Measurements of physical properties at Site 1131 followed the procedures outlined in "Physical Properties" in the "Explanatory Notes" chapter. These included nondestructive measurements of P-wave velocity (every 4 cm; Table T10, also in ASCII format), GRA bulk density (every 4 cm; Table T11, also in ASCII format), magnetic susceptibility (every 8 cm; Table T12, also in ASCII format), and NGR (every 16 cm; Table T13, also in ASCII format) using the MST. The P-wave logger (PWL) was activated only on APC cores. Thermal conductivity was measured in unconsolidated sediment at a frequency of one per core (Table T14, also in ASCII format), with one additional sample per core analyzed after deployments of the Adara tool and the DVTP (Table T15, also in ASCII format). A minimum of two discrete P-wave velocity measurements per section were made on the working half of the split cores (Table T16, also in ASCII format). Measurement frequency was increased to five per section after the PWL was turned off. Standard index properties (Table T17, also in ASCII format) and undrained shear strength (only in unconsolidated sediments) (Table T18, also in ASCII format) were measured at a frequency of one per section. Difficulties occurred with the pycnometer used for determination of dry volume for index properties measurements (see "Index Properties" in "Physical Properties" in the "Explanatory Notes" chapter).
The following sections describe downhole variations in sediment physical properties and their relationships to lithology and downhole logging measurements. Variations in magnetic susceptibility are described in "Paleomagnetism".
Sediment physical properties data reflect the homogeneous sediments recovered at Site 1131. An offset was seen between the discrete bulk density measurements and the GRA densiometry measurements of the MST. This offset was corrected using the equation of Boyce (1976), as described in "Index Properties" in "Physical Properties" in the "Explanatory Notes" chapter. Although high concentrations of gas were present, sediment physical properties were not disturbed to the extent of those at Site 1127. Despite this, small voids caused by gas expansion consistently interfered with the sonic travel through the sediment and, as a result, most of the PWL data are invalid.
A close association was seen between downhole logging data (see "Downhole Measurements") and sediment physical properties measurements. Natural gamma radiation values from both whole core and the gamma-ray downhole log show an excellent correlation that supports the integrity of both data sets. Gamma-ray attenuation and moisture and density data have similar patterns but lower values than the downhole logging data. This difference probably results from the fact that in situ density includes the influence of sediment overburden and hydrostatic pressure, whereas the laboratory measurements do not. A similar effect is seen in the P-wave velocities, particularly below 140 mbsf, where in situ velocities are higher than those measured in discrete samples.
Physical properties data can be divided into two units on the basis of trends in the measured parameters. Physical properties Unit (PP Unit) 1 (0-30 mbsf) is characterized by a rapid increase in NGR (<5-45 cps) to the base of the unit (Fig. F19). This increase appears to result from increasing uranium concentration, as indicated by the spectral gamma-ray log (see "Downhole Measurements"), and to correlate with the disappearance of HMC in the sedimentary section (see "Inorganic Geochemistry"). Bulk density (1.4-1.62 g/cm3) shows a small increase with depth, whereas P-wave velocity remains constant near 1.55 km/s (Fig. F19). Physical properties Unit 1 corresponds to lithostratigraphic Unit I (see "Lithostratigraphy"). High but decreasing porosity values (62%-55%) in this unit correspond to the recovered packstone facies. Overall increases seen in bulk density and P-wave velocity and decreases in porosity within PP Unit 1 are likely to reflect sediment compaction.
Physical Properties Unit 2 (30-530 mbsf) is characterized by distinct cyclicity in NGR data (Fig. F19) with values between 5 and 62 cps. This cyclicity is likely to be related to Milankovitch-induced sea-level variability and the resultant changes in sedimentology at Site 1131. From 30 to 130 m within PP Unit 2, P-wave velocity shows a slight increase (1.6-1.65 km/s) accompanied by a small increase in bulk density (from 1.7 to 2 g/cm3). At 130 mbsf, a sudden increase in P-wave velocity is observed (Fig. F19), which corresponds to a change to the PWS3 measurement probe (see "Sonic Velocity" in "Physical Properties" in the "Explanatory Notes" chapter). Below this offset, P-wave velocity generally increases from average values of 1.8 to 2.0 k/ms (Fig. F19). Within the sedimentary sequence, P-wave velocity often correlates to high NGR values. These instances correspond to firmgrounds within the sediment section. Within PP Unit 2, bulk density shows a general increase from 1.55 to 1.7 with higher frequency variability superimposed on this trend. Accompanying this increase in density is a decrease in porosity from 50% to 30% (Fig. F19). Both of these changes result from lithostatic compaction.
Undrained peak and residual shear strength were measured on unconsolidated sediments from 0 to 155 mbsf (Fig. F20). Shear strength from Site 1131 shows an overall downhole increase (2-30 kPa) resulting from compaction. Increased variability occurs below 30 mbsf, which corresponds to the PP Unit 1/Unit 2 boundary. This variability is caused in part by alternations in sediment lithification, but may also result from drilling disturbance or cracking of the sediment before failure, producing lower values for peak strength.
Thermal conductivity values at Site 1131 range from 0.76 to 1.16 W/(m·K) (Fig. F21). Thermal conductivity is correlated to increases in sediment bulk density at Site 1131 (Fig. F21) and thus is likely to be controlled by compaction and diagenetically controlled changes in sediment induration.
Only three in situ temperature measurements were made at Site 1131, two using the Adara tool and one using the DVTP (Fig. F22). There was considerable variation in estimates of mudline temperature from 11.98° to 13.55°C. An additional estimate of seafloor temperature was obtained using an expendable bathythermograph. This yielded a value lower than those obtained from the in situ temperature tools, possibly because of differences in temperature calibration. None of the in situ temperature measurements were affected by postemplacement movement of the probe, and differences between data fits gave only minor differences in temperature (Table T15).
In situ temperature data define a linear temperature-depth trend that defines a geothermal gradient of 42.6° ± 0.5°C/km (r2 = 0.99; N = 6) (all uncertainties are 1 standard deviation). The geometric mean of thermal conductivity between 0 and 115 mbsf was used for determination of heat flow (0.95 ± 0.089 W/[m·K]) (Fig. F22). Using this value and the geothermal gradient determined above, heat flow at the site is estimated to be 40.5 mW/m2. This value is higher than that determined at Site 1130, which has similar low thermal conductivity sediments and is in a similar position and depth on the shelf margin, although it is lower than the value determined in much deeper water off the platform margin at Site 1128.