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doi:10.2204/iodp.proc.329.104.2011

Physical properties

At Site U1366, physical property measurements were made to provide basic information for characterizing lithologic 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) on whole-round core sections. After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (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. Discrete measurements of electrical resistivity were made on the split sediment sections to calculate formation factor. The Section Half Image Logger and SHMSL were used to collect images and color reflectance of the split surfaces of the archive-half cores. Six holes were cored, but only Holes U1366B–U1366D and U1366F were relatively complete. Hole U1366B was the most complete for physical property logging. These holes have not been correlated and offsets exist.

Density and porosity

Bulk density values at Site U1366 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). A total of 17 discrete samples were analyzed for MAD, 6 samples from Hole U1366B, 8 samples from Hole U1366B, 4 samples from Hole U1366C, 2 samples from Hole U1366D, 1 sample from Hole U1366E, and 2 samples from Hole U1366F.

In general, wet bulk density values determined from whole-round GRA density measurements and measurements from discrete samples agree well (Fig. F14A). Wet bulk density discrete samples have an average value and standard deviation of 1.26 and 0.10 g/cm³, respectively. GRA measurements of bulk density have an average and standard deviation of 1.02 and 0.56 g/cm³, respectively.

Measurements of grain density were determined from mass/volume measurements on discrete samples (Fig. F14B). The mean and standard deviation of grain density is 2.46 and 0.62 g/cm³, respectively. No depth-dependent variation is observed.

Measurements of porosity (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]) were determined from mass/volume measurements on discrete samples (Fig. F14C). The mean and standard deviation porosity is 83% and 4%, respectively. Below ~20 mbsf (lithologic Unit II in Hole U1366F) porosity is >85%.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL for all recovered cores at Site U1366. Uncorrected values of magnetic susceptibility are presented for Holes U1366B–U1366D and U1366F (Fig. F15). The spatial resolution of the WRMSL magnetic susceptibility loop is ~5 cm, and the observed “ringing” of values in Holes U1366D and U1366F is due to edge effects. Measured whole rounds from those holes were generally 10 cm long as a result of whole-round sampling prior to WRMSL measurements.

Natural gamma radiation

NGR results are reported in counts per second (cps) (Fig. F16). NGR counting intervals were ~1 h per whole-core interval, and NGR counts should be reliable. The tops of Holes U1366C and U1366F show high NGR counts at 0 mbsf (Fig. F16B, F16D), consistent with sampling an enriched concentration of radioactive elements at the sediment/water interface. The absence of an NGR peak in Hole U1366B (Fig. F16A) suggests that the mudline was not sampled. This peak is absent from the record of Hole U1366D (Fig. F16C) because a whole-round sample spanning 0 to 10 cm below seafloor of Core 329-U1366D-1H was removed and refrigerated to provide an intact seafloor manganese nodule for microbiological and mineralogical study.

NGR counts are a maximum at the sediment/water interface and decrease with depth (Fig. F16). Distinct peaks that are particularly conspicuous in Hole U1366B are associated with manganese nodules. To quantify the concentrations of uranium, thorium, and potassium in these nodules, the nodules were removed from the cores and their NGR counts were measured. Concentrations of uranium, thorium, and potassium were estimated using a Monte Carlo inversion. Potassium concentration is 0.2–0.3 wt% and the Th/U ratios are >5 for all nodules measured (Table T2). These concentrations are compared with uranium and thorium concentrations in the uppermost section of Hole U1366E. Figure F17 shows that although the sediment/water interface is enriched in thorium, the NGR peak at the seafloor is primarily caused by uranium. Further, the uranium content of the nodule does not contribute significantly to the seafloor peak. In contrast to uranium, the nodule is enriched in thorium relative to the surrounding sediment. Deeper in the section, away from the nodule, sediment is enriched in thorium relative to uranium.

In Hole U1366C, NGR counts in lithologic Unit II are featureless, consistent with core disturbance (Fig. F16). The spatial resolution of NGR is ~12 cm, and the observed ringing of values in Holes U1366D and U1366E is due to by edge effects caused by whole-round sampling prior to WRMSL and NGR measurements. Measured whole rounds from these holes are generally only 10 cm in length.

P-wave velocity

Measurements of P-wave velocity at Site U1366 were determined from measurements on sediment whole cores and mass/volume measurements on discrete samples from the working halves of sediment split cores (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). In general, discrete measurements are somewhat higher than whole-core measurements; this difference is attributed to incomplete saturation of the sediment (Fig. F18). The mean value is 1520 m/s (close to the compressional velocity of water) (Fig. F18B). In Hole U1366C, there is a slight shift to higher values in lithologic Unit II relative to Unit I, but this is likely due to core disturbance that was observed below ~15 mbsf. In Holes U1366D and U1366F, the greater apparent scatter is an artifact caused by edge effects, as these measurements were made after whole-round sampling for biogeochemistry and microbiology.

Formation factor

Measurements of electrical conductivity were measured on working halves of the split sediment cores in Holes U1366C and U1366F. Measurements in Hole U1366C were made at a nominal interval of 10 cm. Because of clear evidence of core disturbance below ~15 mbsf in Hole U1366C, measurements were not made below this depth in this hole. Below this depth, measurements were made on working halves of split cores from Hole U1366F but with wider measurement spacing because of prior whole-round sampling. 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 T3), normally prior to making measurements for that section and then around the 75 cm offset of each section. These measurements were used to compute drift (Fig. F19). The temperature dependence of electrical conductivity was corrected; 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 drift (Table T4). At depths less than ~15 mbsf, the formation factor in Hole U1366C increases with depth (Fig. F20). In Hole U1366F, the formation factor decreases from ~15 to 23 mbsf. This pattern inversely mimics the change in observed porosity (Fig. F14).

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method. Many of the needle-probe measurements on sediment are considered unreliable because temperature-time series of these measurements indicate that the measurements caused fluid to convect within the samples. Convection leads to unreasonably low estimates of thermal conductivity by causing the thermal response to heating to depart from the theoretical prediction. However, a subset of values >0.65 W/(m·K) are considered reliable. These values show similar magnitude and scatter to values determined from a piston core collected during the KNOX-02RR site survey (Fig. F21) (R. Harris, unpubl. data). The mean and standard deviation of these values are 0.7 and 0.1 W/(m·K), respectively.

Color spectrometry

Results from measurement of color reflectance are presented in Figures F22, F23, and F24. L* values are ~40, with some clusters of higher values at ~240. The majority of a* and b* values are ~0 and 15, respectively. Both variables decrease slightly with depth. Color spectrometry values are relatively consistent across unit boundaries.

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