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doi:10.2204/iodp.proc.340.110.2013

Physical properties

Good correlation exists between Holes U1400C and U1400B to ~200 mbsf. Analysis of the correlation suggests Hole U1400C has consistently thicker bedding than Hole U1400B. High magnetic susceptibility, P-wave velocity, and density values in the upper 20 mbsf correlate with higher sandy, volcanic content. P-wave velocity generally increases with depth, with the most significant variability in the uppermost 50 mbsf where we encountered coarse-grained volcaniclastic material. Shear strength measurements show significant variability with values generally increasing with depth. Temperature increases linearly with increasing depth in the uppermost 103 m with a temperature gradient of 52.9° ± 1.6°C/km.

Stratigraphic correlation between Holes U1400C and U1400B

Hole U1400A is shallow and distant from Holes U1400C and U1400B; we therefore only tied Holes U1400C and U1400B. We used both magnetic susceptibility and natural gamma radiation (NGR) data to correlate depths between the two holes (Fig. F8). We trimmed 5.1 cm off each end of core sections in the magnetic susceptibility data and 10 cm off each core section end in the NGR data to minimize edge effects during correlation. High core recovery in both holes resulted in robust correlation markers using both magnetic susceptibility and NGR. Hole U1400B was the reference hole for these correlations because it has the longest continuous record; however, both holes have good, continuous recovery to ~230 mbsf. Except for the upper tens of meters, all correlation shifts for Hole U1400C are positive (upward). Several good correlation tie points exist between Holes U1400B and U1400C. Some of the tie points deeper than ~200 mbsf are offset by >20 m. This offset is unexpected because hole separation is ~20 m. Caution must therefore be used in the interpretation of deeper correlations. The most significant stretch between these holes occurs at 30–50 mbsf. At these depths, Hole U1400C requires a ~10 m upward shift for good correlation between holes. Below ~50 mbsf, Hole U1400C correlates well with Hole U1400B, and we applied only small upward shifts below 50 mbsf. Analysis therefore suggests that below 50 mbsf Hole U1400C has consistently thicker bedding than that found in Hole U1400B. We picked correlation points to ~200 mbsf with confidence. Below this depth, data are sparse and correlation picks are less reliable. Our correlation coefficient using both magnetic susceptibility data and NGR is 0.58. All picked correlation depth shifts are shown in Table T4.

Gamma ray attenuation density, magnetic susceptibility, and P-wave velocity

Changes in magnetic susceptibility correlate with lithologic changes. Where volcaniclastic sediment exists, magnetic susceptibility is high. In Hole U1400B, magnetic susceptibility reaches maximum values of 2500 × 10^–5 SI between 0 and 25 mbsf and at 100 mbsf. In Hole U1400C, maximum values (~3000 × 10^–5 SI) occur at 100 mbsf. At all locations where magnetic susceptibility values are high we see evidence for sandy volcanic turbidites. In hemipelagic sediment, magnetic susceptibility values remain low (mostly <400 × 10^–5 SI). Bulk density also correlates with the cored material, showing high values (2.2 g/cm³) in dense volcanic sands and low values (<1.7 g/cm³) in hemipelagic sediment.

We measured higher P-wave velocities in volcaniclastic sediment (typically ~1750 m/s) in the upper 50 m of the holes and lower P-wave velocities in hemipelagic sediment. Where hemipelagic sediment dominates, P-wave velocity increases steadily with increasing depth.

Shear strength

Because of abundant sand in Hole U1400A, undrained shear strength (S_u) was measured discontinuously in fine-grained intervals within the uppermost 26 m (Cores 340-U1400A-1H through 5H) with the automated vane shear (AVS) and fall cone. S_u increases ~1 kPa/m with increasing depth.

In Holes U1400B and U1400C, S_u measurements were performed with the handheld penetrometer, fall cone, and AVS. Between 25 and 70 mbsf, S_u measurements with the handheld penetrometer and AVS increase ~4 kPa/m downhole, whereas S_u measurements with the fall cone are lower and increase ~1 kPa/m downhole. As a consequence, S_u values provided by the AVS are four times higher than those measured with the fall cone. The presence of sandy intervals between 70 and 105 mbsf leads to scattered S_u data and a relative decrease of S_u in the interbedded fine-grained sediment (as much as 50% for S_u from the handheld penetrometer). This decrease in S_u is in agreement with a relative decrease in GRA density and P-wave velocity.

Below 105 mbsf, S_u measurements from the handheld penetrometer exceed the measurement limit of the tool, indicating that shear strength is >220 kPa. Nevertheless, this increase in strength is not observed in fall cone measurements, which found that the residual strength of the sediment is constant downhole, with a mean value of ~40 kPa to 210 mbsf.

Sediment expansion observed during core cutting on the catwalk (Holes U1400B and U1400C) suggests that decompression reduces the S_u of the sediment substantially after retrieval. Measurements performed with the AVS and fall cone may therefore underestimate in situ conditions. On the other hand, S_u values provided by the handheld penetrometer are generally higher than those with the AVS and fall cone, with the fall cone providing the lowest S_u.

P-wave velocity

Discrete measurements of compressional wave velocity in hemipelagic mud match the general trend in values from the P-wave logger (PWL). Volcaniclastic sediment shows much higher velocities, ranging from 1700 to 1840 m/s. In Hole U1400C, from 210 to >400 mbsf, P-wave velocities in hemipelagic mud increase from 1650 to 1750 m/s and follow the PWL values.

Moisture and density

We collected 88 moisture and density (MAD) measurements (10 from Hole U1400A, 47 from Hole U1400B, and 38 from Hole U1400C), including 71 samples of hemipelagic sediment and 17 samples of volcaniclastic sand (turbidites).

Porosity of hemipelagic samples ranges between 51.5% and 73% (Figs. F9, F10). Like Site U1399, Site U1400 shows a weak negative porosity-depth correlation from ~66% at the mudline to ~60% at 427 mbsf. Volcanic sand samples have porosities between 36% and 51%. As at previous sites, porosity of loose sands may be underestimated because of draining of pore water during sampling or overestimated because of sediment reworking during core recovery. Alternatively, where core recovery, handling, or splitting processes reorganize sand grains, sandy sediment may become undercompacted and yield anomalously high porosities.

Bulk density of hemipelagic samples ranges between 1.5 and 1.82 g/cm³. Bulk density of sandy samples is as high as 2.18 g/cm³, quite distinct from the densities of hemipelagic samples. Grain density of hemipelagic samples typically ranges between 2.53 and 2.77 g/cm³. Volcanic sand has grain density between 2.74 and 2.87 g/cm³, somewhat higher compared to that at Site U1399.

Thermal conductivity

Thermal conductivity was measured at 161 depths on recovered whole-round sections from all three holes. The measured thermal conductivity averaged 1.050 W/(m·K), with a standard deviation of 0.075 W/(m·K) and a standard error on the mean of 0.006 W/(m·K). There is a small but statistically significant increase of thermal conductivity with increasing depth (Manga et al., 2012).

Downhole temperature

Temperature was measured with the APCT-3 at the bottom of Cores 340-U1400C-5H, 8H, and 12H (48.3, 76.1, and 103.3 mbsf, respectively). Downhole temperature was monitored for 642, 645, and 2392 s, respectively. Temperature was calculated from these time series of temperature measurements using TP-Fit (see APCT-3 user manual on the Cumulus/Techdoc database at iodp.tamu.edu/​tasapps/). We assume a thermal conductivity (k) of 1.0 W/(m·K) and ρC = 3.7 × 10⁶ J/m³K. To calculate uncertainty, we assume k ranges from 0.9 to 1.1 W/(m·K) and ρC is between 3.2 × 10⁶ and 4.0 × 10⁶ J/m³K. At the base of Cores 340-U1400C-5H, 8H, and 12H, we obtained temperatures of 6.94° ± 0.03°C, 8.45° ± 0.03°C, and 9.68° ± 0.09°C, respectively. The uncertainties are greater than the error on the best-fit solution and the probe’s measurement accuracy. The temperature of ocean water at the seafloor was 4.26°C.

A best-fit linear relationship between depth and our four temperature measurements gives a temperature gradient of 52.9° ± 1.6°C/km (Fig. F11). Using the measured thermal conductivity the implied heat flow is 56 ± 4 mW/m². The near-surface heat flow at this site, if conductive, is reduced 2% owing to bathymetry and as much as 4% because of sedimentation (Manga et al., 2012). There is no evidence for fluid flow affecting temperature. The linear temperature gradient also requires that mass movement occurred >10⁴ y ago so that the temperature disturbance from emplacement has decayed (Harris et al., 2011).

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