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doi:10.2204/iodp.proc.303306.111.2006

Physical properties

Physical property measurements were carried out at Site U1312 following the procedures described in “Physical properties” in the “Site U1312–U1315 methods” chapter. Magnetic susceptibility measurements were made with both the MSCL and MST system. Additionally, GRA density and NGR were measured with the multisensor system. Discrete P-wave velocities were measured (P-wave sensor 3 [PWS3]–measuring perpendicular to the core axis) on each section, and moisture and density measurements were made on two discrete samples per core, usually at the top of the first and the bottom of the sixth sections.

Whole-core magnetic susceptibility

The magnetic susceptibility records derived during coring operations at Site 1312 show a highly variable record in the upper 40 m because of variations in clay content. Magnetic susceptibility measurements obtained from the MSCL and MST generally show similar trends and display multiple excursions toward higher values in the upper section of each hole (Fig. F29) and consistent with the presence of clays. In the lower section of Hole U1312A, the MST shows more coherent and negative values than the MSCL. Site U1312 has peak magnetic susceptibility values of ~8 × 10^–3 SI units and minimum values approaching the instrument noise level, with most values ranging from 0.4 × 10^–3 to 1.6 × 10^–3 SI.

GRA density

Sediments from Site U1312 show a general increase in bulk density downcore consistent with a downcore decrease in water content (Fig. F30). GRA density varies between 1.5 and 1.8 g/cm³. Discrete bulk densities show similar trends but are offset toward lower values relative to GRA densities.

Porosity

Dry and wet water content, as well as grain densities, were determined from discrete samples (Fig. F31). Both dry and wet water contents decrease in the upper 40 m and then are relatively constant downcore. Grain density values are relatively constant, with a value of ~2.7 g/cm³. Porosity measurements were calculated from the discrete samples. Porosity varies between ~55% and 70% (Fig. F32). Porosity decreases from 70% at the seafloor to ~60% at a depth of 40 mbsf. This pattern is consistent with compaction and is inversely correlated to density.

P-wave velocity

Discrete measurements of P-wave velocity (PWS3) were performed on cores from Site U1312. P-wave velocities generally vary between 1500 and 1600 m/s (Fig. F33); however, there are several departures to lower values of ~1400 m/s. Trends in P-wave velocities are in general correlated with trends in density.

Natural gamma radiation

Natural gamma ray counts range from 10 to 35 cps, with the majority of the values between 10 and 15 cps (Fig. F34) in the lower part of the hole. NGR values also mimic the trends seen in the magnetic susceptibility and GRA density data. NGR counts average ~15–20 cps from 0 to 50 mcd, whereas they average ~10 cps from 50 to 275 mcd.

Discussion

The physical properties measured on cores from Site U1312 show the greatest variability in the upper 40 m consistent with greater clay content. Below 40 m, the carbonate content exceeds 90 wt%.

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