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doi:10.2204/iodp.proc.334.103.2012

Physical properties

At Site U1378, physical properties measurements provide basic information characterizing lithostratigraphic units. After sediment cores reached thermal equilibrium with ambient temperature at ~20°C, gamma ray attenuation (GRA) density, magnetic susceptibility, and P-wave velocity were measured using the Whole-Round Multisensor Logger (WRMSL) on whole-round core sections. After WRMSL scanning, the whole-round sections were logged for natural gamma radiation (NGR). Thermal conductivity was measured using the full-space method on sediment cores and the half-space method on split lithified sediment cores. A photo-image-capture logger and a color spectrophotometer were used to collect images of the split surfaces of the archive-half cores on the Section Half Imaging Logger and the Section Half Multisensor Logger (SHMSL), respectively. Discrete P-wave measurements were made on split sediment cores using the Section Half Measurement Gantry (SHMG). Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split sediment cores.

Density and porosity

Bulk density values at Site U1378 were determined from both GRA measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). Samples were chosen from relatively undisturbed portions and preferentially in clay sediments rather than silty or sandier sections. A total of 366 discrete samples were analyzed for MAD.

In general, wet bulk density values determined from whole-round GRA measurements agree with measurements from discrete samples (Fig. F23A). Wet bulk density values increase with depth. This increase is likely due to dewatering caused by overburden pressure and is well described by a linear trend. A small offset in wet bulk density values marks the boundary between lithostratigraphic Units I and II.

Grain density measurements were determined from mass/volume measurements on discrete samples. Grain density values are relatively constant with depth with an average value of 2.69 g/cm³; however, variability in Unit II is large, with values between ~2.5 and 2.9 g/cm³ (Fig. F23B). No discernible offset is apparent between values characterizing lithostratigraphic Units I and II. On the whole, these values suggest terrigenous composition.

Porosity values (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]) were determined from mass/volume measurements on discrete samples using MAD Method C. Porosity is inversely correlated with bulk density and decreases from ~70% at the seafloor to 40% at the bottom of the hole (Fig. F23C). A slight increase in porosity between Units I and II corresponds to the decrease in wet bulk density at this boundary. An increase of porosity observed at ~440 mbsf corresponds to a sandy interval. Porosity increases a few percent between ~480 and 529 mbsf and may correspond to a decrease in clay content (see “Lithostratigraphy and petrology”).

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made on the SHMSL for all recovered cores from Site U1378. Uncorrected values of magnetic susceptibility are presented (Fig. F24). Magnetic susceptibility values measured with these two methods are in good agreement. In general, magnetic susceptibility in the sedimentary sequence is low, indicating an abundance of non-iron–bearing clays. However, a region of generally higher values lies between ~85 and 195 mbsf, with maximum values <0.5 SI. Other notable regions of relatively high values occur at 335, 355, 440, and 460 mbsf. These excursions exhibit high wave number variability and may be due to lithologic variations between sand, silt, and clay (see “Lithostratigraphy and petrology”).

Natural gamma radiation

NGR results are reported in counts per second (cps; Fig. F25). NGR counting intervals were ~10 min per whole-core interval. Despite the short counting time, NGR counts are considered reliable.

NGR counts have a mean and standard deviation of 21 and 5 cps, respectively. NGR counts show a small positive trend through lithostratigraphic Unit I and are relatively constant through Unit II. Notable excursions to higher values in Unit II occur at ~200 and 480 mbsf. These lowermost NGR highs are associated with scoria (see “Lithostratigraphy and petrology”).

P-wave velocity

Unconfined measurements of P-wave velocity at Site U1378 were determined from measurements on sediment whole cores and on discrete samples from the working halves of sediment split cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). Whole-core P-wave velocity measurements at this site only produced useful results above ~20 mbsf and again between 270 and 440 mbsf (Fig. F26). Based on visual inspection of the core, the unreliable measurements are attributed to the low water content of the clays. The P-wave signal does not seem to penetrate through relatively dry clays. Comparisons between these measurements and measurements taken on empty core liners suggest that the signal travels within the plastic liner around the core instead of through the recovered material, resulting in unrealistically low velocities. Between 270 and 440 mbsf, SHMG measurements of P-wave velocity along the z-axis are ~1700 m/s.

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method and on lithified split cores using the half-space method (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). In general, thermal conductivity increases with depth and is inversely correlated to porosity (Fig. F27). In the uppermost 100 mbsf, variability is significant and likely reflects gas concentrations in the core. Between ~260 and 310 mbsf, both needle-probe and half-space measurements of thermal conductivity were made. In general, needle-probe values are lower and likely reflect cracking in the core as the needle was inserted. Half-space values of thermal conductivity show less scatter than needle-probe values.

Downhole temperature

Downhole temperature was measured using the APCT-3. Four measurements were made between 34 and 110 mbsf in Hole U1378B (Table T7; Fig. F27). All measurements were made in lithostratigraphic Unit I. The Sediment Temperature Tool was not used at this site.

All measurements were made in a good sea state (<1 m swell), and all temperature-time series were recorded with a sample interval of 1 s. The temperature tool was stopped at the mudline for as long as 10 min prior to each penetration. The average bottom water temperature is 22.6°C (Table T7). Temperature-time series for each temperature measurement are shown in Figure F28. Significant frictional heating occurred on all penetrations of the APCT-3, with the temperature-time records exhibiting characteristic probe penetration and subsequent decay. Tool movement while the probe was in the sediment is minimal with the exception of Core 334-U1378B-3H, which indicates slight movement at about the 5 min mark. The effective origin time of the frictional heat pulse was estimated by varying the assumed origin time until the thermal decay pulse best fit a theoretical curve. All of the measurements appear to be reliable. Equilibrium temperatures plotted as a function of depth are relatively linear; coupled with the average bottom water temperature, they give a least-squares gradient of 51.4°C/km (Fig. F27).

Heat flow

Because thermal conductivity appears relatively constant over the depth of temperature measurements, we compute the heat flow as the product of the thermal gradient and thermal conductivity. Using the mean thermal conductivity of 0.86 W/(m·K) gives a heat flow of 44 mW/m². This value is consistent with forearc values of heat flow.

If heat transfer is by conduction and heat flow is constant, the thermal gradient will be inversely proportional to thermal conductivity according to Fourier’s law. This relationship can be linearized by plotting temperature as a function of summed thermal resistance (Bullard, 1939),

where

• T = temperature,

• z = depth,

• T_o = bottom water temperature,

• q = heat flow, and

• [Δz_i/k(z)_i] = thermal resistance.

We assume a linear increase of thermal conductivity with depth and extrapolate temperatures to the sediment/basement interface (Fig. F27). The estimated temperature at this depth is 31°C.

Vane shear

Undrained shear strengths increase approximately linearly with depth through lithostratigraphic Unit I to a maximum value of 154 kPa (Fig. F29). At the top of Unit II, vane shear strength decreases and becomes more erratic. This change in behavior occurs near the depth where the coring method changed from APC to XCB.

Color spectroscopy

Color reflectance measurement results are presented in Figure F30. L* values are generally between 10 and 50. a* and b* values are inversely correlated. a* values are generally between –5 and 0, and those of b* generally vary from –5 to 10.

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