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doi:10.2204/iodp.proc.341.107.2014

Physical properties

Physical properties measurements were taken at Site U1421 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 through the GRA bulk densitometer and magnetic susceptibility loop on the STMSL at 5.0 cm intervals with 2 s measurements. GRA bulk density and magnetic susceptibility were measured with the WRMSL at 2.5 cm intervals with 5 s measurements. Compressional wave (P-wave) velocity was also measured on the WRMSL track for all APC cores at this site. After WRMSL scanning, the 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 measurements on the working half sections of Hole U1421A include P-wave and shear strength using the Section Half Measurement Gantry. Moisture and density (MAD) were also measured on 10 cm³ plugs collected from the working halves from Hole U1421A. No discrete measurements were performed on samples from Holes U1421B and U1421C. Summaries of all the physical properties measured from Holes U1421A and U1421C are provided in Figures F12 and F21, respectively.

Gamma ray attenuation bulk density

Variations in GRA bulk density in the recovered sections of Site U1421 likely reflect changes in mineralogy/lithology, consolidation, and porosity, which are overprinted by variable core recovery. Whole-round GRA bulk density averages ~2.0 g/cm³ at the site, with the lowest values occurring in the uppermost 6 m and large variability downcore (Figs. F12, F21). APC cores from the uppermost portion of the site were largely full to the width of the liner and hence are likely to produce approximately calibrated density. However, as in the other sites, in the reduced-diameter XCB core sections of Hole U1421A the absolute WRMSL bulk density values reflect a minimum limit.

Magnetic susceptibility

We evaluate the consistency between the WRMSL loop magnetic susceptibility meter and the SHMSL point-source magnetic susceptibility meter in the APC splice portion of Site U1421, where full core liners should limit the effect of sediment volume on the loop sensors. Both data sets were smoothed using a Gaussian filter of 10 cm (±3σ) and interpolated to 2.5 cm resolution to accommodate for the differing response functions of the instruments (see “Physical properties” in the “Methods” chapter (Jaeger et al., 2014), prior to comparison. We found an offset in the relative magnitude of the measurements, with loop magnetic susceptibility being an average of ~1.45× greater than that of the point-source data from the Site U1421 composite splice (Fig. F22). In Hole U1421A XCB cores, the reduced core diameter is such that the relationship between loop magnetic susceptibility and point-source magnetic susceptibility is likely to be driven by the volume of sediment in the WRMSL measurement window and cannot be expected to meaningfully reflect instrument reproducibility. As in the case of GRA bulk density, raw WRMSL magnetic susceptibility values should be regarded as a lower limit of sedimentary volumetric magnetic susceptibility.

We normalized loop magnetic susceptibility for changes in sediment recovery and compaction by dividing the volumetric mass susceptibility by the GRA bulk density after smoothing both data sets using a Gaussian filter of 10 cm (±3σ) to correct for the differing response functions of the instruments. This generates a mass magnetic susceptibility (χ) with units of cubic centimeters per gram (Fig. F23). Unlike at the other sites, this normalization appeared to increase the variance of the magnetic susceptibility data; however, this effect was attributed to a single event of up to 3000 IU in the uncorrected WRMSL data spanning 5 cm between 637.175 and 637.225 m CCSF-A, which was magnified in the volume-normalized data. If this event is excluded from the calculation of variance, the reduction in the normalized data set is ~10% relative to the uncorrected magnetic susceptibility data, similar to the effect of normalization on magnetic susceptibility variance at the other expedition sites. Mass magnetic susceptibility averages ~65.5 cm³/g downhole at the site (Fig. F23). The record is characterized by a low-amplitude oscillation of ~100 m in depth with superimposed periods of high variability. The lowest magnetic susceptibility of the recovered record is in the uppermost portion of the core, with an abrupt transition to higher values at ~6 m CCSF-A. Variance is highest in the deepest 300 m of the cores and may reflect the presence of abundant ice-rafted debris clasts.

Compressional wave velocity

P-wave velocity was measured on the WRMSL P-wave logger (PWL) on all APC cores from Holes U1421A and U1421B. We halted PWL measurements in Hole U1421A when we switched to XCB coring with Core 341-U1421A-19H (~95 m CSF-A). PWL values range from ~1500 m/s at the seafloor to ~2000 m/s at ~95 m CSF-A, generally increasing downhole. However, this trend is not strong, and there is increasing scatter deeper than ~60 m CSF-A (Fig. F12). PWL values show velocity peaks at ~15 and ~65 m CSF-A, each associated with intervals of high density.

P-wave measurements using the P-wave caliper (PWC) tool were taken at Site U1421 when core recovery allowed. Because of poor core recovery coinciding with the switch to XCB coring, few PWC measurements were taken deeper than ~95 m CSF-A. Much of the recovered material consists of clast-rich or clast-poor diamict (see “Lithostratigraphy”), the nature of which produces uneven contact with the P-wave calipers, so velocity measurements could not be obtained with regularity. 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 abundant clasts, soft sediment, or bad coupling with the liner. PWC values show no significant overall trend with depth and vary widely, sometimes within the same section, likely because of the varying amounts and lithology of clasts. Though values generally range from ~1500 to ~2000 m/s, occasional measurements of velocities >2200 m/s were also observed (Fig. F12). All discrete measurements at this site were taken within the dominant matrix lithology of the recovered interval.

Comparison of PWL and PWC measurements in the APC interval of Hole U1421A reveals that the PWC measurements are generally lower than the PWL values. A scatter plot shows the mismatch between the values, with a slope of ~0.41 (Fig. F24). Postcruise analysis is required to investigate the velocity discrepancy. Comparison of the PWC and PWL velocity values with the sonic log and vertical seismic profile (VSP) (see “Downhole logging”) should be undertaken before using the data in any interpretations or analysis.

Natural gamma radiation

NGR was measured at 10 cm intervals on all whole-round core sections that exceeded 50 cm in length. NGR measurements show cyclic downcore fluctuations between 14 and 41 counts per second (cps) with a mean and standard deviation of 32 and 5, respectively. Downhole variations in raw NGR values are influenced by changes in porosity and core recovery volume and, consequently, parallel changes in GRA bulk density.

As with WRMSL magnetic susceptibility, we calculate an equivalent activity of the sediment by normalizing to the WRMSL 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. This normalization by GRA bulk density reduces downcore variance in NGR by ~35%. As NGR can only be collected on sections longer than 50 cm, low recovery limits our interpretation of the portion of the NGR record deeper than ~100 m CCSF-A (Fig. F25).

Moisture and density

Bulk density values in Hole U1421A 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., 2014]). Depending on core recovery, quality, and lithology, one to three samples were taken per core. A total of 88 samples were analyzed for MAD.

Discrete bulk densities fit well with GRA bulk densities between ~0 and 100 m CSF-A (Fig. F8). Between ~0 and 25 m CSF-A, density increases from ~1.7 to ~2.1–2.2 g/cm³, approximately following an exponential curve. The interval between ~26 and 45 m CSF-A was not recovered. Deeper than ~45 m CSF-A, density values generally range from ~2.1 to 2.4 g/cm³, except for two lower density samples taken from muddier sediments (See “Lithostratigraphy”). In the intervals between ~290 and 310 m CSF-A and between ~450 and 500 m CSF-A, densities display a wide range from ~1.8 to 2.6 g/cm³ and show no discernible downhole trend. Generally, intervals of mud and diatom ooze have lower densities than intervals of diamict. Clast-rich diamict displays increased scatter relative to clast-poor diamict (Fig. F8). Bulk grain densities display considerable scatter, ranging from 2.7 to 3.1 g/cm³ (Fig. F26).

Porosity measured on discrete samples displays large ranges of ~24%–26% to ~54%–56% from ~280 to 650 m CSF-A. These ranges do not appear to correspond with lithostratigraphic trends or patterns (Fig. F26).

Shear strength

Shear strength measurements were performed on working section halves from Hole U1421A using the automated vane shear testing system (see “Physical properties” in the “Methods” chapter [Jaeger et al., 2014]). 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. We obtained shear strength measurements on all APC cores (to ~95 m CSF-A) and some XCB cores to ~490 m CSF-A. Shear strength measurements were halted when the recovered material was hardened enough to crack upon penetration of the automated vane. Shear strength measurements indicate that sediments within the shallowest ~25 m CSF-A are soft (~5–20 kPa). At ~45 m CSF-A, shear strength values show increasing scatter and do not display a strong trend with increasing depth. We observe two intervals of high shear strength (>60 kPa) at ~47 and ~60 m CSF-A that coincide with increased density and NGR (Fig. F12). All samples reflect the dominant lithology of the recovered sediment.

Heat flow

Temperature measurements were conducted using the APCT-3 during APC coring in Hole U1421C. Although four measurements were planned, only two were actually acquired because of operational difficulties caused by large lonestones. Two temperature measurements were taken in total (Fig. F27A), and a geothermal gradient was obtained from these two (Cores 341-U1421B-4H and 6H) within the depth interval of 29.0–38.2 m CSF-A. We display the best-fit trendline to determine temperature gradient (Fig. F27B):

where T(z) is in situ temperature at depth z (m CSF-A). The estimated geothermal gradient is 20.6°C/km.

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