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doi:10.2204/iodp.proc.342.106.2014

Physical properties

We made physical properties measurements on whole-round sections, section halves, and discrete samples from section halves. Gamma ray attenuation (GRA) bulk density, magnetic susceptibility, P-wave velocity, and natural gamma radiation (NGR) measurements were made on whole-round sections using the Whole-Round Multisensor Logger (WRMSL) and NGR logger. Thermal conductivity measurements could not be performed at Site U1405 because of technical problems. Compressional wave velocity on section halves was also measured at a frequency of two in each section (at ~50 and 100 cm) using a P-wave caliper (PWC). For moisture and density (MAD) analyses, one discrete sample was collected in each section (typically at ~35 cm from the top of a section). The Section Half Multisensor Logger (SHMSL) was used to measure spectral reflectance and magnetic susceptibility on archive section halves.

Magnetic susceptibility

From the top of the sediment column at Site U1405 to ~18 mbsf (lithostratigraphic Unit I), magnetic susceptibility first increases downhole from 50 to 100 instrument units (IU) and then decreases to ~5 IU at the top of Subunit Ib (Fig. F34). This decrease is associated with a decrease in carbonate content at 10 mbsf (see “Geochemistry”). Below, Unit II is largely composed of a uniform sequence of greenish gray biosiliceous ooze and clay, and magnetic susceptibility remains very low, averaging 5 IU in all three holes. A small peak to ~25 IU occurs in Holes U1405B and U1405C at ~80 mbsf and is associated with increased color reflectance a*.

Density and porosity

Bulk density, water content, porosity, and grain density measurements were obtained from 194 discrete samples using the MAD method. Bulk density was also determined using the GRA method on the whole-round sections. Changes in MAD bulk density are consistent with those in the GRA bulk density (Fig. F34). MAD values are ~2% lower than GRA density values throughout the section. The most striking feature in the density profile is the significantly higher values in lithostratigraphic Unit I than in Unit II. The average bulk density is 1.57 g/cm³ in Subunit Ia of Holes U1405B and U1405C, increasing from 1.4 g/cm³ at the top of the hole to 1.7 g/cm³ at ~3 mbsf in Hole U1405A. Subunit Ib is characterized by a decrease from ~1.6 to ~1.3 g/cm³ in all three holes. Bulk density values show a peak at ~4 mbsf in Holes U1405B and U1405C that is not seen in Hole U1405A.

In Unit II, bulk density shows a typical downhole increase from ~1.3 to 1.5 g/cm³ caused by compaction, with several superimposed small peaks and shifts. In Hole U1405A, notable peaks are found at 68, 220, and 238 mbsf. Broadly correlative peaks in bulk density are recorded at 78, 140, 178, and 220 mbsf in Hole U1405B and at 78, 135, 163, and 218 mbsf in Hole U1405C.

Water content and porosity increase downhole in Unit I from 40 to 70 wt% and from 65 to 87 vol%, respectively, with a distinctive shift at the contact between Units I and Unit II. Throughout Unit II, both of these parameters decrease gradually (to 40 wt% for water content and to 67 vol% for porosity) to the bottom of Hole U1405A. Some superimposed shifts appear at 70, 100, and 180 mbsf, and some of these are likely to be associated with stratigraphic hiatuses. Notable decreases in these physical properties occur (water content shifts from ~60 to 40 wt% and porosity from 75 to 63 vol%) at ~270 mbsf, but the link to lithology cannot be established because of the poor recovery below 250 mbsf in Hole U1405A.

Grain density generally varies between 2.6 and 2.8 g/cm³ throughout Hole U1405A with the exception of one data point at ~70 mbsf that shows a specific density of 2.9 g/cm³ and two data points that show values <2.5 g/cm³ at ~218 mbsf.

P-wave velocity

P-wave velocity from whole-round sections and section halves track each other well, with an average offset of ~15 m/s (Fig. F35). Overall, P-wave velocity gradually increases downhole in each of the three holes. Between the top of the sediment column and ~230 mbsf, P-wave velocity increases from 1500 to 1600 m/s and reaches 1650 m/s in the lower part of lithostratigraphic Subunit IIc of Hole U1405A (230–308 mbsf). In Hole U1405A, P-wave velocity measurements are slightly higher in Unit I than the upper part of Unit II. P-wave velocity shows a downhole compaction trend similar to bulk density and water content.

Natural gamma radiation

In all three holes from the top of the sediment column to ~25 mbsf (lithostratigraphic Unit I), NGR values range from 15 to 30 cps (Fig. F35). In Unit II, NGR values fluctuate from 15 to 25 cps and are on average lower than those in Unit I. Notable peaks are observed in all three holes. A peak of ~25 cps is seen at ~82 mbsf in Holes U1405B and U1405C and may be associated with the small peak at ~70 mbsf in Hole U1405A. However, other peaks cannot be traced between holes (see “Stratigraphic correlation”). For example, in Hole U1405B the 30 cps peak at 140 mbsf has no equivalents in Holes U1405A and U1405C. Physical property variations such as these likely reflect changes in sediment composition that do not extend across all holes, which could reflect sedimentation changes associated with drift deposits.

Color reflectance

Reflectance parameters were measured on archive section halves in Holes U1405A–U1405C. For Hole U1405C, the resolution was decreased from 2.5 to 5 cm in order to finish processing all cores from Hole U1405C before arriving at the next site.

Color reflectance parameters a* and b* follow similar trends to one another and characterize lithostratigraphic Units I and II in the three holes (Fig. F36). Within each lithostratigraphic unit, both a* and b* remain relatively constant, and at the unit boundary they drop dramatically. In Unit I, the values fluctuate from 0 to 6 for a* and from 5 to 15 for b*. In Unit II, a* varies between –4 and 0 and b* varies between –2 and 4. The positive values of a* and b* in Unit I correspond to yellow, brown, and gray sediment. The lower values in Unit II indicate higher blue and green content and reflect relatively homogeneous greenish gray sediment (see “Lithostratigraphy”).

In Unit II, a* increases slightly downhole. Slightly more distinct variations occur at ~70–80 mbsf in Holes U1405A–U1405C, which correspond to features in the magnetic susceptibility and NGR data series at those depths (Fig. F35).

L* corresponds to sediment brightness and generally follows pronounced lithologic changes (Fig. F36). In Unit I, the average L* is 52 in all three holes. Unit II is characterized by slightly lower L* values that vary between 40 and 55. In Hole U1405A, L* fluctuates downhole between 40 and 60 from the top of the hole to ~210 mbsf. More than any other physical property, L* profiles show somewhat different patterns, or noise distributions, in Holes U1405A–U1405C. The variations in L* recorded at Site U1405 probably reflect changes in calcium carbonate content.

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