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doi:10.2204/iodp.proc.327.106.2011

Physical properties

The uppermost sediment section in Holes U1363B, U1363F, and U1363G was cored using the APC. The XCB was used to core deeper sediments and the sediment/basalt interface in Holes U1363B and U1363F, as well as in Holes U1363C and U1363D.

Physical property measurements included whole-round magnetic susceptibility, density, and P-wave velocity and discrete measurements of thermal conductivity, P-wave velocity, and MAD properties (e.g., bulk density, grain density, and porosity) (Figs. F5, F6; Table T9). Sampling frequency for the discrete samples was ~1–2 per section, taken at the same locations as thermal conductivity measurements, except in cases of fluidized sandy sections where it was not possible to extract discrete samples.

Magnetic susceptibility

All cores were run through the WRMSL, with measurements made every 2 cm (1 cm for Holes U1363F and U1363G). Magnetic susceptibility values are as high as ~1400 × 10^–6 SI, with the highest values corresponding to sandy turbidite sequences (e.g., Cores 327-U1363B-3H and 4H) and ash layers present near the top of Core 327-U1363B-2H (Fig. F5A). Additionally, Core 327-U1363B-8X has some high magnetic susceptibility values (~1000 × 10^–6 SI) because of the presence of iron oxides close to the sediment/basalt interface. Other cores, including clays and fine turbidite sections toward the top and bottom of the hole, generally have values of <500 × 10^–6 SI.

After the whole-round sections were split, the archive halves were run through the SHMSL. Point measurements were made at 1 cm intervals. Magnetic susceptibility trends determined through this method are similar to those obtained from the WRMSL (Fig. F5A), with values tending to be slightly lower than those determined by the WRMSL, except in the case of turbidite sequences, where SHMSL values are consistently higher.

Gamma ray attenuation bulk density

Bulk density is estimated by GRA density measured on the WRMSL (Fig. F5B). In general, density increases slowly downhole in the uppermost 15 m, below which it varies between ~1.8 and ~2.0 g/cm³ as a result of alternating sand and clay layers. The highest density values (~2.0 g/cm³) occur in the massive sandy turbidites. However, because of flow-in that commonly occurs during APC coring in unlithified sands (particularly if the corer does not stroke out completely during penetration), it is unclear if these higher density values are accurate. Near the bottom of the hole (e.g., Cores 327-U1363B-8X and 9X), average GRA bulk density decreases slightly as a result of a higher clay fraction.

Thermal conductivity

A total of 153 individual measurements were made at 26 locations, yielding values averaging 1.30 ± 0.20 W/(m·K). Individual thermal conductivity measurements are typically filtered for outliers that commonly occur with the needle-probe method. Filtered data are then averaged to produce a single value. The record is most complete in sections cored with the APC because XCB cores are often too disturbed to produce reliable measurements (Fig. F6; Table T9). In general, measurements were taken at a frequency of ~1 per section, with a minimum of three measurements per location. Samples were chosen on the basis of lithology, targeting sand and clay end-members identified from maxima and minima found in WRMSL magnetic susceptibility and GRA bulk density data. The average thermal conductivity value for this site may be slightly biased toward the clay end-member because flow-in was common in sand units cored with the APC, making thermal conductivity difficult to measure. In some cases, especially shallow cores, sections were too fluidized to produce usable data because of induced fluid convection in the material during measurement. Despite these issues, the data compare well with those collected at similar depths from nearby Hole U1301C (Expedition 301 Scientists, 2005). Thermal conductivity values are inversely correlated with MAD porosity (Fig. F7A).

P-wave velocity

In addition to continuous P-wave velocity measurements made on whole-round sections with the P-wave logger (PWL), 130 measurements of P-wave velocity were made on split sections using the P-wave bayonets (PWB) or P-wave caliper (PWC). Because the PWC was unstable, as described in “Physical properties” in the “Site U1362” chapter, we mainly used the PWB (y- and z-axes) on section halves. For lithified sediments, however, we used the PWC (x-axis) because the cracks induced by the y- and z-axis transducers (bayonets) on the PWB precluded the acquisition of reliable velocities.

P-wave velocity measured with the PWL ranges from 1.46 to 1.87 km/s, with an average of ~1.57 km/s (Fig. F6A), excluding the erroneously low values derived from insufficient sediment filling within core liners. P-wave velocity determined by the PWB and PWC ranges from 1.49 to 1.75 km/s, with an average of ~1.52 km/s (Fig. F6A). Velocities measured in clay lithologies consistently display less variability than those measured in sand and also display a systematic increase with depth as a result of compaction. Because of mobilization and redeposition of sand layers in turbidite sequences during coring, we could not determine a statistically significant trend with depth for the sandy units.

Comparing measurements made along horizontal and vertical axes provides evidence for weak velocity anisotropy (Fig. F8). This tendency is consistent with the layered nature of these sediments and may result in part from greater clay compaction with depth.

Moisture and density

Eighty-eight discrete samples collected from Site U1363 were used to determine MAD properties (Figs. F5, F6C). Bulk density values range from 1.4 to 2.1 g/cm³, with an average of 1.7 g/cm³. Grain density exhibits a range of 2.7–3.0 g/cm³, with a mean of 2.8 g/cm³. Porosity values range from 38% to 76%, with a mean of 60%. GRA and MAD densities differ by as much as 7%. Results from Site U1362 (basement section) show that GRA results provide a lower bound on bulk density because of cores partially filling the liner, but the results derived from the sedimentary section at Site U1363 show MAD data that tend to be low relative to GRA results (Fig. F5B). This may be the case because flow-in and resedimentation of coarse material tend to cause bias in the GRA data that is avoided through more careful sample selection for MAD measurements.

P-wave velocity and porosity are clearly inversely correlated, and P-wave velocity and bulk density are positively correlated (Fig. F9). Sections rich in sand tend to have a high and variable P-wave velocity across a narrow range of porosities (V_P = 1.55–1.75 km/s for porosity = 45%–60%).

MAD data are highly variable over small depth ranges because samples were selected from the same locations as thermal conductivity measurements, which targeted end-member clay and sand lithologies. Cross-plots of bulk density and porosity show a clear linear trend, which is consistent with the method used to determine porosity, based on measured bulk and grain densities (Fig. F7B).

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