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Physical properties

Three holes were cored at Site U1357. The physical property measurements program included nondestructive measurements of GRA bulk density and magnetic susceptibility (loop sensor) on all cores and P-wave velocity on the cores from Holes U1357A and U1357B. NGR was measured on selected whole-round core sections from Hole U1357A and on all core sections from Holes U1357B and U1357C. Because cores from Hole U1357A were the only ones split onboard, moisture, density, and porosity measurements were performed on samples taken from the working half of each of these sections.

The suite of measurements was adjusted for each hole from information learned from the previous hole at this site. Hole U1357A cores were measured on the Special Task Multisensor Logger (STMSL), which consists of GRA bulk density and loop sensor magnetic susceptibility. These measurements are done as soon as the core is in the laboratory and were to be used for interhole correlation. After thermal equilibration, Hole U1357A cores were run through the WRMSL, which included GRA bulk density, loop sensor magnetic susceptibility, and a P-wave logger (PWL). All these measurements were made on all APC cores in Hole U1357A to 184.6 mbsf (Cores 318-U1357A-1H through 20H).

Some core sections in Hole U1357A (Section 318-U1357A-1H-2 and Cores 318-U1357A-5H through 10H and 19H) were also run through the Natural Gamma Radiation Logger (NGRL). However, because of the sensitive nature of the detectors (the cores were leaking gas, water, and sediment from holes drilled to release gas), lithology of the sediments characterized by extremely low measurable counts, and time constraints, it was decided to stop the NGR measurements before core splitting and describing for the remainder of the Hole U1357A cores.

For Hole U1357B, the cores were also run through the STMSL with the overall strategy to use these data for quick correlation to Hole U1357A data for potential adjustment of the drilling offset during drilling Hole U1357C. Afterward, the core sections were allowed to equilibrate to room temperature and were run through the WRMSL. These measurements were made on Cores 318-U1357B-1H through 19H. Later in the cruise, when time permitted, all core sections of Holes U1357B were run through the NGRL.

The first two cores of Hole U1357C were extensively sampled for microbiology and optical stimulated luminescence dating. As such, no physical properties measurements could be made on these two cores. Starting with Core 318-U1357C-3H, all cores from Hole U1357C were measured using the WRMSL. However, because of experience with the previous two holes, the PWL was turned off during these runs. As with Hole U1357B, when time permitted later in the cruise all core sections in Hole U1357C were measured using the NGRL; otherwise no other measurements were obtained on board.

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

Gamma ray attenuation bulk density

GRA density was measured at 5 cm intervals on the STMSL and at 2.5 cm intervals on the WRMSL (5 s and 10 s integration time, respectively). GRA density generally reflects variations in lithology and porosity, and values are as high as 1.3 g/cm3 (Fig. F38).

However, GRA density data are generally of poor quality because these sediments have relatively high gas content, resulting in gas voids, core expansion (Fig. F38), and some ruptured core liners. GRA density data were filtered in a two-step process. In order to remove obvious voids and highly gas expanded sections, GRA density values of <0.6 g/cm3 were removed from the data set. A running average was applied with 10 cm measurement spacing. This filtered data are displayed in Figure F38.

Magnetic susceptibility

Whole-core magnetic susceptibility was measured at 2.5 cm intervals (2 s measurement time). Data ranges from about –10 to 2.9 instrument units, with some peaks in Cores 318-U1357A-19H and 318-U1357B-19H, which represent the occurrence of gravel clasts. The magnetic susceptibility of the nearly pure diatom ooze results in data that are near the noise level, except for the basal clay-bearing diatom ooze of Core 318-U1357A-19H. Pilot measurements on discrete samples using the Kappabridge in the paleomagnetic laboratory yield more useful data because the Kappabridge has 100∞ higher sensitivity than the Bartington sensor on the WRMSL and thus better resolves smaller and/or negative values (see “Paleomagnetism”).

Natural gamma radiation

The NGRL measures the NGR of the sediments. However, because of the high purity of these diatom oozes and therefore low levels of clay or other minerals that act as carrier phases for potassium, thorium, or uranium isotopes, a special routine was required to resolve statistically significant results. First, before these analyses were conducted, a site-specific background measurement was obtained. This background measurement requires a 6 h integration time to fully characterize any variation in the site radiation. Additionally, to generate statistically significant results, unusually long integration times were required (600 s per detector position, adding up to a 20 min run time per 1.5 m section). The combination of long integration time and a site-specific background measurement allows the measurement of counts that range from 1 to 18 cps, even while the average background counts are 6 cps. The longer integration time allows for the relative uncertainties in these measurements to be <15% and generally <5%. The NGR data obtained for some cores from Hole U1357A and all cores from Holes U1357B and U1357C were used for an initial attempt to correlate between cores of three adjacent holes (see “Stratigraphic correlation and composite section”).

P-wave velocity

Because of the pervasive gas-induced expansion, no valid data were obtained.

Moisture and density measurements

Measurements of density, porosity, and grain density were undertaken on 130 samples taken from Hole U1357A (Cores 318-U1357A-1H through 20H). Dependent 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 and were taken in undisturbed sediments whenever possible. However, because of pervasive gas pockets these data may be problematic.

Bulk densities (moisture and density [MAD]) from discrete samples are plotted in Figure F39 and range from 1.02 to 1.31 g/cm3.

GRA densities and wet bulk densities from samples (MAD) correlate only in the upper 5 m of Hole U1357A. Below 5 mbsf, the GRA density results are consistently lower then those measured on discrete samples. We interpret this to be a result of significant gas-induced core expansion resulting in systematic underestimation of the bulk density as measured by the GRA density meter. Therefore, GRA density results from all three cores should be taken as highly suspect.

Porosity ranges from 50% to 99% and generally decreases with depth (Fig. F40).

Grain densities are plotted in Figure F40. Grain density as well as wet bulk and dry density (Fig. F41) exhibit a slight shift at ~160 mbsf. We interpret this shift to signify the transition from diatom oozes with millimeter-scale gas release texture, which characterizes lithostratigraphic Unit I, to the clay-bearing to clay-rich olive-green diatom ooze that represents lithostratigraphic Unit II (see “Lithostratigraphy”).

Moisture content and void ratio show similar trends of decreasing with depth, as expected (Fig. F42).