Previous   |   Next

doi:10.2204/iodp.proc.311.106.2006

Physical properties

Site U1328 is located within the Bullseye vent (Riedel et al., 2002). Seismic data suggest the presence of slope basin sediments deposited on older, accretionary complex material. This site was designed to probe the deep structure of this vent.

Physical properties were measured in cores recovered from Holes U1328B, U1328C, U1328D, and U1328E. Most of the samples discussed in this report were from Hole U1328B, which was sampled from 0.7 to 56.2 mbsf, and from Hole U1328C, which was sampled from 56.7 to 279.6 mbsf. Selected cores from gas hydrate–bearing zones were obtained from Hole U1328E, which was dedicated to special tools. Figure F37 presents an overview of the physical property data obtained at this site. Cores from this site were systematically scanned upon arrival on the catwalk to detect IR anomalies indicative of gas hydrate dissociation during core recovery. Many cold temperature anomalies were observed in the GHSZ, and catwalk sampling was conducted based on these scans. Notably, there were also cold thermal anomalies below the seismically inferred BSR at this site. 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 the "Site U1328 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 F38 shows cold thermal anomalies in the concatenated false-color IR images for Holes U1328B and U1328C. This figure also shows the Hole U1327B LWD/MWD resistivity image and the pore water saturation derived from these data for comparison.

As a precursor to quantitative studies on 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. Figure F39 shows a plot of temperature vs. depth for Holes U1328B and U1328C. These temperature profiles, and the definition of the baseline temperature of the core in the absence of gas hydrate, represent a first step toward quantifying the gas hydrate content of cores from IR data (Tréhu et al., 2004). Considerable effort was expended at this site on special experiments to calibrate the IR data by using it to target samples for IW chlorinity analyses.

Strong cold anomalies were detected in the shallowest cores from this site. A considerable amount of gas hydrate had been anticipated in the uppermost 40 m based on LWD/MWD resistivity measurements (see "Downhole logging"). Unfortunately, recovery was poor in this zone. Another zone of low IR temperature anomalies was observed from 105 to 115 mbsf. A high-resistivity zone was observed at a similar but slightly shallower depth in the LWD/MWD data; however, no chlorinity anomalies were detected in this zone. The strongest IR anomalies were detected just above the BSR, at 210–222 mbsf. Large chlorinity anomalies were also observed here. Finally, a few small but distinct cold anomalies were observed below the BSR, one of which was targeted for IW analysis, which showed pore water freshening indicative of gas hydrate. The possible presence of gas hydrate beneath the predicted BSR depth may indicate the presence of Structure II gas hydrate, which is stable to a depth of ~30 m below the base of the pure methane Structure I GHSZ. This hypothesis will be tested postcruise through analysis of chemical and physical property data obtained during Expedition 311.

Figure F40 shows a very strong temperature anomaly in an IR image taken immediately after the sample was removed from the catwalk to the Chemistry Laboratory, where it was extruded from the liner and split prior to analysis of pore water. The temperature in the interior of a gas hydrate–bearing sand layer was –3°C, an 18°C drop from the temperature at the outer edge of the core. This is one of the coldest gas hydrate–related anomalies documented during Expedition 311. The core liner temperature obtained from the IR scan on the catwalk was 6.5°C, a T of 5.5°C. The chlorinity corresponding to this IR anomaly was 411 mM, indicating strong pore water freshening. An adjacent, slightly stronger anomaly (liner temperature = 5.5°C) had a Cl value of 347 mM (see Table T3; Samples 311-U1328C-18X-3, 67–72 and 72–82 cm).

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 T13). Both estimates of density were compared to in situ densities measured by the LWD/MWD tools (Fig. F37). The MAD data are compared to wireline data obtained in the same hole and to LWD/MWD data in Figure F41. The different data sets agree well. The agreement between the MAD and wireline data from the same hole below 100 mbsf is especially good, with matching trends in both data sets (see "Downhole logging" for further comparison of MAD and wireline density data). Low-porosity outliers are interpreted to represent sand-rich intervals. This hypothesis will be tested through postcruise grain-size analysis.

Accurate estimates of porosity are very important for obtaining accurate estimates of the in situ gas hydrate content of sediments recovered in pressure cores, and pressure cores represent the most direct means of determining the absolute amount of gas hydrate. During ODP Leg 204, it was reported that MAD samples from pressure cores sometimes yielded lower porosity estimates than adjacent APC or XCB cores. We tested that observation at this site (Fig. F42). No such correlation was found in seven samples from three different pressure cores.

Magnetic susceptibility

Magnetic susceptibility at this site was generally higher and more variable than at Sites U1327 and U1329 cored previously during Expedition 311 (Fig. F41C). Causes for the intersite variability in magnetic susceptibility will be a topic for postcruise research. It is likely that the variability is related to differences in the amount of silty and sandy turbidites, reflecting different sedimentary environments.

A decrease in magnetic susceptibility was previously reported by Novosel et al. (2005) for shallow sediments within Bullseye vent, compared to shallow sediments on the flanks of the vent. This has been attributed to chemical processes related to gas hydrate formation. At Site U1328, the average magnetic susceptibility in cores from the uppermost 30 m, while strong, is lower than in cores from 30 to 70 mbsf. Whether this is due to gas hydrate or to lithological variability will be examined postcruise.

Compressional wave velocity from the multisensor track and Hamilton frame

Because of the high gas content and limited recovery from the shallow section at this site, only a few poor quality P-wave velocity data were obtained (Table T14).

Shear strength

Shear strength measurements were made routinely throughout Holes U1328B and U1328C using the handheld Torvane (Table T15). A limited number of measurements were also made with the automated vane shear (AVS) system (Table T16). 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 the MAD samples had been extracted. At least two Torvane shear strength measurements were taken per section, and often more where there were visual changes in the sediments (i.e., color and grain size). Two AVS measurements were taken per core.

AVS shear strength measurements were similar to, but generally slightly lower than, those obtained with the handheld Torvane (Fig. F43A; see also "Physical properties" in the "Site U1327" chapter). Shear strength values generally increase with depth and range from 15 kPa at the top of the hole to 120 kPa at the bottom. The ratio of shear strength to overburden pressure is a measure of the consolidation state of the sediments (Fig. F43C). A ratio >0.25 indicates overconsolidation and a ratio <0.25 indicates underconsolidation (Riedel et al., 2006). Except for the uppermost 5 m of the hole, the sediments are underconsolidated. The apparent overconsolidation of sediments in the upper 5 m may be caused by high carbonate content.

Electrical resistivity

Electrical resistivity was measured on core samples by both contact and noncontact methods. The contact resistivity measurement interval varied depending on core quality. MST noncontact resistivity measurements were made every 2.5 cm. Values obtained were very scattered because of gas expansion in the core. Contact resistivity measurements were made to a depth of 260 mbsf and ranged in value from 0.3 to 2.2 m (Table T17; Fig. F44A). Higher resistivities were recorded but these were caused by cracks in the sediment. Resistivity increases with depth but becomes more scattered.

Pore water resistivities were calculated from the IW salinities using equations developed by Fofonoff (1985) and corrected to 20°C (Fig. F44A). These were used to calculate the formation factor (ratio of saturated sediment resistivity to pore fluid resistivity) from the contact resistivities (Fig. F44B). Archie's parameters were then determined by fitting Archie's equation to the formation factor and MAD porosity data (Fig. F44D). The cementation coefficient m = 1.36 and the tortuosity coefficient a = 1.58. Measurements made in sand intervals were not included in the estimation of Archie's parameters. These parameters were then used to determine porosities, which were compared to the porosities measured from the MAD samples. Archie's parameters give a good fit to the data to a depth of 175 mbsf. Below this depth, the Archie's porosities underestimate the MAD porosities, suggesting a change in lithology that requires the calculation of a new set of Archie's parameters.

Thermal conductivity

Thermal conductivity values range from 0.6 to 1.3 W/(m·K) and show no consistent trend with depth (Table T18; Fig. F37). Although we tried to avoid disturbed portions of the core, gas expansion cracks were pervasive. 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. The depth interval from 110 to 140 mbsf, which has high thermal conductivity, high shear strength, high density, and low porosity, may be a more lithified zone similar to that observed from 75 to 90 mbsf at Site U1327. In contrast to Site U1237, however, the magnetic susceptibility is high in this interval.

In situ temperature profile

Although nine deployments of temperature tools were attempted at Site U1328, only three provided reliable data (Table T19). Unfortunately, one tool failed three times before a loose connection was found. The data are shown in Figure F45. Four deployments were seriously degraded by ship heave (56.5, 85.0, 93.0, and 103.0 mbsf), which reached ~9 m during operations in Hole U1328E. During the 93.0 mbsf deployment, the DVTP appears to have broken loose from the seafloor prematurely, so that it primarily measured the temperature of fluid in the drill pipe. Even the best data obtained at this site were affected by heave. The APCT16 deployment at 75.5 mbsf, while providing a "good" in situ temperature estimate (Table T19), has an unusual temperature profile with a very high initial temperature and anomalously fast decay rate.

Extrapolated temperatures are plotted and compared to data from Site U1327 and from Site 889 in Figure F46. The temperatures determined for Site U1328 are generally similar to those at Site U1327 and slightly higher than those determined at similar depths for Site 889 (Westbrook, Carson, Musgrave, et al., 1994). The temperature gradient at Site U1328 was determined to be 53.6 ± 0.4°C/km (Fig. F46). The remarkably good fit of the line to the three reliable (and, coincidentally, one unreliable) data points clearly underestimates the uncertainty in the determination of the subsurface temperature profile. The line falls within the region delimited by the standard error in the slope and intercept of the line fit to all reliable data points from Sites 889, U1327, and U1328, except for at the seafloor, where the intercept of the line fit to three Site U1328 reliable points is ~0.5°C higher than the expected bottom water temperature of ~3.0°C. Although the scatter in the combined data slightly exceeds the scatter expected based on the estimated measurement uncertainties for good- to excellent-quality data, we do not have enough data to constrain the vertical and lateral temperature gradients.

The combined temperature gradient predicts the base of methane hydrate stability at 220–245 mbsf. For the measured background pore water salinity of 3%, the predicted base of the GHSZ drops by ~2 m to 222–247 mbsf. For comparison, the seismic BSR depth in estimated to be 219 mbsf.

Advanced piston corer methane tool

Note: Derryl Schroeder (Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station TX 77845, USA) provided the advanced piston corer methane (APCM) tool shipboard data and plots.

Pressure, temperature, and electrical conductivity histories of Cores 311-U1328C-2H, 3H, and 6H were logged using the APCM tool (Ussler et al., in press). Similar measurements were also made during deployment of the PCS (see "Pressure coring"). Results of the APCM tool deployments are shown in Fig. F47. Pressure, temperature, and conductivity are plotted continuously for all three deployments (Fig. F47A). The principal purpose of the conductivity probe is to detect the presence or absence of gas in the headspace above the core. Conductivity shows a dramatic drop at the time of the APC shot for Cores 311-U1328C-2H and 6H but not 3H, suggesting either that the headspace remained filled with drilling fluid (i.e., there was less methane at this depth) or that the conductivity probe malfunctioned, possibly as a result of clogging of the sensor by sediment. Pressure data show the descent, pause at the seafloor, lowering into the hole, shot of the APC, hold at the bottom of the coring position, pull to and pause at the seafloor, finally followed by ascent to the rig floor. All three deployments were coincident with APCT or APCT-3 measurements of temperature at the cutting shoe of the core. The APCT-3 data are compared to the APCM data in Figure F47B. The two temperature records show both positive and negative correlations. We note that the APCM shows cooling as the tool is fired and the APCT-3 shows the onset of frictional heating. This may be caused by cooling of the core as a result of pumping of fluid through the drill pipe. XCB cores require continuous pumping, typically leading to a cooler core top and a steeper gradient from top to bottom than is obtained with APC coring (Tréhu et al., 2004).

Additional analysis of these data will be conducted postcruise, including integration with core line measurements made on the rig floor (Ussler et al., in press) to precisely reconstruct the pressure and temperature history of the core. The data should ultimately be of great value for understanding processes that affect core recovery, and for providing temperature and pressure boundary conditions for modeling the dissociation of gas hydrate during core recovery. Such modeling is critical for more sophisticated interpretation of IR anomalies.

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 core (Fig. F48). 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 U1328. 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.

Top of page   |   Previous   |   Next