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doi:10.2204/iodp.proc.340.106.2013

Physical properties

We collected physical properties data between 0 and 140 mbsf at Holes U1396A and U1396C. We also collected physical properties data between ~5 and 15 mbsf at Hole U1396B to fill a gap in the data from Hole U1396A. Good correlation exists between holes. Magnetic susceptibility provides the most valuable tool for differentiating stratigraphic changes at this site. P-wave and density measurements are consistent but show little variability with depth. Porosity measurements in sandy sediment are anomalously high, possibly an artifact of the core recovery process. All shear strength measurements increase monotonically with depth, with a few anomalous values. Temperature increases linearly with depth, with a temperature gradient of 69.3° ± 1.5°C/km. There is no evidence for fluid advection disturbing the temperature measurements.

Stratigraphic correlation between Holes U1396A and U1396C

We used magnetic susceptibility to correlate depths between Holes U1396A, U1396B, and U1396C (Figs. F7, F8). Hole U1396C is the reference hole for these correlations because it has the longest continuous record. Core 340-U1396A-2H shattered during coring, and although much of the sediment from the core was recovered, placed into core barrels on deck, and scanned on the Whole-Round Multisensor Logger (WRMSL), there is a chance that a significant amount of the material was disturbed during this process. We therefore removed all WRMSL data for Core 340-U1396A-2H from the correlation analysis. Core 340-U1396B-2H covers most of the depth range missing from Hole U1396A for correlation. By correlating both Holes U1396A and U1396B to Hole U1396C, we were able to make a nearly complete correlation at Site U1396. Core 340-U1396B-1H had all depth values set to 5 mbsf when we downloaded the data (this has since been changed); therefore, we applied a linear interpolation for Core 340-U1396B-1H to correlate this upper core with Hole U1396C. Holes U1396A and U1396B correlate well with Hole U1396C, with correlation coefficients of 0.75 and 0.79, respectively (Figs. F7, F8). Depth shifts for all correlation corrections for Holes U1396A and U1396B never exceeded 3 m and rarely exceeded 1 m. In the uppermost 7 m, we see clear correlation of turbidite units between the two holes. All correlation pick depth shifts are shown in Tables T4 and T5.

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

Magnetic susceptibility identifies volcaniclastic layers embedded in the hemipelagic background. Natural gamma radiation (NGR) shows low-amplitude and low-frequency variations with depth on a wavelength of one to two core lengths. NGR values are generally anticorrelated with magnetic susceptibility anomalies. Gamma ray attenuation (GRA) density does not have any systematic trend or correlation with volcaniclastic layers.

Thermal conductivity

Thermal conductivity was measured on 50 sections. The mean value was 1.041 W/(m·K) with a standard deviation of 0.079 W/(m·K) and a standard error of the mean of 0.010 W/(m·K).

Shear strength

Handheld penetrometer measurements of undrained shear strength (S_u) in Holes U1396A and U1396C are consistent and show a general trend that increases downhole. No measurements were performed in Hole U1396B. S_u measurements conducted with the automated vane shear (AVS) also show a trend of increasing shear strength with depth in both holes. Within this trend, a few low values are observed at 85, 115, and 130 mbsf.

P-wave velocities

Discrete measurements of P-wave velocity measured on the x-axis (PW-X) identify the volcaniclastic layers as having high velocity (1650–1800 m/s) compared to the hemipelagic sediment (1550–1650 m/s). Such measurements were only possible where tephra layers were thicker than the transducer’s caliper separation (3 cm). PW-X measurements generally match P-wave logger (PWL) measurements on the WRMSL.

Moisture and density

We collected 35 moisture and density (MAD) samples (32 from Hole U1396A and 3 from Hole U1396C; Figs. F9, F10). Porosity ranges from ~54% to 70%, with one volcanic sand sample having a porosity of 50%. Porosity shows no clear trend with depth. We caution that our calculated porosity values in high-permeability sediments such as volcanic turbidites may be too low because many of the samples drained water when we removed them from the working half of the split core. When we baked these samples, less water was present for removal, resulting in porosities that will be systematically lower than true values. Normally compacted sands do not typically have porosities >50%. The deepest sediments at Site U1396 are ~4.5 Ma in age and sedimentation rates are low (see “Paleomagnetism”), implying that no significant overpressure or anomalously high porosities should exist at this site. We therefore suggest that the anomalously high porosities we observe in sandy sediment at Site U1396 may result from significant reworking of the sediment during core recovery and splitting. Bulk density ranges from 1.45 to 2.00 g/cm³. Grain density ranges between 2.65 and 2.8 g/cm³.

Downhole temperature

Temperature was measured with the APCT-3 at the bottom of Cores 340-U1396A-3H, 4H, 5H, and 6H (24.6, 34.1, 43.6, and 53.1 mbsf, respectively) and Cores 340-U1396C-6H and 11H (55.9 and 103.4 mbsf, respectively). Downhole temperature was monitored for 320, 316, 346, 654, 943, and 2281 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-U1396A-3H, 4H, 5H, and 6H we obtained temperatures of 7.62° ± 0.02°C, 8.33° ± 0.02°C, 8.79° ± 0.02°C, and 9.64° ± 0.02°C, respectively. At the base of Cores 340-U1396C-6H and 11H we obtained temperatures of 9.87° ± 0.02°C and 13.07° ± 0.01°C, respectively. We have no reliable measure of the temperature of ocean water at the seafloor. Computed uncertainties are similar to the error on the best-fit solution and the probe’s measurement accuracy.

A best-fit linear relationship between depth and our six temperature measurements gives a temperature gradient of 69.3° ± 1.5°C/km (Fig. F10). Using measured thermal conductivity, the implied heat flow, if conductive, is 72 ± 2 mW/m². The near-surface heat flow at this site is reduced by 2% owing to the bathymetry, and sedimentation has a negligible effect on heat flow (Manga et al., 2012). There is no statistically significant deviation of the measurements from a straight line, and hence there is no signature of fluid flow in the temperature measurements (Manga et al., 2012).

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