Previous   |    Next

doi:10.2204/iodp.proc.344.105.2013

Physical properties

At Site U1412, 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) and thermal conductivity was measured using the full-space method on unconsolidated sediments. Following core splitting, color reflectance and magnetic susceptibility were measured on the archive-half cores. Moisture and density (MAD) were measured on discrete samples collected from the working halves of the split sediment cores, generally once per section. P-wave velocity and strength were measured on the working halves of split cores, depending on recovered length and condition. For consolidated sediments in Hole U1412C, thermal conductivity was measured on the split core using the half-space method.

Density and porosity

Bulk density values at Site U1412 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]). Both types of measurements yield similar trends, but the GRA values show greater scatter. GRA values scatter higher and lower than the MAD density values in APC cores and are generally less than or equal to MAD density values for XCB and RCB cores. Gas expansion voids in Hole U1412A and variable core diameter and drilling disturbance in the XCB and RCB cores contribute to the scatter in GRA density values (Fig. F24). Bulk density values increase and porosity values decrease in the uppermost 30 m. Both properties remain stable to the base of Unit I, with average bulk density and porosity values of 1.69 g/cm³ and 60%, respectively. In Unit I, the average grain density is 2.69 g/cm³. Both the GRA and the discrete samples indicate some scattered high bulk density values in Units II and III. Just above the Unit II/III boundary, three discrete samples yielded high bulk densities and abnormally low grain densities. Limited data do not allow us to assess whether these values are real or due to measurement errors.

Bulk density and porosity values in Unit III show considerable variability but suggest slightly more compaction (higher bulk density and lower porosity) than that at the base of Unit I. Unit III samples from Hole U1412D show porosity trends similar to those from Hole U1412C but at a slightly (~15 m) shallower depth.

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 at Site U1412 (Fig. F25). Magnetic susceptibility values measured by these two methods are in good agreement. In Unit I, the background magnetic susceptibility is between 20 and 30 IU. Magnetic susceptibility excursions reach ~200 IU and are sometimes, but not always, associated with tephra layers. The few recovered intervals in Unit II yielded low magnetic susceptibility values (<15 IU). Magnetic susceptibility in Unit III is variable, ranging from near 0 to ~75 IU with one excursion to >160 IU.

Natural gamma radiation

NGR counting periods were 10 min, and measurement spacing was fixed at 20 cm for all holes and sections. NGR results are reported in counts per second (cps) (Fig. F26). NGR values in Unit I generally range between 15 and 25 cps. Lower NGR values (around 10 cps) are observed within Unit II. NGR values in Unit III are higher and have greater variability than those within Units I and II. The highest counts, around 46 cps, were observed within Unit III at 362 mbsf.

P-wave velocity

P-wave velocity at Site U1412 was measured on the working halves of cores using the P-wave caliper (Fig. F27). Because of gas expansion, poor recovery, and drilling disturbance, P-wave velocity data could only be collected from limited intervals. In the uppermost 25 mbsf of Unit I, P-wave velocities vary between 1520 and 1600 m/s. From ~330 mbsf, just above the boundary between Units II and III, to the bottom of Unit III, P-wave velocity shows scattered values around 1500 ± 200 m/s.

Thermal conductivity

Thermal conductivity measurements were conducted on 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]). Thermal conductivity values within Unit I range from 0.7 to 1.2 W/(m·K), with most values between 0.9 and 1 W/(m·K). Thermal conductivity values decrease slightly with depth, possibly because of gas disturbance of the core (Fig. F28A). This trend contradicts the commonly observed inverse correlation with porosity, which is nearly constant within Unit I (Fig. F24). However, thermal conductivities are higher in Unit III compared to Unit I, following the inverse relationship with porosity. Thermal conductivity values within Unit III are higher than those within Unit I, and they rapidly increase with depth from 1 to 1.20 W/(m·K).

Downhole temperature and heat flow

Four downhole temperature measurements were attempted using the APCT-3 between 20 and 60 mbsf in Hole U1412A. All measurements were made in a calm sea state. A least-squares linear fit to the temperature data yields a gradient of 114°C/km and a bottom water temperature of 2.25°C (Fig. F28B). The use of a linear fit implies constant thermal conductivity and basal heat flow. Extrapolation of temperature to the base of the hole was not conducted because of the lack of reliable thermal conductivity values within Unit II.

Sediment strength

Compressive sediment strength was measured by the Geotester STCL-5 pocket penetrometer in Units I and II and by a third-party needle penetrometer in Units II and III (see “Physical properties” in the “Methods” chapter [Harris et al., 2013]). Shear strength in Unit I was measured by the automated vane shear apparatus. In the uppermost 30 m, compressive strength linearly increases with depth from ~2 to 390 kPa and shear strength increases from 20 to ~180 kPa (Fig. F29). From ~30 to ~180 mbsf, compressive strength values become variable, ranging between ~100 and ~470 kPa, and strength does not increase with depth. Values in Unit III are very scattered. The highest strength of 1980 kPa was observed at ~330 mbsf, where the lowest porosity (~30%) was observed (Fig. F24). Some of the scatter in compressive strength below 30 mbsf may reflect variable disturbance from gas expansion in Unit I and from the small surface area tested by the needle penetrometer.

Electrical conductivity and formation factor

Formation factor was obtained from electrical conductivity measurements in the y- and z-axes of the split core from the shallow portion of Hole U1412A (0 to ~107.91 mbsf) and in the z-axis in Hole U1412B (0.1~6.25 mbsf) (Fig. F30). The y and z measurements in Hole U1412A are similar throughout the hole, indicating little anisotropy of electrical conductivity within the sediments. Formation factor increases from 1.5 to 3.2 between 0 and 14 mbsf and becomes stable between 14 and ~65 mbsf, ranging between 2.5 and 4.0. Between ~65 and 107.91 mbsf, formation factor decreases with depth. Core disturbance due to gas expansion likely causes some of the observed scatter.

Color spectrophotometry

Reflectance L* values range between 30 and 40 and are similar in Units I and III, whereas reflectance a* and reflectance b* values show a slight shift at 20 mbsf and excursions just below the Unit II/III boundary (Fig. F31).

Top of page   |    Previous   |    Next