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doi:10.2204/iodp.proc.340.107.2013

Physical properties

Variations in physical properties in Holes U1397A and U1397B are correlated to lithologic variations. In particular, high magnetic susceptibility is correlated with ash layers and volcaniclastic turbidites. High P-wave velocities and densities characterize sandy sediment and turbidites. Shear strength increases linearly with depth in the upper 30 mbsf but becomes more variable at greater depths. In the upper 61 m, temperature increases linearly with depth with a temperature gradient of 70.0 ± 8.8°C/km.

Stratigraphic correlation between Holes U1397A and U1397B

We used magnetic susceptibility and natural gamma radiation (NGR) to correlate depths between Holes U1397A and U1397B (Fig. F9). Hole U1397A is the reference hole for these correlations because it has the longest continuous record. Both holes have poor recovery at depths below 120 mbsf; it is therefore difficult to correlate below this depth. Correlation for the uppermost 25 m between the holes is good, with clearly matching peaks in magnetic susceptibility throughout this range; however, major discrepancies exist below 25 mbsf between the two holes. It was especially difficult to find clear correlations between 25 and 50 mbsf and all depths below 70 mbsf in the magnetic susceptibility data. As a result, we also looked at gamma ray attenuation (GRA) density and NGR to determine if better correlation points exist in these data sets. Ultimately, we used NGR data to tie points between 25 and 50 mbsf. We trimmed 10 cm off of each end of the core sections in the NGR data to ensure minimization of edge effects and found better correlation between 25 and 50 mbsf. We then cross-referenced our correlations using NGR with magnetic susceptibility as a quality control measure. In general, correlations are strongest in the uppermost 25 m and between 50 and 70 mbsf. At all other depths, correlation is poor. Our correlation coefficient is 0.24 using magnetic susceptibility data and 0.37 using NGR data. These values are artificially low because of meter-scale gaps and several core breaks between data sets (Analyseries software includes these zones to calculate correlation coefficient). We suggest that the poor correlation between 25 and 50 mbsf results from real geologic differences between holes, consistent with observed differences between the cores. Geologic differences may also explain discrepancies below 80 mbsf. Where we make correlations, depth shifts for Hole U1397B never exceed 3 m and rarely exceed 1 m. All picked correlation depth shifts are shown in Table T4.

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

Magnetic susceptibility shows large variations (300 × 10^–5 to 6880 × 10^–5 SI), with maximum values more than twice those measured at previously visited sites. High values of magnetic susceptibility correlate with the location of ash layers and volcaniclastic turbidites. GRA density (1.8–2.4 g/cm³) increases slightly with increasing depth. NGR is approximately constant but is reduced at the depth interval (50–70 mbsf) where magnetic susceptibility is also uniformly low. P-wave velocity ranges between 1400 and 1750 m/s with no significant downhole increase. Between 70 and 100 mbsf and between 160 and 185 mbsf, P-wave velocity shows maximum values of up to 2200 m/s related to basal parts of the turbidite units. Magnetic susceptibility typically increases with increasing depth within thick turbidite deposits (e.g., the turbidite unit at 52.5–55.0 mbsf in Hole U1397A). Also, bulk density marks turbidite units well but shows only a slight systematic increase with depth.

Thermal conductivity

Thermal conductivity was measured in 17 sections from Hole U1397A and 16 sections from Hole U1397B. Measured thermal conductivity is 1.037 W/(m·K) with a standard deviation of 0.127 W/(m·K) and a standard error on the mean of 0.022 W/(m·K).

Shear strength

Undrained shear strength (S_u) measurements made with the fall cone and automated vane shear (AVS) apparatus increase downhole (1 kPa/m) in the uppermost ~30 m of Holes U1397A and U1397B. S_u measurements performed with the handheld penetrometer show large fluctuations in Hole U1397A (>50 kPa), whereas in Hole U1397B, S_u values are consistent with those obtained with the fall cone and AVS (<50 kPa). Between 30 and 55 mbsf, although measurements are variable and scattered in both holes, S_u increases with depth (1 kPa/m). From ~55 to 65 mbsf no measurements could be made because of the coarse size of the sediment in Holes U1397A and U1397B (mainly sand). S_u measurements were successfully performed from 65 to 95 mbsf in both holes, and values increase (≤200 kPa) between 65 and 80 mbsf and decrease from 80 to 95 mbsf. Available S_u measurements from 95 to 103 mbsf in Hole U1397A increase rapidly with depth (as high as >100 kPa). From 95 to 150 mbsf, S_u measurements were not made because of the presence of coarse material. The last interval in which S_u measurements could be made, from 150 to 180 mbsf, has scattered values, from low (<50 kPa) to very high (>450 kPa). The scatter is consistent with the lithologic heterogeneity of the sediment type and may also reflect variable consolidation states.

P-wave velocity

Discrete measurements of P-wave velocity measured on the x-axis (PW-X) are consistent with the trend of P-wave logger (PWL) measurements. P-wave velocity increases slightly from 0 to 120 mbsf. Velocities near the seafloor average ~1600 m/s and are typical values for water-saturated sandy silt (Hamilton and Bachman, 1981).

Moisture and density

We collected 46 moisture and density (MAD) samples (40 from Hole U1397A and 6 from Hole U1397B; Fig. F10). Porosity ranges from ~40% to 68%, with two volcanic sand samples having a porosity of <40%. Porosity shows no trend with depth; however, in the upper 120 m porosity shows a larger range (40%–68%) compared to cores below this depth (60%–68%). Hemipelagic sediment has higher porosities than turbidites. Porosity of loose sands may be underestimated because of the 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 (ρ) for hemipelagic sediment ranges between 1.55 and 1.94 g/cm³. Dark-colored volcaniclastic turbidites have systematically higher bulk densities (1.75–2.40 g/cm³).

Grain density ranges between 2.48 and 3.17 g/cm³. Hemipelagic sediment has grain densities between 2.6 and 2.85 g/cm³ (with one outlier at 2.48 g/cm³).

Downhole temperature

Temperature was measured with the APCT-3 at the bottom of Cores 340-U1397A-5H, 6H, and 7H (36.5, 46.0, and 55.5 mbsf, respectively) and Cores 340-U1397B-3H, 4H, and 7H (25.6, 35.1, and 61.3 mbsf, respectively). Downhole temperature was monitored for 652, 626, 671, 647, 685, and 647 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-U1397A-5H, 6H, and 7H we obtained temperatures of 6.48° ± 0.06°C, 6.66° ± 0.03°C, and 7.91° ± 0.02°C, respectively. At the base of Cores 340-U1397B-3H, 4H, and 7H we obtained temperatures of 5.61° ± 0.05°C, 7.01° ± 0.03°C, and 8.83° ± 0.04°C, respectively. These reported uncertainties are greater than the error on the best-fit solution and the probe’s measurement accuracy, and they are dominated by uncertainties in the thermal properties of the sediment. The temperature of ocean water at the seafloor was 4.26°C.

A best-fit linear relationship between depth and our six temperature measurements gives a temperature gradient of 70.0° ± 8.8°C/km (Fig. F11). Using measured thermal conductivity, the implied heat flow, if conductive, is 72 ± 9 mW/m². The measured near-surface heat flow at this site is lowered, relative to that at depth, by 4% owing to bathymetry and up 4% owing to sedimentation (Manga et al., 2012). There are no statistically significant deviations of measurements from a straight line that would be indicative of fluid flow (Manga et al., 2012).

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