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doi:10.2204/iodp.proc.341.104.2014

Physical properties

Physical properties measurements were taken on each of the six holes drilled at Site U1418 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 core sections longer than ~30 cm were measured with 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 after ~4 h, GRA bulk density and magnetic susceptibility were measured with the WRMSL at 2.5 cm intervals with 5 s measurements. Core disturbance, due to gas expansion between ~100 and ~200 m CCSF-B, complicated the correlation between holes and compromised the collection of core logger compressional wave (P-wave) velocity data. Because of this, P-wave velocity (measured on the P-wave logger [PWL]) was only partially measured in Holes U1418A–U1418C within the upper ~200 m CSF-A. 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. Color spectrometry, color reflectance, and magnetic susceptibility were measured on the split cores using the SHMSL at 2.5 cm resolution. Discrete P-wave and shear strength measurements were made on the working-half sections of split sediment cores from Holes U1418A, U1418B, U1418D, and U1418F on the Section Half Measurement Gantry. Moisture and density (MAD) were measured on 10 cm³ plugs collected from the working halves of Holes U1418A, U1418D, and U1418F. Summaries of all physical properties measured with the multisensor loggers on each hole (excluding Hole U1418B), as well as discrete bulk density and P-wave velocity, are provided in Figures F30, F31, F32, F33, and F34.

Gamma ray attenuation bulk density

Variations in GRA bulk density can reflect changes in mineralogy/lithology, consolidation, and porosity. Whole-round GRA bulk density averages ~1.96 g/cm³ in the APC cores and displays downhole cyclic variability on the order of ~0.4 g/cm³ (Figs. F30, F31, F32, F33, F34). A trend toward higher values with depth in the APC cores is consistent with increasing sediment compaction/consolidation. However, an abrupt decrease in whole-round GRA bulk density values corresponds to the depth at which we transitioned from APC core collection to XCB in Hole U1418D (~250 m CSF-A; Fig. F32). Furthermore, GRA bulk density measured on cores retrieved with RCB coring is generally lower than the GRA bulk density of sequences retrieved with APC (see WRMSL GRA bulk density deeper than ~250 m CCSF-B on Fig. F35). As outlined in “Physical properties” in the “Site U1417” chapter (Jaeger et al., 2014b), the relatively low apparent WRMSL wet bulk density partially reflects incomplete core recovery associated with the XCB and RCB coring methods, as the width of the recovered sediments fails to completely fill the core liner and the GRA bulk density measurements are therefore calibrated for a larger sediment volume than is actually contained in the XCB and RCB cores.

Magnetic susceptibility

As outlined in “Physical Properties” in the “Site U1417” chapter (Jaeger et al., 2014b), to facilitate comparison both the point-source and WRMSL loop magnetic susceptibility data sets were smoothed with a Gaussian filter of 10 cm (±3σ) and then interpolated to constant 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 1.58× greater than the point-source measurements in the Site U1418 APC splice (Fig. F36). 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., 2014a]).

A decrease in WRMSL loop magnetic susceptibility and an increase in the difference between loop and point-source magnetic susceptibility deeper than ~250 m CCSF-B (Fig. F35) are associated with the transition to XCB/RCB 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 the 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, as observed in “Physical properties” in the “Site U1417” chapter (Jaeger et al., 2014b) (Fig. F35). After smoothing with a Gaussian filter of 10 cm (±3σ) to correct for the differing response function of the instruments, we can use this relationship to calculate the volume-corrected, or mass, magnetic susceptibility (χ) with units of cubic centimeter per gram (Fig. F35). Similar to the observations at Site U1417, there is a reduction in variance of χ of ~25% relative to the uncorrected magnetic susceptibility record normalized by the mean core GRA bulk density, attributable to the effects of variable sediment volume. Mass magnetic susceptibility averages ~50 cm³/g downhole at the site (Fig. F35). High-amplitude cyclic variability between 60 and 120 cm³/g is present at the site shallower than ~250 m CCSF-B, transitioning to reduced variability centered at ~40 cm³/g in the deeper sections. This corresponds with the approximate depth of APC refusal and may reflect a concurrent change in lithology and/or incomplete volumetric correction of the WRMSL volume magnetic susceptibility data. A return to higher magnetic susceptibility of >60 cm³/g deeper than 810 m CCSF-B may reflect the transition to a mass transport deposit, identified and described in “Lithostratigraphy.”

Compressional wave velocity

P-wave velocity was measured on the WRMSL PWL in Holes U1418A–U1418C (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014a]). In Holes U1418A and U1418C, velocities were measured at 0–200 m CSF-A, with reasonable measurements as deep as 110 m CSF-A (Fig. F37). Between ~110 and 180 m CSF-A, gas expansion prevented accurate P-wave measurement on the PWL because of the development of void spaces that decoupled the core material from the liner. This is an interval of elevated methane content (reaching 40,000 ppmv; see “Geochemistry”). WRMSL P-wave velocity values gradually increase downhole, tracking GRA bulk densities (Figs. F30, F31), ranging from ~1450 m/s at the seafloor to ~1650 m/s at ~200 m CSF-A (Fig. F37). After switching to XCB coring in Hole U1418D, we halted PWL measurements because of inaccuracies caused by the resulting void spaces in the core.

Discrete P-wave measurements using the P-wave caliper (PWC) tool (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014a]) were taken in Holes U1418A, U1418C, U1418D, and U1418F. Discrete measurements from Holes U1418A and U1418C overlapped PWL measurements for depths shallower than ~200 m CCSF-B (Fig. F38). PWC values were automatically picked where possible and manually picked when the automatic picker encountered errors because the calipers did not have sufficient contact with the sample because of very soft sediment or bad coupling with the liner. The PWC values appear systematically slower than the PWL measurements. A scatter plot shows that the two measurements are positively correlated but tend toward slower PWC values (Fig. F38). 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; however, all discrete measurements at this site were taken within the dominant lithology of the recovered interval. Measured PWL velocities show increased scatter in the interval between 110 and 180 m CCSF-B, an effect probably associated with gas expansion in these cores that causes the core material to become decoupled from the liner (see “Geochemistry”). Deeper than ~200 m CCSF-B, the range of values narrows and slightly increases with depth to ~560 m CCSF-B, where there is an ~20 m/s shift in the value range to ~1700 m/s (Fig. F39). There are inflection points to higher rates of change in velocity at ~700 m CCSF-B (~1750 m/s) and ~800 m CCSF-B (>1800 m/s), with P-wave velocities reaching >2000 m/s at the bottom of the site at ~900 m CCSF-B.

Natural gamma radiation

NGR measurements were analyzed 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., 2014a]). 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 cyclical downcore fluctuations between 16 and 45 counts per second (cps) with a mean and standard deviation of 33 and 3, respectively (Fig. F40). High-frequency variations in NGR values are likely coupled with changes in clay lithologies and consequently parallel trends in GRA bulk density and P-wave velocity from the core logger measurements, particularly shallower than 230 m CCSF-B in Holes U1418A and U1418C.

The low-frequency variability in NGR is roughly characterized by a downhole increase between 0 and 260 m CCSF-B in Holes U1418A–U1418D, although interpretation is complicated beyond this point by a decrease in recovered sediment diameter within the core liner (which depresses raw NGR counts) associated with the transition to XCB and RCB recovery. As in the case of WRMSL magnetic susceptibility, we can calculate an equivalent activity of the sediment by normalizing to the raw GRA bulk density after smoothing the data sets with a Gaussian filter of 50 cm (±3σ) to accommodate for the varying response functions of the instruments. Although this treatment reduces the overall variance in the NGR record by ~20% relative to the Gaussian-smoothed uncorrected NGR data normalized by the mean core GRA bulk density, the decrease in NGR observed deeper than ~200 m CCSF-B persists in reduced form. As in the case of magnetic susceptibility, this difference may reflect a change in lithology coincident with APC refusal and/or imperfect volume correction (Fig. F40). A transition to lower volume-normalized NGR values deeper than 810 m CCSF-B corresponds to lithologic changes at the Unit II/III boundary, as described in “Lithostratigraphy.”

Moisture and density

Bulk density values in Holes U1418A, U1418D, and U1418F were calculated from mass and volume measurements on discrete samples taken from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014a]). Depending on core recovery, quality, and lithology, one to three samples were taken per core. A total of 298 samples was analyzed for MAD: 80 samples from Hole U1418A, 36 samples from Hole U1418D, and 182 samples from Hole U1418F.

MAD values correspond well with WRMSL GRA bulk densities in Holes U1418A and U1418D. MAD densities increase downhole from ~1.5–1.8 g/cm³ at the seafloor to ~1.8–2.0 g/cm³ at ~320 m CCSF-B. Deeper than ~320 m CCSF-B, density continues to increase at a slower rate, reaching 2.2 g/cm³ by ~900 m CCSF-B. Deeper than ~900 m CCSF-B, the densities become more variable and increase to ~2.2–2.4 g/cm³ (Fig. F41).

Bulk grain density was observed to be the product of normal consolidation at ~300 m CCSF-B (Fig. F42). Between the seafloor and ~260 m CCSF-B, grain density values are fairly constant, at ~2.8 g/cm³, with a small number of higher density intervals of between ~3.2 and 3.6 g/cm³. Between ~260 and 400 m CCSF-B, a secondary population of lithologies with lower bulk grain densities of ~2.6 g/cm³ is present. Deeper than 400 m CCSF-B, scatter increases, with bulk grain density values ranging from ~2.5 to ~3.0 g/cm³.

Porosity (percent pore space of wet sediment volume) measured on discrete samples decreases with depth, showing a normal consolidation curve. Discrete porosity values decrease rapidly from >60% at the surface to ~40%–44% at ~70 m CCSF-B and then continue a more gradual decrease to ~35% at ~910 m CCSF-B (Fig. F42).

Shear strength

Shear strength measurements were performed on working section halves from Holes U1418A and U1418D using the automated vane shear testing system (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014a]). Efforts were taken 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 and PWC measurements.

Shear strength indicates that sediments range from very soft (0–20 kPa) to very stiff (120–180 kPa). Generally, the rate of change of shear strength with depth is constant downhole. Values drop slightly off the trend between ~100 and ~130 m CCSF-B. These cores are within the interval of high methane gas levels (see “Geochemistry”), and values are slightly more scattered deeper than ~150 m CCSF-B (Fig. F43). All samples were taken in the dominant lithology of dark gray–greenish mud, so it is unlikely that the increasing variance 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. This effect is especially apparent in the low values measured deeper than ~280 m CCSF-B. Shear strength measurements were halted at Core 341-U1418D-37X (~297 m CCSF-B) 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 U1418A. Four temperature measurements were taken in Hole U1418A (Fig. F44), and a geothermal gradient was successfully obtained (Cores 341-U1418A-4H, 7H, 10H, and 13H) within the depth interval of 33.3–118.4 m CSF-A. The best-fit line of the temperature with depth measurements is shown in Figure F44B:

where T(z) is in situ temperature at depth z (meters CSF-A). The estimated geothermal gradient is therefore 59°C/km. Again, note that this geothermal gradient was established for the depth interval shallower than 118 m CSF-A.

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