IODP Proceedings    Volume contents     Search


Physical properties

Physical properties at Site U1368 were measured on whole cores, split cores, and discrete samples. 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 with the Whole-Round Multisensor Logger (WRMSL). After WRMSL scanning, the whole-round sections were logged for NGR. Thermal conductivity measurements were made on sedimentary whole rounds using the full-space method and on basement split cores using the half-space technique. Compressional wave velocity measurements on sedimentary split cores were made once per core and moisture and density (MAD) analyses on discrete core samples were made at a nominal frequency of every other section. On basaltic basement cores, these measurements were made twice per core. The Section Half Multisensor Logger (SHMSL) was used to measure spectral reflectance on archive-half sections and take point measurements of magnetic susceptibility. Additionally, discrete measurements of electrical resistivity were made on the split sediment sections at approximately every 10 cm. These measurements were used to calculate formation factor. The Section Half Imaging Logger and a color spectrophotometer were used to collect images of the split surfaces of the archive-half cores. Four holes targeted the sedimentary cover, Holes U1368B–U1368E. The most complete sedimentary sequence for logging physical properties comes from Holes U1368B and U1368E. In Hole U1368F, we targeted basement but also recovered a few meters of sediment above basement. Cores from the different holes through the sediment have not been correlated and offsets exist.

Density and porosity

Sedimentary values of bulk density at Site U1368 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 329 Scientists, 2011a]). A total of 29 discrete sedimentary samples were analyzed for MAD, 7 samples from Hole U1368B, 9 samples from Hole U1368C, 4 samples from Hole U1368D, 7 samples from Hole U1368E, and 2 samples from Hole U1368F.

Wet bulk density values determined from whole-round GRA measurements from Hole U1368B display conspicuous density inversions correlated with section breaks that indicate core disturbance (Fig. F31A). Discrete bulk density values plotted on these disturbed trends suggest that fluids drained from the sediment during core handling, possibly because of the longer than normal time before WRMSL measurements. Prior to WRMSL measurements, these cores were measured for dissolved oxygen, accounting for the time delay. The departures in density have magnitudes of up to 0.3 g/cm3 or almost 20%. Because the perturbing trends are well delineated, we detrended segments and adjusted the density data (Fig. F31B). One sign that these corrections may be appropriate is that the magnitudes of discontinuities across section breaks are substantially decreased. The adjusted data show greater continuity across section breaks (Fig. F31C).

Similar perturbations caused by core handling also likely affected data from Holes U1368C and U1368D (Fig. F32), but because whole rounds were removed prior to GRA measurements, trends are less clear. Density inversions in the uppermost sections of Hole U1368E also suggest disturbance. In detail, it appears that the measurements in the uppermost part of each section are fairly constant but then oscillate in the lower part. We have not attempted to remove these density inversions. In both Holes U1368B and U1368E, the perturbations suggest that fluids preferentially drained from the lower part of each section. It remains puzzling why the perturbations preferentially affected the bottoms of sections in all cases rather than the tops. However, this observation implies that MAD samples taken from the tops of sections, as they were in Hole U1368E, may be more representative of in situ values than samples taken from the lower portions of sections, as exhibited in Hole U1368B. Neglecting the low values between 1.8 and 3.0 mbsf because of a collapsed core liner, the adjusted density in lithologic Unit I is 1.6 g/cm3 in both Holes U1368B and U1368E. Bulk density values in Unit III are generally much more variable than in the upper two units.

Relationships between discrete measurements of bulk density, grain density, and porosity in Holes U1368B–U1368E are shown in Figure F33. Given the likely core disturbance discussed above, the most robust MAD measurements may be those of grain density based on dry weights and volumes. Within lithologic Unit I, the mean and standard deviation grain density is 2.78 and 0.05 g/cm3, respectively. Porosity reduction may have also occurred, but the MAD measurements are too sparse to make a quantitative assessment.

Density values and MAD samples from Hole U1368F are shown in Figure F34. The WRMSL density calculation assumes that the core liner is entirely filled with rock. This assumption is appropriate for sediment cores, but basement cores have a smaller maximum diameter of 58.5 mm, narrower than the 66 mm internal diameter of the plastic core liner. WRMSL density values are adjusted by a factor of 66/58.5 = 1.18. Peak values of WRMSL density are slightly greater than 3 g/cm3. MAD density values are somewhat lower than maximum WRMSL values with an average density of 2.85 g/cm3. Porosity values of basement have an average value of 4.5%.

Magnetic susceptibility

Volumetric magnetic susceptibilities were measured using the WRMSL, and point measurements were made using the SHMSL on all recovered cores from Site U1368. Uncorrected values of magnetic susceptibility are presented for Holes U1368B–U1368E (Fig. F35). Values collected on the WRMSL and SHMSL are in general agreement. The spatial resolution of the WRMSL magnetic susceptibility loop is ~5 cm, and the observed ringing in Holes U1368C–U1368E is caused by edge effects.

Magnetic susceptibility values in Holes U1368B and U1368E show high wave number variations in the upper portion of lithologic Unit I. In the lower portions of Unit I, values slowly increase with depth to Unit II. Values in Unit II are generally lower than in Unit I. Magnetic susceptibility in Unit III is more variable and generally higher than the upper units. At ~14.5 mbsf in Hole U1368B, magnetic susceptibility attains an extremely high value of 8.222 × 10–5 SI. These high values have a spatial association with titanomagnetite (see “Lithostratigraphy”).

Magnetic susceptibility in Hole U1368F is shown in Figure F36. Values in the basalt are generally higher than in the sediment and attain a maximum at ~40 mbsf.

Natural gamma radiation

NGR results are reported in counts per second (cps) (Fig. F37). NGR counting intervals were ~1 h per whole-core interval for Hole U1368B but decreased to 0.5 h per whole-core interval for Holes U1368C–U1368F. NGR counts are considered reliable. NGR at the tops of all holes is high, with the exception of Hole U1368D, where a whole round was sampled. These high counts indicate that the sediment/water interface was sampled.

Below the sediment/seawater high, NGR counts are uniformly low but tend to increase with depth and are somewhat higher in lithologic Unit III. Ringing is more prevalent in cores from Holes U1368C and U1368D because only short core pieces remained of them after whole-round sampling prior to NGR measurements.

NGR counts in Hole U1368F basalt are low but increase slightly with depth (Fig. F38). Between 100 and 110 mbsf, counts associated with breccia are higher, with values >10 cps.

P-wave velocity

Compressional wave velocity was measured by the P-wave logger (PWL) on all whole cores and by the insertion and contact probe systems on sedimentary split cores in Hole U1368B. The contact probe system allows determination of velocities in the y-, z-, and x-directions. In general, compressional wave velocities on split cores are somewhat higher than on whole cores, with a mean offset of ~40 m/s (Fig. F39). This offset may have resulted from excessive wetting of sediment to ensure good contact between the sensor and the split core sediment, a water-rich rind inside the core liner, and possible variations in pressure/probe contact between the PWL and discrete x-direction transducers. The mean value of the PWL measurements is ~1497 m/s (close to the compressional velocity of water) (Fig. F39B). Below ~8–10 mbsf in all holes, P-wave velocities decrease perceptibly through lithologic Unit II. Values of P-wave velocity in Unit III are more variable than in either Unit I or II. Cores from Holes U1368C and U1368D exhibit greater apparent scatter than cores from Holes U1368B and U1368E; this increased scatter is an artifact caused edge effects, as these measurements were made after whole-round sampling for chemistry and microbiology.

Point measurements of compressional wave velocity for Hole U1368F are shown in Figure F40. The mean P-wave velocity through the basalt has a mean value of 5480 m/s.

Formation factor

Electrical conductivity was measured on working halves of the split sediment cores from Hole U1368B, and the formation factor was calculated. For each measurement, the temperature of the section was also noted. Surface seawater was used as a standard and measured at least twice per section (Table T6), normally prior to making measurements for that section and then around the 75 cm offset of each section. These measurements were used to compute the drift (Fig. F41), which was very low at this site. The temperature dependence of electrical conductivity was corrected and all reported measurements correspond to a temperature of 20°C. Electrical conductivity measurements were transformed to a dimensionless formation factor by dividing the measurements for the drift (Table T7).

Within lithologic Unit I, the formation factor is relatively uniform (Fig. F42). Formation factor values in lithologic Unit II increase slightly with depth, and values in Unit III are more scattered (Fig. F36).

Thermal conductivity

Thermal conductivity measurements were conducted on sediment whole-round cores using the full-space method. Measurements were made at a frequency of once per section. The mean and standard deviation of the sediment thermal conductivity measurements are 0.80 and 0.16 W/(m·K), respectively (Fig. F43A). These values are compared with thermal conductivity values measured on returned material from a piston core collected during the KNOX-02RR site survey cruise. The mean and standard deviation of these values are 0.96 and 0.14 W/(m·K), respectively (R. Harris, unpubl. data).

Thermal conductivity measurements were collected on recovered basalt using the half-space method (see “Physical properties” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). Measurements were made at an approximate frequency of once per section if suitable pieces were present. Thermal conductivity values are relatively uniform with depth in the basalt and have a mean and standard deviation of 1.66 and 0.11 W/(m·K), respectively (Fig. F43B).

Color spectrometry

Spectral reflectance was measured on split archive-section halves from Holes U1368A–U1368F using the SHMSL. The parameters L* (luminescence) (Fig. F44) and a* (green–red) (Fig. F45) have limited relationships with lithologic variation downhole. The parameter b* (blue–yellow) (Fig. F46) decreases through lithologic Units II and III with increasing clay and sand content as the units become increasingly darker.

Spectral reflectance parameters a* and b* in Hole U1368F (Fig. F47) have lower values in the basalt than in the sediment (0–10 mbsf). L* shows greater variability in the basalt than in the sediment. In general, spectral reflectance values are relatively uniform for parameters L* and a*. Maximum values of b* increase slightly with depth within the basalt.