Previous   |    Next

doi:10.2204/iodp.proc.342.109.2014

Physical properties

We made physical property 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). Thermal conductivity was measured on one section per core for Cores 342-U1408A-1H through 10H. 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 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 half sections.

Magnetic susceptibility

Overall, magnetic susceptibility ranges from 0 to 160 IU (Fig. F29). From the top of the sediment column at Site U1408 to ~12 mbsf (lithostratigraphic Unit I; see “Lithostratigraphy”), magnetic susceptibility is characterized by high values and varies in magnitude between 40 and 160 IU. At the contact with underlying Unit II, magnetic susceptibility decreases sharply to ~10 IU. Magnetic susceptibility throughout Units II–IV remains constant and averages 14 IU. In Unit III, two superimposed sharp peaks appear and correlate among all three holes. The first peak (~25 IU) occurs just below the transition between Units II and III at ~30 mbsf. The second peak, with a magnitude of ~60 IU in Holes U1408A and U1408C and ~40 IU in Hole U1408B, occurs at ~60 mbsf. In all three holes between ~80 and 115 mbsf, magnetic susceptibility data show pronounced variability associated with distinct color cycles. Cycles in magnetic susceptibility also occur between ~140 and 170 mbsf in all holes.

Density and porosity

Two methods were used to evaluate bulk density at Site U1408. The GRA density method provided a bulk density estimate from whole-round sections. The MAD method applied to 173 discrete samples gave a second, independent measure of bulk density, as well as dry density, grain density, water content, and porosity from discrete samples.

Overall, bulk density values vary between 1.4 and 1.95 g/cm³ (Fig. F29). MAD bulk density values are on average ~3% lower than GRA bulk density downhole to 180 mbsf. Below ~180 mbsf, the offset increases to 5% as a result of the change in coring method from APC to XCB. In lithostratigraphic Unit I, bulk density increases from 1.5 to 1.75 g/cm³. An abrupt decrease to ~1.4 g/cm³ occurs at the contact with underlying Unit II; this change correlates with the transition between clayey silt, nannofossil foraminiferal ooze, and nannofossil clay with silt (see “Lithostratigraphy”). Bulk density throughout Units II and III increases downhole at Site U1408 to ~1.95 g/cm³ at ~225 mbsf. In Holes U1408B and U1408C, this trend toward higher values changes to more constant values that average 1.8 g/cm³ at 75 mbsf. Between 225 mbsf and the bottom of Hole U1408A, bulk density decreases from 1.9 to 1.6 g/cm³.

Water content and porosity vary between 20 and 60 wt% and between 45 and 80 vol%, respectively, throughout Site U1408. Water content and porosity show greater variability in Units I and II than in underlying sediment. Throughout Unit III (below 22 mbsf), both of these properties decrease gradually downhole from 50 to 25 wt% for water content and from 75 to 50 vol% for porosity. In Unit IV, water content and porosity both increase downhole to ~55 wt% and ~65 vol%, respectively, at the bottom of the hole.

Throughout Hole U1408A, grain density remains relatively constant and averages 2.77 g/cm³. Units I and II are characterized by higher amplitude variability in grain density (2.6 and 2.9 g/cm³) than Unit III (2.7 and 2.8 g/cm³). Unit IV is characterized by a downhole decrease in grain density from 2.8 to 2.68 g/cm³ at the bottom of the hole.

P-wave velocity

P-wave velocity was measured with the P-wave logger (PWL) on all whole-round sections and by the PWC on undisturbed section halves from Holes U1408A–U1408C. Whole-round and section-half data track each other well, but PWC values are consistently lower than PWL values by ~18 m/s (Fig. F30). Overall, P-wave velocity gradually increases downhole from 1500 to 1800 m/s, which we attribute to downhole compaction; bulk density and water content show similar trends. P-wave velocities are slightly higher in lithostratigraphic Unit I than in the upper part of Unit II, which corresponds to the transition between clayey silt, nannofossil foraminiferal ooze, and nannofossil clay with silt (see “Lithostratigraphy”). We did not complete PWL measurements for the lower part of Unit III and all of Unit IV, because sediment rarely filled the liner in these intervals, a consequence of coring soft chalks with the XCB. PWC-measured velocities increase to 1900 m/s in the nannofossil chalk and thin chert layers associated with Unit IV.

Natural gamma radiation

NGR was measured on the whole-round sections in Holes U1408A–U1408C. NGR values range from 10 to 55 cps in all three holes (Fig. F30). In lithostratigraphic Unit I, NGR data show large variations between 15 and 55 cps, with the largest values occurring at the peak defining the contact between Units I and II. NGR values increase from ~20 to ~30 cps in Unit II and reach ~40 cps at the top of Unit III (~22 mbsf). The upper part of Unit III (between 22 and 40 mbsf) is characterized by a trend toward lower values, from 40 to 25 cps. Between ~40 and 225 mbsf, the NGR trend remains constant, with an average of ~25 cps, significant low-amplitude variability throughout, and several larger peaks and troughs (see “Stratigraphic correlation”). NGR values decrease from 25 to 5 cps at the contact between Units III and IV and remain low to the bottom of the hole.

Color reflectance

Color reflectance was measured on archive section halves from Holes U1408A–U1408C. Four general trends can be defined in all three holes (Fig. F31). In lithostratigraphic Unit I, values show relatively large fluctuations between 3.5 and 8 for a* and from 3 to 10 for b*. In Unit II, a* has decreased, constant values of 2.2 and b* decreases from ~11 to ~8. The change in color reflectance across the Unit I/II boundary is associated with the transition from brown and gray clayey silt and nannofossil foraminiferal ooze to pale yellow and light gray nannofossil clay with silt (see “Lithostratigraphy”). At the transition between Units II and III at ~22 mbsf, distinct decreases occur in a* (from 2 to –1) and b* (from 4.5 to 2). These decreases correspond to the transition from yellow, pale brown, and greenish gray sediment. Throughout Unit III, color reflectance parameters a* and b* remain constant downhole (a* averages 0.5 and b* averages –0.35). Two superimposed peaks that correlate among all three holes occur at ~30 and ~135 mbsf. In Hole U1408A at the top of Unit IV (230 mbsf), a pronounced step occurs where a* and b* increase from –1 to 2.2 and from –1 to 1.75, respectively. These changes correlate to increases in water content and in radiolarian content and decreases in bulk density and NGR. This step also correlates to the transition from greenish gray to light greenish gray nannofossil oozes to underlying pinkish brown nannofossil oozes.

The peaks in a* and b* at 115 mbsf in Hole U1408A and at 85 mbsf in Hole U1408C are spurious measurements attributable to technical problems during data collection.

L* corresponds to sediment brightness and generally follows pronounced lithostratigraphic changes (Fig. F31). L* records show similar trends in all three holes. In lithostratigraphic Unit I, L* values average 53 and abruptly increase to 65 in Unit II, clearly recording the transition from brown and gray clayey silt and nannofossil foraminiferal ooze to pale yellow and light gray nannofossil clay with silt (see “Lithostratigraphy”). From ~15 mbsf to the bottom of Unit II, L* values decrease to 45 and remain constant throughout Unit III with an average value of 54, although distinctive peaks in all three holes are observed at ~95 and ~155 mbsf. At the top of Unit IV, L* values increase to 80 and remain high but variable to the bottom of the hole. The major variations in L* values recorded at Site U1408 appear to correlate with changes in calcium carbonate content (see “Geochemistry”).

Thermal conductivity

Thermal conductivity was measured on Section 3 in Cores 342-U1408A-1H through 10H, usually near the middle of the section (~75 cm), using the full-space probe (Table T21). Thermal conductivity increases downhole from ~1.0 to 1.2 W/(m·K) and follows an approximately linear trend (Fig. F32). Thermal conductivity measurements show some correlation with bulk density (R² = ~0.61) (Fig. F33). Thermal conductivity increases downhole as porosity decreases, which we attribute to lower interstitial spacing that attenuates the applied heat from the probe.

Top of page   |    Previous   |    Next