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doi:10.2204/iodp.proc.340.109.2013

Physical properties

Magnetic susceptibility generated good correlation between Holes U1399B with U1399A to 180 mbsf. Analysis of the correlation suggests no significant differences in lithology between the two holes. P-wave velocities show a significant amount of scatter over short (meter scale) depth intervals but in general increase slightly with depth. Both density and magnetic susceptibility increase where higher concentrations of sand exist in sediment. Shear strength generally increases with depth in both holes, with higher values typically measured with the handheld penetrometer. The measured temperature gradient in the upper 81 m is 65.4° ± 0.9°C/km. There is no evidence in the temperature measurements for either vertical or horizontal fluid flow.

Stratigraphic correlation between Holes U1399B and U1399A

Good correlation exists between Holes U1399A and U1399B. We used magnetic susceptibility to correlate depths between the two holes (Figs. F5). We trimmed 5.1 cm off each end of the core sections in the magnetic susceptibility data to minimize edge effects during correlation. Hole U1399A was the reference hole for these correlations because it has the longest continuous record. Both holes have good, continuous recovery down to ~180 mbsf. Unlike previous sites that had limited core recovery, high core recovery at Site U1399 resulted in clear correlation markers with depth. Correlation for the uppermost 20 m is strong, with only a marginal shift in depth between the holes. At depths below 20 mbsf, however, significant stretching is necessary for Hole U1399B to match observations in Hole U1399A. Despite this stretching, correlation between holes is still very good, suggesting that sedimentary units in each of the holes are similar at these depths but several meters thicker in Hole U1399A. In particular, the 30–60 mbsf interval required significant stretching (in excess of 10–20 m) for the best correlation; however, as Figure F5 demonstrates, a strong correlation exists between sites when we apply this stretching. A similar amount of stretching is also implied from analysis of stratigraphic logs, which further supports the significant offset in correlation picks. Because of this, we were able to pick correlation points to 180 mbsf. Our correlation coefficient using magnetic susceptibility data is 0.66, with the poorest correlation at 30–60 and 150–180 mbsf, where the most significant stretching of Hole U1399B occurs. Where we make correlations, depth shifts for Hole U1399B are primarily negative (downward) and generally exceed 10 m at all depths below 20 mbsf. All picked correlation depth shifts are shown in Table T4.

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

Magnetic susceptibility shows background values <1000 × 10^–5 SI with spikes that are correlated with increased density and the presence of coarse-grained volcaniclastic material. Gamma ray attenuation (GRA) density shows significant scatter in both holes but averages ~1.7 g/cm³. The significant scatter in GRA density makes it very difficult to recognize a detailed trend; in general, bulk density increases with depth. A sharp drop in GRA density is observed near the base of the proposed slide at ~150 mbsf.

Hemipelagic sediment shows high values (>15 cps; maximum = 50 cps) of natural gamma radiation (NGR) in Holes U1399A and U1399B. In contrast, volcaniclastic sediment has comparatively low (<15 cps) values. P-wave velocity measurements from the Whole-Round Multisensor Logger (WRMSL) show a significant degree of scatter, making it very difficult to ascertain clear velocity trends throughout both holes. P-wave velocities vary by 20% over short (meter scale) intervals. In general, P-wave velocity increases slightly with depth (Fig. F6).

Shear strength

Measurements of undrained shear strength (S_u) were successfully performed on fine-grained sediment in Holes U1399A and U1399B with the fall cone, automated vane shear (AVS), and handheld penetrometer (Fig. F6). As observed at Site U1398, S_u measurements from the handheld penetrometer are higher than those obtained from the AVS and fall cone, which suggests that S_u measured after core splitting is likely underestimated compared to conditions in the core liner immediately after core recovery.

Another divergence in these measurements is that the fall cone provides lower S_u measurements than the AVS. These differences are mostly found below 60 mbsf in Hole U1399A and below 47 mbsf in Hole U1399B (Fig. F7). This discrepancy is due to the presence of lower remolded shear strength (represented by fall cone measurements) than peak strengths (corresponding to AVS measurements). The ratio between peak and remolded strength, known as sensitivity (S_t), is ~3, but it can be >4 in both holes (Fig. F8). Therefore, fall cone measurements below 60 mbsf in Hole U1399A and 47 mbsf in Hole U1399B provide a lower bound of shear strength.

S_u measurements performed with the handheld penetrometer and AVS increase downhole in both holes, from ~10 kPa at the top to ~100 kPa at 90 mbsf in Hole U1399A and 75 mbsf in Hole U1399B. Below 60 and 47 mbsf in Holes U1399A and U1399B, respectively, we have fewer measurements because of the increase of coarser materials.

Low values of S_u relative to the trend of S_u increasing with depth are identified in some deformed hemipelagic sediment. The most significant anomaly is found at 140 mbsf (S_u < 60 kPa, in a highly deformed hemipelagic interval) in Hole U1399A and at 125 mbsf (S_u < 60 kPa) in Hole U1399B. Other anomalies result from local deformation or the presence of softer hemipelagic intervals, as at 95 mbsf in Hole U1399A (S_u < 20 kPa in a mud clast within a debritic matrix, according to core description) and 157 mbsf (S_u < 80 kPa, undeformed sediment, according to visual core description) in Hole U1399B. The decreases in S_u values at 140 mbsf in Hole U1399A and at 125 mbsf in Hole U1399B may result from a major shear surface (Fig. F7).

No S_u measurements are available between 140 and 150 mbsf in Hole U1399A (the interval is characterized by coarser sediment). At greater depths, S_u measurements were only performed with the fall cone in Hole U1399A (150–185 mbsf) because the sediment was too firm for the handheld penetrometer and AVS to make measurements. The fact that S_u measurements performed by the fall cone are ~50 kPa and do not increase below 150 mbsf, as expected in firm sediment, implies that the sediment is more sensitive than that at more shallow depths.

P-wave velocity

P-wave velocity values measured on the x-axis using the caliper match the P-wave logger (PWL) values, which increase downhole. Velocities are correlated with sediment composition: average velocities in hemipelagic mud are generally lower than in volcaniclastic sediment.

Moisture and density

We collected 60 moisture and density (MAD) measurements (45 from Hole U1399A, including 9 volcaniclastic sand samples, and 15 from Hole U1399B, including 4 clay samples; Fig. F6). The four clay samples match all MAD characteristics of the hemipelagic sediment.

Porosity of 47 hemipelagic sediments ranges between 53% and 73%. Volcanic sand samples have porosities between 40% and 50%. However, pumice-rich sand at 179.1 mbsf and finely bedded sand at 267.2 mbsf have porosities of ~60%. As at other sites, the porosity of loose sand may be underestimated as much as 20% due to draining of pore water during coring, splitting, and MAD sampling. 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 range between 1.5 and 1.8 g/cm³. Sandy samples have bulk densities as high as 2.05 g/cm³. Grain density of hemipelagic samples has a narrow range between 2.5 and 2.8 g/cm³. The volcanic sand has a grain density similar to that of hemipelagic sediment.

Thermal conductivity

Thermal conductivity was measured at 103 depths on recovered whole-round sections. Measured thermal conductivity had a mean value of 1.045 W/(m·K), with a standard deviation of 0.080 W/(m·K) and a standard error on the mean of 0.008 W/(m·K).

Downhole temperature

Temperature was measured with the APCT-3 at the bottom of Cores 340-U1399A-3H, 4H, 5H, and 6H (24.1, 33.6, 43.1, and 52.6 mbsf, respectively) and Cores 340-U1399B-3H, 6H, and 9H (24.7, 53.2, and 81.2 mbsf, respectively). Downhole temperature was monitored for 650, 638, 637, 825, 796, 760, and 686 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-U1399A-3H, 4H, 5H, and 6H we obtained temperatures of 5.91° ± 0.05°C, 6.56° ± 0.01°C, 7.09° ± 0.02°C, and 7.82° ± 0.04°C, respectively. At the base of Cores 340-U1399B-3H, 6H, and 9H we obtained temperatures of 5.96° ± 0.02°C, 7.72° ± 0.03°C, and 9.60° ± 0.03°C, respectively. Uncertainties are similar to the error on the best-fit solution and the probe’s measurement accuracy. The temperature of ocean water at the seafloor is 4.25°C.

A best-fit linear relationship between depth and our eight temperature measurements gives a temperature gradient of 65.4° ± 0.9°C/km (Fig. F9). Using the measured thermal conductivity the implied heat flow is 68 ± 1 mW/m². Bathymetry will not affect the near surface heat flow; however, the high sedimentation rate may reduce the measured heat flow by up to 4% (Manga et al., 2012). Despite a horizontal separation of 410 m between Holes U1399A and U1399B, all temperature measurements lie on a single line. This implies that fluid flow is not disturbing the temperature gradient.

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