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doi:10.2204/iodp.proc.342.108.2014

Physical properties

Physical property measurements were made 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 the whole-round sections using the Whole-Round Multisensor Logger (WRMSL). Thermal conductivity measurements could not be performed at Site U1407 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

Overall, whole-round magnetic susceptibility ranges from –3 to 185 instrument units (IU) (Fig. F34). In lithostratigraphic Unit I, magnetic susceptibility displays high and variable values (30–185 IU) followed by an abrupt decrease at the boundary with Unit II. Throughout Units II–IV, the values become more uniform (5–30 IU). In Subunit Va (~120–175 mbsf), magnetic susceptibility values are low (–1–7 IU), but display a few distinct peaks >50 IU at 147 and 162 mbsf in Hole U1407A, at 140 and 164 mbsf in Hole U1407B, and at 143 and 166 mbsf in Hole U1407C. In the middle of Subunit Vb (~190–230 mbsf), magnetic susceptibility is relatively high (10–60 IU) and shows prominent and superimposed peaks. The interval of high magnetic susceptibility can be traced among all three holes (see “Stratigraphic correlation”). Magnetic susceptibility is low (1–5 IU) below 230 mbsf in Subunit Vb and Unit VI.

Density and porosity

Two methods were used to measure bulk density at Site U1407. The GRA density method provides an estimate from whole-round sections. The MAD method, applied to 144 discrete samples in Hole U1407A, provides an independent measure of bulk density as well as dry bulk density, grain density, water content, and porosity.

At Site U1407, bulk density ranges from 1.5 to 2.1 g/cm³ and, overall, gradually increases downhole. MAD bulk density is, in general, consistent with GRA bulk density (Fig. F34). The average offset between MAD and GRA bulk density is small (0.03 g/cm³), but a larger offset is seen in the lower intervals of lithostratigraphic Subunit Vb (250–270 mbsf in Hole U1407A), where the reduced diameter and fracturing of XCB cores affected the GRA density measurements. From the top of the holes at Site U1407 to the base of Unit I, density increases from ~1.5 to >1.8 g/cm³. Throughout Units II–IV, bulk density gradually increases again from 1.5 to 1.8 g/cm³. From the top of Unit V to 150 mbsf, density is lower than in Unit IV, which is likely related to the change in coring methods from APC to XCB. MAD bulk density in Unit V increases downhole from 1.5 to 2.1 g/cm³. At the bottom of Subunit Vb, bulk density is ~2.0 g/cm³.

Sediment water content at Site U1407 ranges from 5 to 19 wt%, and porosity varies from 39 to 74 vol%. Water content and porosity generally decrease downhole. In lithostratigraphic Unit I, both properties display low values (30–47 wt% and 55–70 vol%), which we interpret as an artifact of measuring foraminiferal sand with a method designed for saturated fine-grained sediment (see “Physical properties” in the “Methods” chapter [Norris et al., 2014b]); therefore, these values do not represent accurate measurements of water content and porosity. From Unit II to IV, water content and porosity gradually decrease downhole, falling from 45 to 30 wt% for water content and from 60 to 50 vol% for porosity. These trends are expected as a result of increased sediment compaction with depth. Water content and porosity are more variable in Unit V (19–48 wt% and 39–70 vol%, respectively).

Grain density ranges from 2.5 to 2.9 g/cm³ at Site U1407. Grain density decreases from 2.8 g/cm³ at the top of the sediment column to 2.6 g/cm³ at 100 mbsf. In the interval from 130 to 210 mbsf, grain density is more variable, ranging from 2.5 to 2.9 g/cm³. In sediment below 220 mbsf, grain density is relatively invariant and varies from 2.7 to 2.9 g/cm³).

P-wave velocity

P-wave velocity was measured using the P-wave logger (PWL) on all whole-round sections and using the PWC on undisturbed section halves from Holes U1407A–U1407C.

P-wave velocity varies from 1450 to 1670 m/s for PWL data and from 1480 to 3310 m/s for PWC data in all three holes. P-wave velocity measured with the PWL is slightly higher than that measured by the PWC in the APC-cored interval and slightly lower in the XCB-cored interval, with the offsets averaging ~20 m/s (Fig. F35). P-wave velocity exhibits a gradual increase toward the bottom of all three holes. In sediment below 120 mbsf, P-wave velocities both increase as a result of lithification and become more variable downhole (from 1650 to ~2000 m/s), possibly caused by the change from APC to XCB drilling. In the upper part of Subunit Vb, P-wave velocity data show abrupt increases at ~180 and 206 mbsf.

Natural gamma radiation

NGR was measured on whole-round sections in Holes U1407A–U1407C and ranges from 2 to 56 cps (Fig. F35). In lithostratigraphic Units I–III, NGR values average 20 cps, with prominent peaks in all three holes at ~9 mbsf (to ~50 cps; Unit I/II boundary) and at ~25 mbsf (> 30 cps) within Unit II. NGR values drop to 2 cps at the boundary between Units III and IV (82 mbsf in Hole U1407A, 85 mbsf in Hole U1407B, and 76 mbsf in Hole U1407C). The decrease in NGR counts at the Unit III/IV boundary corresponds to a large increase in carbonate content. From Unit IV to V, NGR displays low values (2–5 cps), with notable peaks of >20 cps. In Subunit Va, NGR data show a peak at ~140 mbsf that occurs only in Holes U1407B and U1407C. Other prominent events were observed lower in all three holes: at the Subunit Va/Vb boundary (~175 mbsf) and at 200, 232 (>50 cps), and 238 mbsf. These last three peaks correspond to the black shale associated with OAE 2 (see “Lithostratigraphy” and “Geochemistry”).

Color reflectance

Reflectance values were measured on archive section halves in all three holes. For Hole U1407C, the resolution of measurements decreased from 2.5 to 5.0 cm to speed up core processing. Changes in color reflectance a* and b* are consistent among all three holes except for an interval of poor data quality between 130 and 210 mbsf in Hole U1407A caused by a technical problem with the SHMSL (Fig. F36). For all three holes, a* and b* range from –3 to 21 and –25 to 29, respectively.

Both a* and b* decrease from ~10 to ~5 from the top of the sediment column to ~40 mbsf. In lithostratigraphic Unit III, a* and b* display high values at the top of the unit and then decrease to near-zero values at ~42 mbsf. This decrease in Unit III is caused by a change in sediment color from light brown to greenish (see “Lithostratigraphy”). Reflectance parameter a* increases through Unit IV. In Subunit Va, both a* and b* have high values, ranging from 4 to 11. Both reflectance parameters decrease between ~170 and ~180 mbsf in Hole U1407B and U1407C and between 230 and 240 mbsf in all three holes. These fluctuations correspond to alternations of light and dark brown sediment. Correlative minima in the a* records are observed at ~230 mbsf in Hole U1407B (–3.5) and in Hole U1407C (–4.8); this interval corresponds to the greenish white chalk layer just above the black shale of OAE 2 (see “Lithostratigraphy”).

Color parameter L* exhibits the same trends in all three holes, and values range from 13 to 95. From the top of the sediment column to the upper part of lithostratigraphic Unit III, L* increases from ~50 to ~75. At the top of Unit IV, L* increases sharply from 75 to 90, corresponding to a downhole transition to white sediment and a notable increase in carbonate content (see “Lithostratigraphy” and “Geochemistry”). In Subunits Va and Vb, L* averages ~75, whereas in the interval from 180 to 200 mbsf, L* shows substantial fluctuations and a peak to ~90 at 190 mbsf in Holes U1407A and U1407B and at 185 mbsf in Hole U1407C. A minimum was observed at ~232 mbsf in all three holes; this interval is associated with the black shale of OAE 2.

The sharp, high-amplitude expression of the Unit III/IV boundary in color reflectance as well as in NGR and other high-amplitude correlative features in Unit Vb lead us to speculate that an error occurred on the rig floor with the pipe tally. Most likely, at the time the 7.7–10.7 mbsf interval was drilled without coring in Hole U1407C, a 9.5 m pipe was added but not accounted for (a human error possible because no core was taken), leading to sample depth registrations that are too shallow by that amount between 10.7 and 96.0 mbsf. After another interval was drilled without coring from 96 to 117 mbsf, the error apparently corrected itself.

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