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doi:10.2204/iodp.proc.329.107.2011

Physical properties

At Site U1369, physical properties measurements were made to provide basic information characterizing lithologic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, GRA density, magnetic susceptibility, and P-wave velocity were measured with the Whole-Round Multisensor Logger (WRMSL). After WRMSL scanning, the whole-round sections were logged for NGR. Thermal conductivity was measured using the full-space method on sediment cores. Discrete P-wave measurements were made on split sediment cores using the Section Half Measurement Gantry. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split sediment cores. Additional discrete measurements of electrical resistivity were made on the split sediment sections to calculate formation factor. The Section Half Imaging Logger was used to collect images of the split surfaces of the archive-half cores, and a color spectrophotometer was used to measure color reflectance on the Section Half Multisensor Logger (SHMSL). Holes U1369B–U1369E targeted the sedimentary cover. Hole U1369D was impeded by manganese nodules and did not proceed. The most complete hole for logging physical properties was Hole U1369B. Holes through the sediments have not been correlated and offsets exist.

Density and porosity

Bulk density values at Site U1369 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores in Hole U1369B (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). A total of 10 discrete samples were analyzed for MAD.

Bulk density values in all holes are shown in Figure F13. The mean value of bulk density in Hole U1369B is 1.3 g/cm³, and the density is slightly greater in lithologic Unit II than in Unit I. The scattered values in Hole U1369B deeper than 15.5 mbsf likely reflect core disturbance. Gaps and scatter in bulk density values for Holes U1369C and U1369E are caused by whole rounds being taken for geochemical and microbiological sampling prior to WRMSL measurements.

Bulk density, grain density, and porosity values were calculated from Hole U1369B (Fig. F14). Bulk density values are systematically offset relative to values derived from WRMSL measurements. The reason for this offset is unclear. The mean grain density is 2.5 g/cm³ and increases slightly with depth. Porosity values vary between 86% and 76% and decrease slightly with depth. A step toward lower porosity values occurs between lithologic Units I and II.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL in all recovered cores from Site U1369. Uncorrected values of magnetic susceptibility are presented for Holes U1369B, U1369C, and U1369E (Fig. F15). Point measurements from archive halves are much more scattered than whole-core measurements. The spatial resolution of the WRMSL magnetic susceptibility loop is ~5 cm, and the observed ringing in Holes U1369C and U1369E is due to edge effects.

Magnetic susceptibility varies between ~0 and 100 × 10^–5 SI in Hole U1369B. Magnetic susceptibility values are somewhat higher in lithologic Unit II than in Unit I, likely because of the higher RSO content in Unit II (see “Lithostratigraphy”).

Natural gamma radiation

NGR results are reported in counts per second (Fig. F16). NGR counting intervals were ~30 min per whole-core interval for Hole U1369B and decreased to 15 min per whole-core interval for Holes U1369C and U1369E. NGR counts are considered reliable. NGR at the tops of Holes U1369B and U1369E are high, indicating that the sediment/water interface was sampled. A whole round was removed from the top of Hole U1369C before that section was measured.

In general, NGR counts are higher in lithologic Unit II than in Unit I. Ringing is more prevalent in cores from Holes U1369C and U1369E because only short core pieces remained after whole-round sampling prior to NGR measurements.

P-wave velocity

P-wave velocity at Site U1369 was determined from measurements on whole-round sediment cores (Fig. F17) and on discrete samples from the working halves of sediment split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Only two discrete measurements of P-wave velocity were measured. The mean value of all whole-core measurements is ~1530 m/s (Fig. F17B). In Hole U1369B, compressional wave velocities are relatively uniform except between 6 and 11 mbsf, where velocities show modest variation. The lowermost portion of Hole U1369B appears noisy.

Formation factor

Electrical conductivity was measured on working halves of the split sediment cores from Hole U1369B. Measurements in Hole U1369B were made at a nominal interval of 10 cm. For each measurement, the temperature of the section was also noted. Surface seawater was used as a standard and measured at least twice per section (Table T2), normally prior to making measurements for that section and then around the 75 cm offset of each section. These measurements were used to compute the drift, which is small for this set of measurements (Fig. F18). The temperature dependence of electrical conductivity was corrected and all reported measurements correspond to a temperature of 20°C. Electrical conductivity measurements were transformed to a dimensionless formation factor by dividing the measurements for the drift (Table T3).

Within lithologic Unit I, the formation factor generally increases with depth (Fig. F19). Formation factor values in Unit II are >2 and also increase with depth. This pattern is generally inversely correlated with the porosity (Fig. F14).

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method. The mean and standard deviation of thermal conductivity values are 0.7 and 0.07 W/(m·K), respectively (Fig. F20). These values compare favorably with needle-probe measurements from the KNOX-02RR site survey cruise (R. Harris, unpubl. data). A best fitting trend to these measurements shows a slight increase with depth, which is supposed to be related to the decrease in porosity with depth.

Color spectrometry

Spectral reflectance was measured on split archive-half sections from Holes U1368B–U1369D. Measurements from Hole U1368B are shown in Figure F21. L* varies between ~20 and 40 with a minimum at ~9 mbsf, a* varies between 0 and 10, and b* varies between 0 and 20. Spectral reflectance values tend to show more scatter in lithologic Unit I than in Unit II. Values of L* and a* appear inversely correlated to a modest extent, whereas values of b* decrease with depth.

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