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doi:10.2204/iodp.proc.341.103.2014

Physical properties

Physical properties measurements were taken on samples from each of the five holes drilled at Site U1417 to provide basic information for characterizing the drilled section using whole-round cores, split cores, and discrete samples. After cores were divided into sections, all whole-round sections longer than ~30 cm were measured through the GRA bulk densitometer and magnetic susceptibility loop on the STMSL at 2.5–5.0 cm intervals with 2 s measurements. After reaching thermal equilibrium with ambient temperature (~4 h), GRA bulk density, magnetic susceptibility, and compressional wave (P-wave) velocity were measured with the WRMSL at 2.5 cm intervals with 5 s measurements. Some damaged sections with split and/or patched core liners were too wide to fit through the WRMSL and/or the STMSL loop magnetic susceptibility meters and therefore could not be logged with any of the core logger instrumentation. After WRMSL scanning, whole-round sections were logged for NGR at 10 cm intervals with eight detectors. Color spectrometry, color reflectance, and magnetic susceptibility were measured on the split cores using the SHMSL at 2.5 cm resolution. Discrete P-wave measurements and shear strength measurements were made on split sediment cores from working-half sections on the Section Half Measurement Gantry. Moisture and density (MAD) were measured on 10 cm³ plugs collected from the working halves. Summaries of all the physical properties measured with the multisensor loggers on each hole, as well as discrete bulk density and P-wave velocity, are provided in Figures F31, F32, F33, F34, and F35.

Gamma ray attenuation bulk density

Variations in GRA bulk density can reflect changes in mineralogy/lithology, consolidation, porosity, and coring disturbance. Whole-round GRA bulk density averages ~1.8 g/cm³ in the APC cores and displays downhole cyclic variability on the order of ~0.5 g/cm³ (Figs. F31, F32, F33, F34, F35). A trend toward higher values with depth in the APC cores is consistent with increasing sediment compaction/consolidation. However, whole-round GRA values abruptly decrease corresponding to the depth at which we transitioned from APC core collection to XCB core collection in Holes U1417B and U1417D (Figs. F32, F34). This transition to decreasing values partially reflects incomplete core recovery associated with XCB and RCB coring methods, as the width of the recovered sediments fails to completely fill the core liner and the GRA measurements are therefore calibrated for a larger sediment volume than is actually contained in XCB- and RCB-acquired cores.

The negative bias in core logger physical properties measurements associated with a partially filled core liner is confirmed by plotting discrete MAD bulk density values against the equivalent WRMSL GRA bulk density values from corresponding depths in the same core (Fig. F36). When all discrete MAD values from the combined APC, XCB, and RCB cores are considered, there is considerable scatter in the relationship with WRMSL GRA bulk density and trend toward lower WRMSL GRA bulk densities relative to discrete samples, with a slope of 0.92. However, when only APC cores (which were almost uniformly recovered with full liners) are considered, the relationship between discrete MAD and WRMSL GRA bulk densities collapses onto a linear trend with a slope of ~1.

Magnetic susceptibility

Because the WRMSL loop magnetic susceptibility meter has a response function with a ~4.5 cm width at half height (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]), whereas the point-source magnetic susceptibility data is sensitive over ~1 cm, both the point source and loop magnetic susceptibility data sets were smoothed with a Gaussian filter of 10 cm (±3σ) and then interpolated to constant resolution (note that data in the LIMS database are recorded at their raw measurement resolution). Although the relationship between point-source magnetic susceptibility and loop magnetic susceptibility appears to be linear, there is an offset in the overall magnitude of the measurements, with loop magnetic susceptibility being on average 1.68× greater than the point-source measurements (Fig. F37). We evaluate all volumetric magnetic susceptibility measurements in instrument units (IU) because of the lack of available absolute calibration standards (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]).

Volumetric WRMSL loop magnetic susceptibility (κ) averages around ~120 IU downhole at the site (Figs. F31, F32, F33, F34, F35). A few anomalously high values >1000 IU in Hole U1417D are associated with fragments of tungsten carbide drill bit teeth that broke off of XCB cutting shoes. Cyclic variability between 50 and 100 IU is present throughout the core, and a trend toward higher values with depth in the APC cores may reflect sediment compaction. A decrease in WRMSL loop magnetic susceptibility and an increase in the difference between loop and magnetic susceptibility deeper than ~190 m CCSF-B (~225 m CSF-A) in Holes U1417B and U1417D (Figs. F32, F34) are associated with the transition to XCB core collection and likely reflect reduced sediment diameter in the core liner (as discussed in “Gamma ray attenuation bulk density”). The negative bias observed in loop magnetic susceptibility associated with incompletely filled core liners is consistent with decreases in measured GRA bulk density also driven by the deviation from calibration volume (Fig. F38). After smoothing with a Gaussian filter of 10 cm (±3σ) to correct for the differing response functions of the instruments, we can use this relationship to generate a volume-corrected mass magnetic susceptibility (χ) with units of cm³/g (Fig. F38). This mass (GRA normalized) magnetic susceptibility can be used to evaluate changes in lithology independent of variable porosity, different coring techniques, and recovery efficiencies. Correcting for sediment volume reduces variance in χ by ~10% relative to the equivalently smoothed raw WRMSL magnetic susceptibility data normalized by the mean core GRA bulk density.

Compressional wave velocity

P-wave velocity was measured on the WRMSL P-wave logger (PWL) (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]) in Holes U1417A–U1417D to ~220 m CSF-A at a resolution of 2.5 cm (Fig. F39). Reasonable measurements could only be obtained for core depths shallower than ~220 m CSF-A because of the development of void spaces within the core liners after switching to XCB coring in Holes U1417B and U1417D. WRMSL P-wave velocity values gradually increase downhole, closely following GRA bulk densities (Figs. F31, F32, F33, F34), ranging from ~1470 m/s at the seafloor to ~1650 m/s at ~220 m CSF-A (Fig. F36).

Discrete P-wave measurements using the P-wave caliper tool (PWC) were taken in Holes U1417A, U1417D, and U1417E with ~20 m overlap of measurements between holes. In Holes U1417A and U1417D, PWL and PWC measurements overlapped at all depths shallower than ~220 m CCSF-B (Fig. F40). PWC values were automatically picked where possible and manually picked when the automatic picker encountered errors when the calipers did not have sufficient contact with the sample because of very soft sediment or bad coupling with the liner. In cases where both automatic and manual picks were recorded, the manual picks are systematically faster than the automatic picks (Figs. F40, F41). The PWL and automated PWC velocities track well deeper than ~15 m CCSF-B, and a scatter plot shows that the two measurements are significantly correlated (Fig. F40). However, care must be taken when making interpretations based on the discrete velocity data because the sampling is biased by both core recovery and sampling rate in different lithology types.

Deeper than ~200 m CCSF-B, the rate of velocity change with depth increases to ~420 m CCSF-B (Fig. F41). The inflection point at ~200 m CCSF-B may be associated with the transition from lithostratigraphic Unit I to Unit II, where the abundances of lonestones and biosiliceous oozes decrease (see “Lithostratigraphy”). Deeper than ~420 m CCSF-B, elevated velocities (~2420–5700 m/s) are correlated to cemented intervals, whereas lower values (<1650 m/s) are associated with diatom ooze (Fig. F41) (see “Lithostratigraphy”).

Natural gamma radiation

We analyzed NGR at 10 cm intervals on all whole-round core sections that exceeded 50 cm in length, with minimum section length limited by the response function of the sodium iodide detectors (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). Each measurement reflects the integration of 5 min of counts (i.e., 10 min of counting per section, consisting of 5 min at each of two positions separated by 10 cm). NGR values show downhole cyclic variability between 7 and 50 counts per second (Figs. F31, F32, F33, F34, F35) with a mean and standard deviation of 31 and 6, respectively. As NGR counts reflect minerals that fix K, U, and Th, high-frequency variations are likely associated with changes in lithology. NGR variability parallels trends in GRA bulk density and P-wave velocity, which also suggests the dominant influence of terrigenous input. Low NGR values frequently correspond with low magnetic susceptibility and increased lightness (L*).

The high-frequency variability in NGR is superimposed on a long-term increase downhole between 0 and 220 m CCSF-B in Holes U1417A–U1417D, interrupted by a decrease in counts between 220 and 360 m CCSF-B, and partially recovering to higher values below 360 m CCSF-B (Fig. F42). Low NGR counts deeper than 220 m CCSF-B may partly be attributed to the smaller diameter of recovered sediment in the core liner and/or section voids during XCB coring at Holes U1417B and U1417D. For WRMSL magnetic susceptibility, after smoothing the data with a Gaussian filter of 50 cm (±3σ) to accommodate for the varying response functions of the instruments, we can normalize the volumetric NGR to the WRMSL GRA bulk density to correct for variability in sediment volume. Although this treatment reduces the overall variance in the NGR record by ~50% relative to the Gaussian-smoothed uncorrected NGR data normalized by the mean core GRA bulk density, the decrease in NGR observed between 225 and 360 m CCSF-B persists in a reduced form deeper than 300 m CCSF-B (Fig. F42). We therefore propose that the lower volume-normalized NGR values between 300 and 360 m CCSF-B correspond to a lithology change in the late Pliocene (see “Lithostratigraphy”).

Moisture and density

MAD bulk density values in Holes U1417A, U1417D, and U1417E were calculated from mass and volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). Depending on core recovery, quality, and lithology, one to three samples were taken per core. The lithologies associated with samples were recorded in the log sheet if distinct from the dominant sediment (i.e., a sandy event deposit or a biosiliceous layer). Where lithology is undefined in these records, the sample was taken from the dominant lithology within that core. A total of 220 samples were analyzed for MAD: 58 samples from Hole U1417A, 97 samples from Hole U1417D, and 65 samples from Hole U1417E.

MAD values correspond well with GRA bulk densities measured on the WRMSL in Hole U1417A (Fig. F43). Starting at ~225 m CCSF-B, GRA bulk density values are less than the MAD densities because of reduced sediment diameter recovery in the XCB and RCB cores (see “Gamma ray attenuation bulk density”). MAD densities increase downhole to ~300 m CCSF-B, ranging from ~1.5–1.8 g/cm³ at the seafloor to ~1.8–2.1 g/cm³ at ~300 m CCSF-B. From ~300 to 430 m CCSF-B a population of low-density samples ranges from ~1.4 to ~1.8 g/cm³. At ~430 m CSF-A, densities increase to ~1.7–2.0 g/cm³. Between ~430 and ~700 m CCSF-B, density remains relatively constant, with increased scatter below ~620 m CCSF-B associated with biosiliceous layers, sand-rich intervals, and cemented sections (see “Lithostratigraphy”).

Bulk grain density values are fairly constant between the seafloor and ~300 m CCSF-B, ranging between ~2.8 and 3.0 g/cm³ (Fig. F44). Variability increases slightly between ~300 and 430 m CCSF-B, with a population of low bulk grain densities corresponding to low bulk densities. Between ~430 and 620 m CCSF-B, variability again decreases, averaging between ~2.8 and 2.9 g/cm³. A secondary population of elevated scatter occurs deeper than 620 m CCSF-B, with grain densities up to ~3 g/cm³.

Porosity (percent pore space of wet sediment volume) measured on discrete samples decreases with depth to ~310 m CCSF-B, showing a normal consolidation curve. At ~310 m CSSF-B, discrete values increase to ~52%–62% and then decrease to ~45% at ~470 m CCSF-B (Fig. F44). Deeper than ~620 m CCSF-B, more scatter was observed because of measurements associated with biosiliceous material and cemented sections. Deeper than ~450 m CCSF-B, sand-rich sediment and clay and silt sediment show normal consolidation. Diatom-rich sediment shows high porosity, which reflects the presence of the porous diatom frustules (Fig. F44).

Shear strength

Shear strength measurements were performed on working section halves from Holes U1417A and U1417D using the automated vane shear (AVS) testing system (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). Efforts were made to avoid the locations of obvious drilling disturbance or cracks in the half-core sample. Measurements were taken as close as possible to the positions of the MAD samples.

Shear strength indicates that sediments range from very soft (0–20 kPa) to very stiff (120–180 kPa). The rate of change of shear strength with depth decreases at ~40 m CCSF-B (Fig. F45). Values are more scattered deeper than 100 m CCSF-B. All samples were taken in the dominant lithology of dark gray-greenish mud, so it is unlikely that the increasing variability of shear strength measurements is due to lithology alone. Rather, as the mud gets stiffer with depth, other factors such as cracking may affect the measurement. Shear strength measurements were halted at Core 341-U1417D-43X when samples were sufficiently hardened to break destructively upon penetration of the vane.

Geothermal gradient

Temperature measurements were conducted using the APCT-3 during APC coring in Hole U1417A. Three temperature measurements were taken in total (Fig. F46A), and a geothermal gradient was successfully obtained (Cores 341-U1417A-4H, 10H, and 13H) between 33.9 and 104.4 m CSF-A. The best fit line to temperature versus depth data was derived from the results (Fig. F46B):

where T(z) is in situ temperature at depth z (m CSF-A). The estimated geothermal gradient is therefore 61°C/km. Note that this geothermal gradient was established for depths shallower than 120 m CSF-A.

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