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doi:10.2204/iodp.proc.303306.112.2006

Physical properties

Physical properties of marine sediments are good indicators of composition, formation, microstructure, environmental conditions, and depositional processes. Physical properties are also important for correlating depths between holes. Marine sediment physical properties at Site U1313 were measured following the procedures described in “Physical properties” in the “Site U1312–U1315 methods” chapter. Magnetic susceptibilities were measured using both the MSCL and the multisensor track (MST) system. GRA densities, P-wave velocities, and NGR were also measured using the MST. Discrete P-wave velocities were measured (P-wave sensor number 3—measuring perpendicular to the core axis) on each section. Finally, moisture and density were measured on two discrete samples per core, usually at the bottom of the first and sixth sections.

Whole-core magnetic susceptibility measurements

Magnetic susceptibility measurements measured on the MSCL and with the MST generally show similar trends and display multiple excursions toward higher values in the upper section of all holes (Fig. F32). Magnetic susceptibility values at Site U1313 are highly variable in the upper ~140 m because of variations in clay content. Below 140 m, carbonate content exceeds 95 wt% and magnetic susceptibilities are very low. In the lower sections (below ~140 m) of Holes U1312A, U1313B, and U1313C, the values of the magnetic susceptibility measured by the MST show more coherent and negative values. Site U1313 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 units.

Gamma ray attenuation density

Sediments from Site U1313 show a general increase in bulk density with depth (Fig. F33), consistent with a downhole decrease in water content due to compaction. GRA density varies between 1.4 and 1.9 g/cm³. Discrete bulk densities (Fig. F34) show similar increase with depth.

Natural gamma radiation

NGR counts range from 15 to 30 cps in the upper 125 m of all sections as shown in Figure F35. Below 125 m, the values are relatively stable and range from 12 to 14 cps, consistent with the high-carbonate fraction.

Density and porosity

Porosity measurements were calculated from discrete samples. Porosity decreases from 80% at the seafloor to ~55% at 200 mbsf. Discrete dry density values increase downhole from 0.5 g/cm³ to 1.2 g/cm³ at 200 mbsf. The porosity pattern is consistent with compaction and shows an inverse relationship to density. Porosity and dry density values are shown in Figure F34.

P-wave velocity

P-wave velocities were measured for Holes U1313A and U1313B using the MST (Fig. F36). MST measurements of P-wave velocities were found to adversely affect magnetic susceptibility measurements, so P-wave velocities were not measured on subsequent holes. Additionally, discrete measurements of P-wave velocity were made on each section of Hole U1313A. In general, the measurements of P-wave velocity vary between 1500 and 1600 m/s and increase downhole. Trends in P-wave velocity are in general correlated with trends in density.

Discussion

The physical properties measured at Site U1313 are generally well correlated and show the greatest variability in the upper 140 m, consistent with greater variability in clay content. This variability in clay content is thought to reflect glacial–interglacial changes, with clay content being highest during glacial times. Below 140 mbsf, the carbonate content exceeds 95 wt% and the variability in physical properties is small.

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