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doi:10.2204/iodp.proc.334.106.2012

Physical properties

At Site U1381, physical properties measurements were made to 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). For basement cores, only GRA density and magnetic susceptibility were measured. 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 basement cores. A photo-image-capture logger and color spectrophotometer were used to collect images of the split surfaces of the archive-half cores on the Section Half Image Logger and Section Half Multisensor Logger (SHMSL), respectively. Discrete P-wave measurements were made on split sediment cores. Moisture and density (MAD) were measured on discrete subsamples collected from the working halves of the split sediment cores. MAD was not measured on basement cores because of time limitations.

Density and porosity

Bulk density values at Site U1381 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]). A total of 25 discrete samples were analyzed for MAD.

In general, wet bulk density values determined from whole-round GRA measurements and measurements from discrete samples agree well (Fig. F14A). Gaps in the measurements are due to incomplete core recovery with the RCB system. Wet bulk density values within the sedimentary sequence have an average and standard deviation of 1.40 and 0.14 g/cm³, respectively. Within the basement, GRA-derived bulk densities are highly scattered because of the variable filling of the core liner. Within basement, maximum bulk density values are ~2.3 g/cm³.

Grain density measurements were determined from mass/volume measurements on discrete samples within the sedimentary sequence. Although scatter in grain density values are high, grain densities generally decrease with depth from ~2.7 to 2.5 g/cm³ (Fig. F14B).

Porosity was determined from mass/volume measurements on discrete samples using MAD Method C on sediment cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). Porosity is relatively constant with depth, with a mean value of 76% (Fig. F14C). In general, porosity is expected to decrease with depth. The constant values may reflect disturbances caused by RCB coring. A low porosity value, caused by silicification, was measured at 52 mbsf in an ash layer (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 U1381. Uncorrected values of magnetic susceptibility are presented in Figure F15. Magnetic susceptibility values measured with these two methods are in good agreement. In the sediment, mean magnetic susceptibility was low, with a mean and standard deviation of 0.009 and 0.016 SI, respectively.

Within basement, values increased from the sediment/basement interface to 140 mbsf and slowly decreased to the bottom of the hole (Fig. F15). Maximum values of magnetic susceptibility varied between 1 and 2 SI.

Natural gamma radiation

NGR results are reported in counts per second (cps; Fig. F16). NGR counting intervals in the sediments were ~10 min per whole-core interval, and NGR counts are considered reliable. Basement counting times were 1 h per channel. In the shallow part of the basement, 12 channels were used, but in the deeper basement only six channels were used because of time constraints.

NGR counts within the sediment vary between ~10 and 30 cps, with maximum values occurring at ~30 mbsf. NGR counts within the basement are generally <5 cps and increase slightly with depth.

P-wave velocity

Measurements of P-wave velocity at Site U1381 were determined from measurements on sediment whole cores and mass/volume measurements on discrete samples from the working halves of sediment and basement split cores (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). In general, whole-core and discrete measurements on sediments are in good agreement (Fig. F17). The mean value is 1558 m/s, close to the compressional velocity of water.

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the needle-probe method and on basement split cores using the half-space method (see “Physical properties” in the “Methods” chapter [Expedition 334 Scientists, 2012]). The mean and standard deviation in sediment thermal conductivity measurements are 0.80 and 0.07 W/(m·K), respectively (Fig. F18A).

The mean and standard deviation of thermal conductivity in the basalt is 1.45 and 0.07 W/(m·K), respectively. These values are low for basalt and can be attributed to the fact that the samples were not water saturated before measurement because of time constraints.

Downhole temperature

Downhole temperature was measured using the SET. The advanced piston corer temperature tool is not compatible with the RCB coring system and was not used. Four measurements were attempted between 30 and 90 mbsf in Hole U1381B. All measurements were made in sediment.

All measurements were made in a low sea state (<1 m swell), and all temperature-time series were recorded with a sample interval of 1 s. The SET was stopped at the mudline for as long as 10 min prior to each penetration. The average bottom water temperature was 2.4°C (Table T5). Temperature-time series for each temperature measurement are shown in Figure F19.

Significant frictional heating occurred only on penetration 3 of the SET, with the temperature-time record exhibiting characteristic probe penetration and subsequent decay. Tool movement was observed in all temperature records as sudden shifts in temperature both before and after the usable section of the temperature-time series. Tool movement is attributed to using the SET in high-porosity unlithified sediments. 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. As a result, tool movement fits to the equilibrium curve are relatively short (Table T5). Nevertheless, the measurements appear to be reliable and are likely good to a few tenths of a degree Celsius. 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 222°C/km (Fig. F18B).

Heat flow

Because thermal conductivity appears relatively constant and the thermal gradient is linear, we compute heat flow as the product of the thermal conductivity and thermal gradient. Using the mean thermal conductivity of 0.8 W/(m·K) gives a heat flow of 178 mW/m². This value is significantly larger than the half-space prediction for 15 Ma crust of ~130 mW/m² and much larger than the observed global average heat flow for this age crust of 77 mW/m² (Stein and Stein, 1992). This high heat flow value suggests significant fluid flow within the underlying crust.

Vane shear

Undrained shear strengths increase with depth (Fig. F20). The trend is approximately linear in the uppermost 40 mbsf, with a maximum value of 40 kPa. Values decrease and display more erratic behavior downhole. This change may correspond to a change in lithostratigraphy (see “Lithology and petrology”).

Color spectrometry

Results from color reflectance measurements are presented in Figure F21. In the sediment sequence, L* values vary between ~35 and 50 and vary in the basement between 10 and 50. Variability in a* decreases with depth through the sediment section and are between approximately –3 and 2. Mean basement values of a* vary between about –2 and 1. Mean b* values appear inversely correlated to a* and vary between –5 and 10 in the sediment and –5 and 0 in the basement.

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