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doi:10.2204/iodp.proc.342.107.2014

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 NGR measurements were made on whole-round sections using the Whole-Round Multisensor Logger (WRMSL). Thermal conductivity measurements could not be performed at Site U1406 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. For 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 was used to measure spectral reflectance and magnetic susceptibility on archive section halves.

Magnetic susceptibility

Whole-round magnetic susceptibility ranges from 0 to 60 IU at Site U1406 (Fig. F28). From the top of the sediment column to ~2 mbsf (lithostratigraphic Unit I; see “Lithostratigraphy”), magnetic susceptibility is characterized by high values between 30 and 60 IU. At the contact with underlying Unit II, an abrupt step down to ~5 IU occurs. Throughout Unit II, magnetic susceptibility averages 9 IU and increases gradually downhole to ~20 IU at ~182 mbsf. An isolated peak of up to 25 IU occurs at ~30 mbsf in Holes U1406A–U1406C and corresponds to diffuse dark green beds in the core images. Units III and IV are characterized by larger variations in magnetic susceptibility (from 0 to 50 IU). Three notable peaks appear in Unit III and correlate between all three holes. The first peak, to ~25 IU, occurs at the transition between Units II and III at ~182 mbsf. The second peak, to 50 IU, appears at 205 mbsf. The third peak, to ~26 IU, appears at ~235 mbsf (not recovered in Hole U1406C). This third peak correlates with a darker brown-gray color in intervals 342-U1406A-28X-1, 46 cm, and 342-U1406B-28X-5, 62 cm.

Density and porosity

Two methods were used to evaluate bulk density at Site U1406. The GRA method provided a bulk density estimate from whole-round sections. The MAD method, applied to 191 discrete samples, provided a second, independent measure of bulk density, as well as dry bulk density, grain density, water content, and porosity. Changes in MAD bulk density are consistent with those observed in GRA bulk density records (Fig. F28). MAD density values vary between 1.2 and 1.9 g/cm³ and are on average <1% lower than the GRA densities in the APC-cored section and 1.6% higher than GRA densities in the XCB-cored interval. These differences are readily explained by the fact that the same core diameter is assumed for the two core types in the GRA calculations when in fact they are not exactly the same.

In lithostratigraphic Unit I, bulk density averages 1.6 g/cm³, whereas at the top of Unit II at ~2.5 mbsf, bulk density decreases to 1.4 g/cm³. Bulk density then increases downhole to reach ~1.9 g/cm³ at ~210 mbsf, a trend typical of compaction. Between 210 mbsf and the bottom of Hole U1406A, bulk density is relatively constant except for two discrete low intervals at 245 and 270 mbsf.

Water content and porosity vary throughout Site U1406 between 25 and 60 wt% and between 50 and 80 vol%, respectively. Lithostratigraphic Unit I shows lower values (~45 wt% water content and 72 vol% porosity) than the top of Unit II (60 wt% water content and 80 vol% porosity). Below 2.5 mbsf, both of these physical properties decrease gradually downhole to 25 wt% and 50 vol% at ~210 mbsf. Some superimposed shifts are observed at 85, 100, 223, 245, and 270 mbsf; some of these shifts may be associated with stratigraphic hiatuses (see “Age-depth models and mass accumulation rates”).

Grain density generally varies between 2.6 and 2.8 g/cm³ throughout Hole U1406A. The upper part of lithostratigraphic Unit II is characterized by higher variability in grain density than the lower part of the unit, with some values as low as 2.3 g/cm³.

P-wave velocity

P-wave velocity from whole-round sections and section halves are generally consistent and have an average offset of ~20 m/s (Fig. F29). Overall, P-wave velocity gradually increases downhole from 1500 to 1800 m/s. We attribute this trend, which is similar to those observed in bulk density and water content, to downhole compaction. Lithostratigraphic Unit I shows slightly higher values than the top of Unit II. PWL measurements were not performed for most of Units III and IV because sediment rarely filled the liner in these XCB-recovered intervals.

Natural gamma radiation

At Site U1406, NGR values range from 2 to 35 cps and increase downhole to ~230 mbsf (Fig. F29). Over this interval, NGR values shift from 18 to 10 cps at the contact between lithostratigraphic Units I and II. In Unit II, NGR values increase from 10 to 35 cps. Units III and IV are characterized by a generally decreasing downhole trend from 30 to 2 cps, with an abrupt decrease to <10 cps over a <20 m interval centered around ~240 mbsf. Throughout the sediment column, distinctive variations are readily correlated between the three holes, with peaks at ~20, 110, 184, and 240 mbsf (see “Stratigraphic correlation”).

Color reflectance

Color reflectance was measured on archive section halves from Holes U1406A–U1406C. For Hole U1406C, the standard operating resolution of data acquisition was decreased from 2.5 to 5 cm except for key intervals (e.g., for the EOT at ~200 mbsf, color reflectance was measured every 2.5 cm). Color reflectance parameters a* and b* follow similar trends among all three holes (Fig. F30). Four general trends can be defined. In lithostratigraphic Unit I, values vary from 4.5 to 8 for a* and from 7 to 15 for b*. At the contact between Units I and II, a* decreases from 6 to 4.5 and b* increases from 12 to 14. This change in color reflectance values corresponds to the transition from dark brown nannofossil foraminifer ooze to light brown nannofossil ooze (see “Lithostratigraphy”). At ~17 mbsf in the upper part of Unit II, a* and b* values decrease abruptly, with a* shifting from 3.3 to –2 and b* from 12 to –1. This decrease corresponds to the transition in sediment color from light yellow to greenish gray, interpreted as change in sedimentary redox conditions in otherwise homogeneous nannofossil ooze (see “Lithostratigraphy”). Throughout Units II and III (downhole to ~240 mbsf), a* increases slightly from –2 to 1.5 downhole, whereas b* remains relatively constant with an average of 0. Small but distinctive peaks occur within this interval at 30, 180, and 235 mbsf; magnetic susceptibility data show small peak values in these intervals, as well (Fig. F28). At ~245 mbsf in both Holes U1406A and U1406B, a* and b* abruptly increase from 1.5 to 6 and 2 to 10, respectively; this step corresponds to the top of Unit IV. This increase also correlates to increases in magnetic susceptibility and water content, a decrease in bulk density, the transition from greenish gray to grayish brown color, and a substantial increase in radiolarian content (see “Lithostratigraphy”).

L* shows similar trends in all three holes (Fig. F30). In lithostratigraphic Unit I, L* averages 52. L* fluctuates between 50 and 67 in the top ~14 m of Unit II. Distinctive peaks at 17, 160, 195, and 245 mbsf correlate among all three holes. Some other peaks cannot be traced between all holes. For example, peaks at ~45 and ~90 mbsf in Hole U1406B are not observed in Holes U1406A and U1406C. The subtle variations in L* recorded at Site U1406 appear to correlate with changes in carbonate content (see “Geochemistry”).

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