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doi:10.2204/iodp.proc.339.106.2013

Physical properties

The shipboard physical properties program at Site U1388 included high-resolution nondestructive measurements of gamma ray attenuation (GRA) density, magnetic susceptibility (loop sensor), P-wave velocity (mostly in 2.5 cm steps), and NGR on whole-round core sections measured each 20 cm. Thermal conductivity was not obtained at this site because of poor recovery and sediment disturbance. Discrete measurements of P-wave velocity were determined on every other working-half section in Hole U1388B between Cores 12X and 20X. Moisture and density (MAD) were measured once in every section as 10 cm³ discrete samples. Color reflectance spectrometry and split-core point-logger magnetic susceptibility were obtained for every section in each hole in 5 cm steps. Based on physical properties, three intervals were distinguished. The first interval (physical properties Unit I), between 0 and 100 mbsf, is characterized by low core recovery and high-frequency variations of all parameters. A second interval (physical properties Unit II), between 100 and ~150 mbsf, is characterized by high core recovery and a marked positive relationship between GRA density and magnetic susceptibility values. A third interval (physical properties Unit III), from ~150 mbsf to the bottom of the hole, displays low magnetic susceptibility values and indistinct relations between the different parameters.

Whole-Round Multisensor Logger measurements

After allowing the cores to equilibrate for 3 h, GRA bulk density and magnetic susceptibility were measured in all core sections at 2.5 cm intervals at Site U1388 using the WRMSL (Fig. F24).

Gamma ray attenuation density

Variations in GRA density may reflect changes in lithology, consolidation, cementation, and porosity. Measured GRA density ranges between 1.26 and 2.01 g/cm³ in Hole U1388A, showing a steady increase in the upper intervals that probably reflects the presence of a coarse sand layer. A marked decrease observed in the lower part of the hole can be linked with an increase in fine sand content (Fig. F24).

In Hole U1388B from 0 to 100 mbsf, core recovery was very poor and GRA density ranges between 1.7 and 2.2 g/cm³. Despite the poor recovery, distinct high-frequency variations can be recognized (e.g., Cores 339-U1388B-9X and 10X). These high-frequency variations are also present in the upper part of Core 339-U1388B-12X, in which recovery is higher. The boundary between this high-frequency to lower frequency variation further downhole is in Core 339-U1388B-12X at ~110 mbsf. This boundary also can be distinguished by a change in lithology, concomitant with a decrease in the number of sandy turbidite layers (see “Lithostratigraphy”).

GRA density below 110 mbsf has almost the same values as those obtained from discrete samples. Density values range between 1.9 and 2.1 g/cm³ in the interval between 110 and ~150 mbsf for MAD and between 1.6 and 2.1 g/cm³ for GRA. There was high core recovery in this interval, which allows us to identify a strong link between GRA density, magnetic susceptibility, and the presence of silt and sands. Despite the positive correlation between GRA density and magnetic susceptibility, the relationship between NGR and the color reflectance parameters L* and a* is complex. Comparing GRA density to L* shows that high GRA density is not clearly associated with distinct changes in sediment lightness. In general, GRA density and NGR show a negative correlation in this interval, a relationship contrasting with that at other sites. This complex pattern can be linked with the abundance of detrital material including mica, zircon, glauconite, opaques, heavy minerals, and feldspars (see “Lithostratigraphy”) in both muddy and sandy layers.

In the lower part of Site U1388, GRA density values range between 1.6 and 2.1 g/cm³. The GRA density variations are difficult to correlate with changes in grain size (e.g., Cores 339-U1388B-19X and 20X). This could be linked with changes in the nature or fabric of the grains, as indicated by grain density variations (Fig. F25). The highest GRA density values in this interval correspond to layers with coarse grain size and low NGR values. Typically, sandy beds are associated with low GRA density, so this unexpected result could be explained by an unusual mineralogical composition.

Magnetic susceptibility

The most notable aspects of the magnetic susceptibility records at Site U1388 are the cyclical and smooth variations observed between 100 and ~150 mbsf and the low values below ~150 mbsf (Fig. F24). In the upper 100 m (physical properties Unit I), where core recovery was poor, susceptibility values range from 10 × 10^–5 to 30 × 10^–5 SI. In physical properties Unit II, where core recovery was high, magnetic susceptibility long-term trends show three major cycles that become progressively more intense, with average values between 10 × 10^–5 and 40 × 10^–5 SI. Other minor cycles and spikes are superimposed on the major cycles and reach 80 × 10^–5 SI, the highest magnetic susceptibility value. These cyclic variations are mirrored by GRA density values and are associated with the presence of silty/sandy layers with high magnetic susceptibility.

Magnetic susceptibility values are low (<20 × 10^–5 SI) from ~150 mbsf to the bottom of the hole (physical properties Unit III). This observation is also supported by low NRM values (see “Paleomagnetism”). Despite these low values, the composition of the sediment appears to be almost the same as in the overlying intervals, suggesting that diagenetic processes are controlling magnetic susceptibility in this interval. A likely factor for the main decrease of magnetic susceptibility is the reduction of fine-grained magnetite to iron sulfides below the sulfate reduction zone. However, the present sulfate–methane transition (SMT) zone, occurring around 40 mbsf and recognized in Hole U1388B (see “Geochemistry”), shows no correlation between low magnetic susceptibility and this boundary.

P-wave velocity

No reasonable P-wave velocity measurements were retrieved using the WRMSL. An attempt was made to determine P-wave velocities on split cores in each section of Hole U1388B (Fig. F24). Because of low core recovery and poor sediment to liner coupling, reasonable results from split cores were recovered only between ~100 and ~180 mbsf and values range between 1550 and 1730 m/s. P-wave velocities determined by the split-core logger do not show a clear correlation with any other physical property or lithology. Caution must be taken in interpreting the manually picked data, in which greater subjectivity and poorer signal quality lead to higher scatter in the data. The method of manually picking data tends to overestimate velocities.

Natural gamma radiation

NGR was measured on all core sections at 20 cm spacing and run on an integration time of 7 min per section. Measured values range from 20 to 45 cps, with a single peak reaching 88 cps in Core 339-U1388B-12X, which was not plotted because it appears to be an artifact. No distinctive explanatory feature can be found in the sediment (Fig. F26).

In Hole U1388A there is an interval of coarse sediment, in which NGR has low values around 19 cps. In the high-recovery interval from ~100 to ~150 mbsf in Hole U1388B, we can observe three cycles with progressively increasing amplitudes in NGR and a marked increase at the base of Core 339-U1388A-16X. Along this interval, NGR shows mainly negative correlations with magnetic susceptibility and GRA density. However, NGR values do not always reflect observed changes in grain size. This can be related to the described homogeneous mineral composition (see “Lithostratigraphy”).

Below ~150 mbsf core recovery is poorer, but major variations in NGR values can be distinguished in Cores 339-U1388B-18X, 20X, and 22X. In these cases, NGR values increase and decrease by 10 cps over ~2 m. Lows in NGR counts appear to be related to the presence of coarse sand deposits, probably with a lower content of K-feldspars or any other mineral enriched in potassium and thorium contents. This marked decrease in NGR is coincident with an increase in GRA density and relatively high values in grain density. No obvious correlation can be found in this interval between NGR and color reflectance parameters.

Moisture and density

Measurements of bulk density, porosity, and grain density were undertaken on 2 samples from Hole U1388A, 41 samples from Hole U1388B, and 4 samples from Hole U1388C. One MAD sample was taken every other section. The samples were taken at the same depths, approximately 60 cm from the top of the section, when available. Care was also taken to avoid locations with obvious drilling disturbance. These samples were measured for wet mass and dry density to calculate bulk density and grain density.

The MAD bulk densities from discrete samples are plotted in Figure F24 and show a range of 1.9–3.2 g/cm³. In the upper 110 mbsf, GRA and MAD bulk densities show certain offsets, where discrete sample densities are occasionally higher than those measured with the WRMSL. In the interval of high recovery between 110 and 150 mbsf, densities obtained by both techniques are in agreement. For the lowermost interval, below 150 mbsf, WRMSL values are lower because of poor core recovery and incompletely filled core liners, which affect the accuracy of the WRMSL measurements.

The patterns of porosity and moisture content in the interval between 110 and 150 mbsf stand out with respect to those of the other physical parameters, as they have only two amplitude cycles instead of three (Fig. F25). These two cycles appear to be related to long-term compositional changes interrupted by a coarser layer in Core 339-U1388B-15X. In general, samples taken from predominantly silt/sandy sections show lower values in porosity and moisture content. In addition, wet bulk densities appear to be higher in coarse-grained levels. This observation is not sufficiently resolved. Notably, only the dry volume of the sample was determined, not the wet bulk volume. Hence, the difference between the two density estimates might be related to the precipitation of salt crystals in the pore space during sample drying at 105°C, which would affect the determination of dry volumes by pycnometer and all further calculations.

Porosity ranges from 38% to 50%, decreasing with depth in the upper ~100 mbsf (Fig. F25). Bulk densities do not exhibit clear increasing values with depth for Hole U1388B (Fig. F24). The lack of compaction-related depth trends could be a result of poor core recovery and the relatively shallow depths reached in this site. NGR values indicate compositional variations for certain intervals, although grain density values are relatively constant in the range from ~2.6 to ~2.8 g/cm³ without clear trends (Figs. F25, F26). A density peak of 3.2 g/cm³ is found in Core 339- U1388B-24X. Grain densities are on average slightly higher than quartz density (2.65 g/cm³) and probably reflect the varying degree of heavy minerals, opaques, and zircon in the sediment.

Summary of main results

The most remarkable observation at Site U1388 with respect to the previous sites is the negative correlation between GRA density and NGR and the inconsistent correlation between GRA density, magnetic susceptibility, and NGR with color reflectance parameters. When we compare physical properties with lithology, we observe that high GRA densities and magnetic susceptibility are associated with layers of coarse sediment. Nevertheless, NGR is not consistently sensitive to grain size variations (e.g., coarse grain layers), which are in some cases coincident with low NGR values but in other cases not. This relationship between physical properties was not observed at the previous sites and could be related to a homogeneous mineral composition in both fine and coarse layers. In any case, the described changes require more detailed analyses in order to be sufficiently explained.

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