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doi:10.2204/iodp.proc.318.104.2011

Physical properties

The shipboard physical properties program at Site U1356 included nondestructive measurements of gamma ray attenuation (GRA) bulk density, magnetic susceptibility (loop sensor), P-wave velocity, and natural gamma radiation (NGR) on whole-round core sections. P-wave velocities were determined on archive-section halves in Hole U1356A. Additional moisture, density, and porosity measurements were performed on samples taken from the working half of each suitable section in Hole U1356A. Multiaxis P-wave velocity measurements were performed either with the velocity probes inserted into the unconsolidated or semisoft sediments or on cube samples that were also used for moisture, density, and porosity determinations afterward. Measurements were made on all RCB cores.

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

All core sections in Hole U1356A were measured using the Whole-Round Multisensor Logger (WRMSL) (Figs. F30, F31). Four cores (318-U1356A-47R and 91R through 93R) were also analyzed on the Special Task Multisensor Logger, as these data were of special interest at an early stage for various planning of measurement settings and for sedimentology.

Gamma ray attenuation bulk density

Using the WRMSL, GRA density was measured at 2.5 cm intervals (10 s integration time). Variations in GRA density may reflect variations in lithology, consolidation, cementation, and porosity. The measured values are as much as 2.7 g/cm3. In Figure F31, we plotted the filtered data.

GRA density data generally change at the identified lithostratigraphic boundaries (Fig. F31) and also support the location of the main seismic unconformities WL-U5–WL-U3. We observed an initial increase in bulk density from the seafloor to ~179 mbsf, where silty claystones with clasts dominate. Below this horizon, GRA densities are slightly lower and then increase with depth to the bottom of the hole. However, below the main reflectors in the section (e.g., ~460 mbsf [unconformity WL-U5], ~695 mbsf [unconformity WL-U4], and ~880 mbsf [unconformity WL-U3]; see “Site summary”) the regular increase in GRA density is interrupted by steps of slower change (Fig. F31).

Magnetic susceptibility

Whole-core magnetic susceptibility was measured at 2.5 cm intervals (2 s measurement time). The raw data values range from 0 to 7783.5 instrument units (Fig. F30). The highest values represent individual gravel clasts (igneous or metamorphic). In addition, many core tops contain gravel- to cobble-sized clasts that are interpreted as fall-in from higher locations in the hole. These clasts are usually associated with peak values in magnetic susceptibility; therefore, huge peaks occur at the top of most cores. The magnetic susceptibility values for these “artifacts” have been removed from the data of each of these cores for a more realistic plotting of the data in the barrel sheets of individual cores as well as in Figure F30.

Additionally, many gravel-sized clasts are found within the recovered cores. These clasts have a variety of lithologies (see “Lithostratigraphy”) and frequently represent local peaks in magnetic susceptibility. These were retained in all plots.

Magnetic susceptibility data exhibit rhythmic changes especially visible in the cores with improved recovery, starting at Core 318-U1356A-47R but even more so in Core 68R downhole. These changes in magnetic susceptibility likely reflect variations in the input of biogenic and terrigenous sediments and were used for prediction of the sediment composition (bioturbated carbonate-bearing units versus finely laminated carbonate-free intervals) on a centimeter to decimeter scale before the cores were split to assist the sedimentologists (see “Lithostratigraphy”).

Overall, the lithostratigraphic units show a clear correlation to the magnetic susceptibility measurements. Lithostratigraphic Units I, III, and X all correspond to pronounced low magnetic susceptibility.

Natural gamma radiation

Natural gamma radiation was measured on all core sections at 10 cm intervals and run on an integration time of 20 min per section (600 s per measurement). Measured values range from 18.5 to 83 cps (Fig. F30).

The NGR measurements show several intervals of increasing values with depth from the seafloor to 200 mbsf (lithostratigraphic Unit I and Subunit IIa), followed by a gradual shift in decreasing counts in line with the diatom-rich clay ooze of Subunit IIb. Below 275 mbsf, counts increase downhole, reaching higher values in the bioturbated and laminated claystones of Unit VII (~775 mbsf). A distinct decrease is observed in the lower part of Unit VIII and Subunit IXa (850 to ~900 mbsf). The maximum values occur in the claystones of Cores 98R through 104R of lithostratigraphic and Units X and XI.

P-wave velocity

P-wave velocity measurements were made at 5 cm intervals. Measured velocities are as high as 3452 m/s (Fig. F32). However, beginning with Core 39R, there were very few WRMSL P-wave velocity measurements. This is likely due to the increasingly reduced diameter with depth of the RCB cores within the liner. The P-wave logger instrument of the WRMSL was no longer reliably able to transmit the ultrasonic wave through the core at lower core diameters.

P-wave velocities were also measured on the working half of the split core or on cube samples using the P-wave gantry. In softer sediments, when the WRMSL was still collecting data, a single measurement was taken per section. As the WRMSL data degraded in quantity, P-wave measurements on the split core were increased. Depending on core recovery and lithification, up to 6 measurements per section were made with the Section Half Velocity Gantry. These measurements range from 1090 to 5483 m/s. However, the highest velocities correspond to the gravel clasts, whereas the lowest velocities reflect measurements in cracked or otherwise unsuitable deformed material. If we remove these outlier velocity measurements, the data range between 1500 and 2800 m/s, as is plotted in Figure F32.

Additionally, when the degree of lithification of the samples was high enough, multi-axis P-wave measurements were performed on the samples also used for moisture and density (MAD) measurements. These measurements were made before the start of MAD analysis. Additionally, deionized water was not used during these measurements to avoid contaminating the MAD measurements. Preference was made to measure the z-axis P-wave velocities, and they range from 1633 to 2500 m/s. Measurements for the y-axis have a similar range, from 1638 to 2680 m/s (Fig. F32).

Moisture and density measurements

Measurements of density, porosity, and grain density were undertaken on 249 samples taken from Hole U1356A. Depending on core recovery and quality, one sample was taken per section. These samples were carefully selected to cover the representative lithology of each core section. Care was also taken to avoid locations of obvious drilling disturbances. These samples were measured for wet mass, dry mass, and dry volume and, by using these measurements, porosity, percent water mass, dry density, bulk density, and grain density were calculated.

The bulk densities (MAD) from discrete samples are also plotted in Figure F31 and have a raw data range of 1.05–2.60 g/cm3 (Fig. F31).

In the upper 225 m of the hole, both GRA densities and bulk densities of samples (MAD) are in good agreement. However, below 225 mbsf, the GRA density results are consistently lower than those measured from physical samples. We interpret this to be a result of the reduced diameter of the RCB cores recovered in the lower, more lithified portions of the hole. This reduced diameter likely systematically underestimated the bulk density as measured by the GRA density meter and is especially pronounced in the diamictites and mudstones with clasts with alternating bioturbated and laminated mudstones of lithostratigraphic Unit IV (459.4–593.8 mbsf).

Grain density values increase from ~2.6 g/cm3 at the seafloor to ~2.8 g/cm3 at 1000 mbsf. Porosity ranges from 76% to 12% and generally decreases with depth (Fig. F33). Bulk and dry densities exhibit increasing values with depth (Fig. F34). In detail, the trend of compaction reflects variations depending on the lithostratigraphic unit. Moisture content and void ratio show similar trends to porosity, which is expected (Fig. F35).

Thermal conductivity

Where possible, thermal conductivity was measured once per core using the full-space probe on the cores, usually near the middle of sections. It was frequently not possible to measure thermal conductivity because the core was too lithified to insert the full-space needle probe. Consequently, no thermal conductivity measurements were made on cores from below 560 mbsf.

Thermal conductivity in fine-grained sediments is, as a first approximation, a linear combination of the conductivities of the grains and the interstitial water and therefore depends upon porosity or water content and lithology. Overall, thermal conductivity values (W/[m·K]) increase with depth, which, as expected, parallels decreasing moisture content, (Fig. F36).