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doi:10.2204/iodp.proc.344.104.2013

Physical properties

At Site U1414, physical properties measurements were made to characterize lithostratigraphic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured using the Whole-Round Multisensor Logger (WRMSL). After WRMSL scanning, whole-round sections were logged for natural gamma radiation (NGR). For soft-sediment cores, thermal conductivity was measured using the full-space method prior to core splitting. Following core splitting, color reflectance and magnetic susceptibility were measured on archive-half cores using the Section Half Multisensor Logger (SHMSL). Moisture and density were measured on discrete samples collected from the working halves of split sediment cores, generally one per section. For indurated sediments, thermal conductivity was measured on split cores. P-wave velocity and strength were measured on the working halves of split cores.

Density and porosity

Bulk density values in Hole U1414A were determined from both GRA measurements on whole-round cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). Wet bulk density values determined from discrete samples range between 1.4 and 2.2 g/cm³, and trends generally agree with the whole-round GRA density (Fig. F34). Grain density measurements were determined from mass/volume measurements on dry discrete samples within the sedimentary sequence. Site U1414 grain density values are relatively constant between the seafloor and 200 mbsf (Unit I–Subunit IIA), with an average value of 2.70 g/cm³. Grain density values are more variable within Subunit IIB and Unit III, with average values of 2.63 and 2.62 g/cm³, respectively.

Porosity data show gradual compaction within Unit I, with values decreasing to 69% at 145 mbsf. Compaction greatly increases within Subunit IIA, and porosity values reach a local minimum of 54% between 185 and 190 mbsf. Porosity then gradually increases to >75% near 225 mbsf in Subunit IIB before decreasing to the base of the hole. Porosity values are scattered within Unit III.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL for all core sections longer than ~20 cm (Fig. F35). Magnetic susceptibility values measured by these two methods are in good agreement. Background magnetic susceptibility values generally decrease with depth, reaching near- to subzero values below 350 mbsf. Excursions to high magnetic susceptibility values generally correspond to tephra layers recovered in the cores.

Natural gamma radiation

NGR counting periods for sediment cores were 10 min with measurement spacing of 20 cm (Fig. F36). NGR values increase to ~21 cps in the uppermost 30 m and remain approximately constant to 110 mbsf. Between 110 and 130 mbsf, NGR values rise to a peak at 130 mbsf and then decrease to below 10 cps at the Unit II/III boundary. NGR values are scattered in Unit III, with peak values near 340 and 360 mbsf.

P-wave velocity

P-wave velocities from the WRMSL show similar trends but higher values than measurements taken on the working halves of sediment split cores using the P-wave caliper (Fig. F37). P-wave velocities from split cores decrease in the uppermost 25 m and then gradually increase with depth. Overall, measured P-wave velocities in Unit I and Subunit IIA are low, averaging 1520 m/s. A local maximum at ~170 to 190 mbsf corresponds to a bulk density maximum and porosity minimum (Fig. F34). Few reliable P-wave velocities could be obtained from the split core in Unit IIB. Between 300 and 340 mbsf, P-wave velocities average 1850 m/s and then sharply increase below 340 mbsf to values as high as 3260 m/s.

Thermal conductivity

Thermal conductivity measurements were conducted on soft-sediment whole-round cores using the needle-probe method and on lithified split cores using the half-space method (Fig. F38A). Thermal conductivity trends generally correlate to those observed in bulk density and mirror those observed in porosity (Fig. F34). Thermal conductivity increases from 0.85 to 1.0 W/(m·K) within Unit I. In Subunit IIA, thermal conductivity increases rapidly with depth. Between ~200 (the transition from Subunit IIA to IIB) and 240 mbsf, thermal conductivity decreases with depth. Below ~240 mbsf, thermal conductivity steadily increases to reach values >2.5 W/(m·K) at the base of the sediment column.

Downhole temperature and heat flow

Four successful downhole temperature measurements between 16 and 73 mbsf were taken using the APCT-3. The four measurements yield a least-squares best-fit gradient of 168°C/km (Fig. F38B). Thermal conductivity throughout the depth interval of the temperature measurements is nearly constant, and the mean thermal conductivity of 0.89 W/(m·K) from 0 to 85 mbsf yields a heat flow of 149 mW/m².

We use the measured thermal conductivities and the estimated heat flow value of 149 mW/m² to extrapolate temperatures at greater depths using the Bullard (1939) method (see “Physical properties” in the “Methods” chapter [Harris et al., 2013b]). This method assumes steady-state conductive heat flow in the sediments. A least-squares fit yielded values of 3.22° and 53.51°C, respectively, for the sediment/water interface and the top of the basalt in Hole U1414A (Fig. F38B).

Sediment strength

Sediment strength was measured both by the automated vane shear and pocket penetrometer (Fig. F39). To compare the two measurement types, unconfined shear strength can be estimated as one half of unconfined compressive strength (Blum, 1997). Both shear and compressive strength values generally increase with depth in the uppermost 200 m to 200 and 400 kPa, respectively. A few needle penetrometer measurements between 200 and 250 mbsf suggest that there may be a slight compressive strength decrease that corresponds to the observed high porosities within Subunit IIB.

Color spectrophotometry

Reflectance L* values are stable around 40 in Subunit IA and gradually increase to 50 with depth in Subunit IB (Fig. F40). In Subunit IIA, values increase more sharply, and then they fluctuate within Subunit IIA and Unit III with peaks exceeding 70. After some increase near the seafloor, reflectance a* and b* values are generally constant throughout Units I and II. In Unit III, a* values increase to a maximum of ~5 at ~350 mbsf and b* values decrease to the base of the sediment column.

Electrical conductivity and formation factor

Formation factor was obtained from electrical conductivity measurements in the y- and z-directions on the split core for APC cores from Hole U1414A (Fig. F41). No systematic anisotropy is observed. After a sharp rise in the upper several meters, values increase slightly with depth, mirroring the general trend of decreasing porosity.

Igneous basement

The limited time available following basalt recovery made resaturation and measurements on discrete samples infeasible. As a result, physical properties measurements were limited to those on the WRMSL and SHMSL. NGR counting periods for basalt cores were increased as much as possible (30–60 min) based on core recovery and penetration rate. Resulting data show variations that can potentially be correlated to the nature of emplacement and the degree of alteration of the igneous basement (Figs. F42, F43). The data are displayed using a modified depth scale that accounts for unreasonable core overlaps caused by tides (see core recovery in Table T1). Because of the variable size of the rock pieces, volumetric effects should be considered when interpreting the NGR, magnetic susceptibility, and GRA density data. The elevated values of NGR at ~440 mbsf correspond to recovered sediment within the igneous basement.

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