/MWD data from Hole 1327A. Because the MST and MAD data generally agree and because no unusual core features that can explain this difference were noted by the sedimentologists, we interpret this discrepancy as additional evidence for lateral heterogeneity. Magnetic susceptibilityMagnetic susceptibility data enabled us to compare the depth of Core 311-U1327B-1H, which missed the mudline, relative to Cores 311-U1327C-1H and 2H (Fig. F39A). Deeper in Hole U1327C, the magnetic susceptibility data also support inferences about interhole variability at this site. Magnetic susceptibility is low at 152–162 mbsf in Hole U1327C, whereas it is high only 30 m away in Hole U1327D (Fig. F39B). This change is associated with a change in lithology (Fig. F39B) and is possibly caused by a deposit containing numerous rounded and angular rocks of variable composition and size (see "Lithostratigraphy"). Large differences in magnetic susceptibility between Holes U1327C and U1327D also occur from 135 to 137 mbsf. These observations provide yet more evidence of the geologic heterogeneity on a scale of tens of meters that appears to be characteristic of Site U1327. Although the magnetic susceptibility differences may not be directly related to gas hydrate occurrence, a linkage between lithologic differences detected by the magnetic susceptibility and the occurrence of gas hydrate is possible. Compressional wave velocity from the multisensor track and Hamilton frameP-wave velocity was measured using the Hamilton frame and MST only on near-seafloor cores from Holes U1327B and U1327C (Table T12). Because of extensive cracking caused by gas exsolution during core recovery, no meaningful measurements could be made deeper than 20 mbsf. MST velocity measurements were made every 2.5 cm along Cores 311-U1327B-1H and 311-U1327C-1H and 2H (Sections 1–3). Measurements were made on the Hamilton frame in all three directions, taking care to measure undisturbed sediment. P-wave velocities could be measured to greater depth (20.4 mbsf) in the x-direction with the Hamilton frame (P-wave Sensor 3; PWS3) than with the MST because the waveforms were hand-picked. Velocities from both methods show a linear increase in velocity with depth. Measurements made with the Hamilton frame are generally higher than those made with the MST and have greater scatter than those observed at other sites. There is a linear correlation between velocity and density MST and MAD measurements (Fig. F40). Shear strengthShear strength measurements were made routinely throughout Hole U1327C using the handheld Torvane (Table T13) and the automated vane shear (AVS) (Table T14). Measurements were made on the working half of the split core after resistivity and velocity measurements had been completed and after MAD samples were 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 visible changes in the sediments (i.e., color and grain size). Shear strength data are shown in Figure F41. Measurements were made with the small Torvane from 75 to 90 mbsf (Table T13). Above and below this depth, measurements were made with the medium Torvane (see "Physical properties" in the "Methods" chapter). Comparison of data obtained with both vanes from the same section at Site U1325 (see "Physical properties" in the "Site U1325" chapter) indicates that Torvane measurements have large uncertainties at high shear strengths (see "Physical properties" in the "Site U1325" chapter). Nonetheless, a comparison between results obtained with the AVS and the handheld Torvane on two sections from this site, which were retrieved from storage for this experiment, reveals a similar range of shear strength values, providing some confidence in the results. Shear strength increases with depth. The ratio of shear strength to overburden pressure decreases from 0 to 40 mbsf and then becomes constant at a value of ~0.045 to 75 mbsf, indicative of sediment underconsolidation. A large but variable increase in shear strength is observed between 75 and 90 mbsf. Although no lithostratigraphic boundary has been defined at this depth, this depth represents a major change in physical properties as porosity decreases, thermal conductivity and contact resistivity increase, and grain density becomes less scattered (Figs. F34, F42). The cores are too deformed for reliable shear strength measurements deeper than 90 mbsf, the start of XCB coring. Electrical resistivityAlthough contact resistivity used to be routinely measured on Deep Sea Drilling Project and ODP legs, this measurement has not been made in recent years. During Expedition 311, these measurements were made using a standard four-pin Wenner-type probe array (Table T15). Contact resistivity measurements were made on all cores to 145 mbsf (Hole U1327C). At greater depths, the cores were too disturbed to yield meaningful results. Noncontact resistivity measurements were made to greater depth using the MST. Resistivity values show considerable scatter because of cracks within the sediments resulting from gas expansion during core retrieval. For the MST, measurement interval was 2.5 cm; for the contact resistivity, the measurement interval varied depending on core quality.
Pore water resistivities were calculated from the IW salinities using equations developed by Fofonoff (1985) and corrected to 20°C (Fig. F42A). Values of the whole sediment resistivity ranged from 0.3 to 2.4 Thermal conductivityThermal conductivity ranges from 0.6 to 1.2 W/(m·K) and shows no trend with depth (Table T16; Fig. F43). Values below 30 mbsf are low because of gas expansion cracks, which resulted in poor contact between the sediment and the thermal conductivity probe. The highest values 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. Attempts to evaluate the quality of individual measurements by comparison with core photographs and barrel sheets will be undertaken postcruise. In situ temperature profileAlthough seven deployments of temperature tools were attempted in Hole U1327C, only four provided usable data. Unfortunately, two tools failed and one was subjected to excessive heave. The data are shown in Figure F44. In situ temperatures extrapolated from the data are shown in Table T17 and compared to data from Site 889 in Figure F45. The temperatures determined for Hole U1327C are slightly higher than those determined at similar depths at Site 889 (Westbrook, Carson, Musgrave, et al., 1994) but are generally consistent with those data. The difference between the gradient determined from the Site U1327 data and the combined Sites 889 and U1327 data is not statistically significant. For the observed in situ salinity, the temperature gradient predicts the base of pure gas hydrate stability at 225–250 mbsf, consistent with other observations. Additional comparisons between the in situ temperatures recorded at this site and at other sites drilled during Expedition 311 are discussed in "In situ temperature profile" in the "Site U1326" chapter. 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. F46). 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 U1327. 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) are observed between 142 and 160 mbsf, offset from the high-resistivity and low-density zone observed on LWD/MWD logs in Hole U1327A. In particular, Core 311-U1327C-16X (124–132 mbsf) might be expected to exhibit anomalies similar to those in Core 19X (151–159 mbsf). Instead, the anomalies in Core 311-U1327C-16X are weak. In the selected cores obtained in Hole U1327D, IR images bracket a similar range as in Hole U1327C, but the numbers of anomalies and intensities are lower, except in Core 311-U1327D-11X.
m. These were used to calculate formation factor (ratio of saturated sediment resistivity to pore fluid resistivity) from the contact resistivities (Fig.