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doi:10.2204/iodp.proc.340.105.2013

Physical properties

Holes U1395A and U1395B were cored to 121 and 124 mbsf, respectively, with the APC; low recovery below 124 mbsf constrains most of our data and interpretations to depths shallower than 120 mbsf. Depths generally refer to those of Hole U1395A. High magnetic susceptibility and low NGR identify volcaniclastic beds. GRA density distinguishes between volcaniclastic, bioclastic, and pumice-rich turbidites. Lower values of S_u obtained from the automated vane shear (AVS) and fall cone compared to results from the handheld penetrometer can be related to strength degradation after core splitting because the handheld penetrometer measures shear strength while the core is still in the core liner, immediately after the core arrives on deck. The sediment may not be fully depressurized from in situ conditions and therefore may have higher shear strength. The temperature gradient in the upper 44 m is 98.2° ± 8.8°C/km. The implied heat flow of 101 mW/m² is the highest measured at all Expedition 340 sites.

Stratigraphic correlation between Holes U1395A and U1395B

We used both magnetic susceptibility and gamma ray attenuation (GRA) density data sets to correlate depths between Holes U1395A and U1395B (Fig. F7), with magnetic susceptibility providing the most robust correlation. Hole U1395A generally produced a cleaner, higher resolution magnetic susceptibility data set; we therefore used Hole U1395A as the reference and stretched Hole U1395B data as needed. Correlation is best in the uppermost 50 m, where both holes had ~100% recovery rates and clearly identifiable turbidite units (Fig. F7A). Between 50 and 115 mbsf, some discrepancy exists between the holes despite the use of the APC over this interval in both holes. We therefore attribute correlation discrepancies between 50 and 115 mbsf to slight differences in stratigraphy, as noted in the core description logs. Stratigraphic correlation between 60 and 80 mbsf and 90 and 110 mbsf shows the most significant discrepancies between the holes (Fig. F7B). Below ~120 mbsf, we used the XCB to core, resulting in poor recovery. We were unable to make correlation ties below 115 mbsf. Although we did not use them for correlation, natural gamma radiation (NGR) data also show consistent trends between holes; it may be valuable in the future to use this to improve correlation at this site. The largest depth shift for the correlation picks is 3 m, with most <1 m. All correlation pick depth shifts are shown in Table T4. Our correlation coefficient using these picks is 0.78.

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

Magnetic susceptibility (Fig. F8) clearly identifies volcaniclastic beds and turbidites with pronounced positive anomalies as high as 2800 × 10^–5 SI (e.g., 13–20, 72–75, and 113–120 mbsf). Magnetic susceptibility typically increases downhole through turbidites. Ash layers have clear but smaller magnetic susceptibility signatures, with typical values of 700 × 10^–5 to 1500 × 10^–5 SI. Hemipelagic sediments have low and relatively constant magnetic susceptibility (<350 × 10^–5 SI). Magnetic susceptibility is relatively low in the bioclastic turbidite at 50–60 mbsf (mostly <700 × 10^–5 SI).

NGR measurements (Fig. F8) are usually anticorrelated with magnetic susceptibility. NGR values are low for volcaniclastic beds (around 5 counts per second [cps]) and relatively high for hemipelagic sediment (around 10 cps).

GRA density values (Fig. F8) are as high as 2.0 g/cm³ in turbidites that contain dense volcanic clasts but relatively low (~1.4 g/cm³; e.g., 56–52 mbsf) for bioclastic sediment. Whole-Round Multisensor Logger (WRMSL) P-wave velocity (Fig. F8) generally increases with depth. Overall, volcaniclastic turbidites have higher velocities (1750–1800 m/s) than hemipelagic sediment (1500–1600 m/s).

Thermal conductivity

Thermal conductivity was measured on 36 sections. The mean thermal conductivity was 1.029 W/(m·K) with a standard deviation of 0.100 W/(m·K) and a standard error of the mean of 0.017 W/(m·K).

Shear strength

Undrained shear strength (S_u) was measured with three techniques (Fig. F8). The handheld penetrometer generally gives higher values than the AVS and the fall cone. Measurements of S_u obtained from the handheld penetrometer increase linearly downhole (~6 kPa/m) to 40 mbsf in Hole U1395B, whereas in Hole U1395A, S_u decreases from 25 to 45 mbsf to 35 kPa. Below 45 mbsf, measured S_u ranges from 50 to 220 kPa (the limit of the instrument).

The AVS shows a downhole linear trend from 15 to 86 kPa in the 0–40 mbsf interval (~2 kPa/m). AVS measurements could not be performed in firm sediment and loose coarse-grained turbidities. AVS values are scattered below 60 mbsf. Fine-grained sediment on top of turbidite sequences has low shear strength (13–40 kPa between 40 and 100 mbsf).

Shear strength measured with the fall cone increases from 0 to 45 mbsf (about 2 kPa/m). No clear trend is observed from 45 to 65 mbsf because of the presence of positive incursions (>400 kPa) and some relatively low shear strength values (<50 kPa). Below 65 mbsf, S_u increases downhole (about 2 kPa/m).

P-wave velocity

Discrete P-wave values measured on the x-axis (PW-X; Fig. F8) match P-wave logger (PWL) values, with velocities increasing with depth. Few discrete PW-X measurements exist for coarse, unconsolidated sediment (e.g., turbidites). Turbidites generally have higher P-wave velocities (1750–1800 m/s) compared to hemipelagic sediment (1500–1600 m/s).

Moisture and density

We collected 53 moisture and density (MAD) samples (45 from Hole U1395A and 8 from Hole U1395B; Fig. F8). Porosity ranges from ~48% to 67%, showing a weak negative correlation with depth. Bulk density ranges from 1.55 to 1.95 g/cm³ and increases with depth. 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.

Grain density of hemipelagic samples range between 2.65 and 2.8 g/cm³. Grain density of pumice-rich turbidites is lower, ranging between 2.45 and 2.55 g/cm³. Dense volcaniclastic turbidites have grain density similar to hemipelagic sediment at Site U1395. Porosity decreases slightly with depth.

Downhole temperature

Temperature was measured with the advanced piston corer temperature tool (APCT-3) at the bottom of Cores 340-U1395B-3H, 4H, and 5H (24.9, 34.4, and 43.9 mbsf, respectively). Downhole temperature was monitored for 683, 697, and 744 s, respectively. Temperature was calculated from these time series of temperature measurements using TP-Fit software (Heeseman et al., 2006; see APCT-3 user manual on the Cumulus/Techdoc database at iodp.tamu.edu/​tasapps/). We assumed a thermal conductivity (k) of 1.0 W/(m·K) and ρC = 3.7 × 10⁶ J/m³K. To calculate uncertainty, we assumed 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-U1395B-3H, 4H, and 5H, we obtained estimated equilibrium temperatures of 7.11° ± 0.03°C, 8.44° ± 0.02°C, and 9.48° ± 0.02°C, respectively. These reported uncertainties are similar to the error on the best-fit solution and the probe’s measurement accuracy.

Temperature of ocean water at the seafloor was 5.15° ± 0.04°C. A best-fit linear relationship between depth and our four temperature measurements gives a temperature gradient of 98.2° ± 8.8°C/km (Fig. F9). Using the measured thermal conductivity, the implied heat flow, if conductive, is 101 ± 9 mW/m². No correction for sedimentation and bathymetry are needed. The high permeability of the coarse turbidites may also disturb subsurface temperatures by advection; however, there is no statistically significant deviation of the measurements from a straight (conductive heat transfer) line, at least in the upper 44 m where we have data (Manga et al., 2012).

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