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doi:10.2204/iodp.proc.344.107.2013

Physical properties

At Site U1413, physical properties measurements were made to help characterize lithostratigraphic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured using the Whole-Round Multisensor Logger (WRMSL). After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR). For soft-sediment cores, thermal conductivity was measured using the full-space method prior to core splitting. Following core splitting, color reflectance and magnetic susceptibility were measured on the archive-half cores using the Section Half Multisensor Logger (SHMSL). Moisture and density were measured on discrete samples collected from the working halves of the split sediment cores, generally once per section. For indurated sediments, thermal conductivity was measured on the split cores using the half space method. P-wave velocity and strength were measured on the working halves of split cores. Physical properties data are compared to Site U1379, which was drilled during Expedition 334 and is located 11 km from Site U1413 (Expedition 334 Scientists, 2012).

Density and porosity

Bulk density values at Site U1413 were determined from both GRA measurements on whole-round cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). In general, wet bulk density values determined from discrete samples agree with the whole-round GRA measurements for APC cores (0–140.6 mbsf) and are higher than the whole-round GRA measurements for XCB and RCB cores (>140.6 mbsf) (Figs. F25, F26). Grain density measurements were determined from mass/volume measurements on dry discrete samples within the sedimentary sequence (Fig. F26). Site U1413 grain density values average 2.72 g/cm³ and are similar to those from Site U1379 in the upper 150 m. From 150 to 450 mbsf, grain density values exceed those of Site U1379 for similar depths; below 450 mbsf, they are similar to the highest grain densities observed at Site U1379. Between 0 and 180 mbsf, Site U1413 porosity values decrease rapidly from 70% to 50% and more gradually below 180 mbsf to 40% at the bottom of Site U1413. Both trends observed in Site U1413 data have different slopes than those found in porosity data from Site U1379.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL for all core sections longer than ~20 cm (Fig. F27). Magnetic susceptibility values are reported in instrument units (IU). Values measured by these two instruments are in good agreement. Magnetic susceptibility values vary from <20 to ~230 IU in Unit I and from <20 to ~520 IU in Unit II above 200 mbsf. The background value from the seafloor to ~200 mbsf is ~25 IU. Unlike other sites, where magnetic susceptibility excursions are often associated with tephra layers, peaks at Site U1413 appear to coincide with sand-rich layers that contain detrital magnetite (see “Lithostratigraphy and petrology”). In the lower portion of Unit II (200 to ~366 mbsf), magnetic susceptibility remains generally between ~20 and ~50 IU throughout, with few high excursions. Magnetic susceptibility in Unit III is slightly higher and slightly more variable (~30 to ~93 IU) than in Units I and II but has no excursions above 100 IU.

Natural gamma radiation

NGR counting periods were 10 min, and measurement spacing was fixed at 20 cm (Fig. F28). In Unit I, Site U1413 NGR values increase within the uppermost 6 m and then decrease slightly with depth. In Unit II, NGR values gradually increase with depth to peak between 180 and 220 mbsf, below which they range between ~14 and ~38 cps. Poor correlation of NGR values between Sites U1413 and U1379 is likely due to mass movement in Unit I and significantly different thicknesses of Units II and III at the two sites (see “Lithostratigraphy and petrology”).

P-wave velocity

P-wave velocity at Site U1413 was measured on the working halves of sediment split cores using the P-wave caliper (Fig. F29). From 0 to 20 mbsf, P-wave velocities average 1600 m/s. Because of core disturbance, only a few measurements were taken between 20 and 200 mbsf, and values are variable. Below 200 mbsf, P-wave velocities average 1960 m/s. Velocities obtained from Site U1379 are slightly lower than those from Site U1413.

Thermal conductivity

Thermal conductivity measurements were conducted on soft-sediment whole-round cores using the needle-probe method and on lithified split cores using the half-space method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). Above 200 mbsf, thermal conductivity increases with depth from 0.9 to 1.25 W/(m·K) (Fig. F30A). Low thermal conductivity values are observed from 40 to 50 mbsf and may reflect disturbance caused by gas expansion. Below 200 mbsf, thermal conductivity values slightly increase with depth to 1.35 W/(m·K) at the bottom of Hole U1413C. Best-fit linear relationships for the thermal conductivity values above and below 200 mbsf are shown in Figure F30A. Thermal conductivity values below ~200 mbsf are similar to the higher end of those measured at Site U1379 during Expedition 334.

Downhole temperature and heat flow

Five downhole temperature measurements were attempted using the APCT-3 between 21 and 65 mbsf in Hole U1413A; however, two of the measurements could not be used because of a calibration problem with the tool. The remaining three measurements, which were taken with a different APCT-3, provide a least-squares gradient of 49°C/km (Fig. F30B). The mean thermal conductivity of 1.15 W/(m·K) yields a heat flow of 56 mW/m².

We use the measured temperatures and the estimated heat flow value of 56 mW/m² to estimate temperatures at greater depths using the Bullard (1939) method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). This method assumes steady-state conductive heat flow in the sediment. We compared two models for the variation of thermal conductivity with depth, k(z). Model 1 assumes linear increases of thermal conductivity with depth. Model 2 is based on the accurate trace of each thermal conductivity value we measured. Both models yield similar results. Least-squares fits for Models 1 and 2 yielded values of 7.40° and 7.36°C, respectively, for the sediment/water interface and 35.29° and 35.31°C, respectively, at the bottom of Hole U1413C.

Sediment strength

Sediment strength was measured with both the automated vane shear and penetrometers. To compare the two measurement types, unconfined shear strength can be estimated as one half of unconfined compressive strength (Blum, 1997). Results are shown in Figure F31. Strength values generally increase with depth. Vane shear strength increases linearly from ~12 kPa near the seafloor to ~44 kPa at 15 mbsf. From 15 to 40 mbsf, values decrease to ~17 kPa. From ~40 to ~140 mbsf, vane shear strength increases again to a maximum of ~173 kPa. Compressive strength from penetrometer measurements shows similar trends, with a linear increase from 0 to 15 mbsf to a maximum value of ~80 kPa. From 15 to ~40 mbsf, values become variable and decrease to a minimum of ~39 kPa. From 40 to 140 mbsf, compressive strength shows an approximately linear increase to ~500 kPa. Below 140 mbsf, compressive strength values measured by the needle penetrometer are highly variable but generally increase with depth to 320 mbsf, with lower values measured between 320 and 370 mbsf. In Unit III, values range between ~674 and >1500 kPa. High values occur within the sand-rich section below 540 mbsf (see “Lithostratigraphy and petrology”).

Color spectrophotometry

Reflectance L* values generally range between 23 and 50 throughout the site (Fig. F32). Reflectance a* and b* values range between –2 and 2 and between –7 and 0, respectively, and are generally more variable within Units I and II than within Unit III.

Electrical conductivity and formation factor

Formation factor was obtained from electrical conductivity measurements in the y- and z-directions on the split cores from most of Hole U1413A (0–140 mbsf) and all of Hole U1413B (Fig. F33). The y and z measurements are similar to each other in both holes, indicating little anisotropy in the sediments. Values in Hole U1413A and Hole U1413B are consistent. Formation factor values increase from 2 to ~3.5 in the uppermost 20 m of sediments. From 20 to ~100 mbsf, high scatter may reflect core disturbance from gas expansion. Below ~100 mbsf, formation factor shows less scatter, with values ranging between ~4 and ~5, and trends are inversely correlated to variations in porosity, suggesting that data are probably reliable below the zone of gas disturbance.

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