Previous   |    Next

doi:10.2204/iodp.proc.346.103.2015

Physical properties

Physical properties at Site U1422 were expected to be highly variable based on previous results at Site 795 (Shipboard Scientific Party, 1990), reflecting variable lithologies (with organic-rich to various types of hemipelagic sediment, including dust and ice-rafted material) and accompanying diagenetic processes. To this expected variability, Site U1422 added another layer of complexity: the presence of turbidites with variable composition in the lower part of the drilled sequence. Physical properties at Site U1422 were measured on whole- and split-core sections and discrete samples to describe this complexity, as per the strategy outlined in “Physical properties” in the “Methods” chapter (Tada et al., 2015b). Modifications to that plan are addressed herein.

The Special Task Multisensor Logger (STMSL), which measures gamma ray attenuation (GRA) bulk density and magnetic susceptibility, was used for stratigraphic correlation purposes immediately after core recovery at a scanning interval of 5 cm. The WRMSL, which measures GRA bulk density, magnetic susceptibility, and compressional wave velocity, was used at high resolution (2.5 cm) after the cores reached room temperature. Thermal conductivity (one per core) and NGR measurements (eight per full section) completed the suite of whole-core measurements. Compressional wave velocity measurements on split cores were attempted in Hole U1422A (one per core) and subsequently abandoned because of poor results. Shear stress measurements (one per core) were taken in Hole U1422A with more success. Spectral diffuse reflectance (mostly 1 cm intervals) and point magnetic susceptibility (mostly 2 cm intervals) were measured on the SHMSL. Moisture and density (MAD) analyses were undertaken on discrete core samples (2–3 per core) in Hole U1422A. Physical properties measurements are presented in Figures F34, F35, and F36; because a majority of the physical properties are driven by lithology, there was no need to define units independent of the lithologic units.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually near the middle of Section 4. Thermal conductivity in fine-grained sediment is, as a first approximation, a linear combination of the conductivities of the sedimentary grains and the interstitial water and therefore depends upon porosity, water content, and lithology (Hyndman et al., 1974). Overall, thermal conductivity values range from 0.7 to 1.0 W/(m·K) without a clear increase with depth.

Moisture and density

GRA bulk sediment density at Site U1422 displays no clear trend with depth but strong high-frequency variability (i.e., decimeter- to multimeter-scale) within a general range between 1.2 and 1.8 g/cm³ (Fig. F34). Discrete wet bulk density agrees well with the primary trends in GRA bulk density and has similar values (Fig. F35). In lithologic Unit I, variability in GRA bulk density appears to come from the occurrence of massive very dark layers (characterized by low L* values) exhibiting lower bulk density than the rest of the sediment (Fig. F36). This relationship was confirmed at Site U1425 (see Fig. F57 in the “Site U1425” chapter [Tada et al. 2015d]). The cyclical variability seen in GRA bulk density breaks down in Unit II, although the strong variability is still present but with a higher scatter. The region of high scatter located deeper than ~125 m CSF-A (see the “Methods” chapter [Tada et al., 2015b]) coincides with degraded core quality (e.g., microfractures) responsible for low-density readings, whereas the turbidite-rich sequence in Unit II may explain high-density values. Density readings in this sequence reach nearly 1.8 g/cm³ more often than in the sediment above it, probably because of increased terrigenous clastics in certain intervals. With the exception of grain density, most other MAD-derived parameters (i.e., porosity and water content) show no trends downhole. The increase in grain density downhole may be explained by the occurrence of terrigenous-rich turbidites.

Magnetic susceptibility

Whole-core magnetic susceptibility measured on the STMSL immediately after recovery closely tracks WRMSL susceptibility measured on cores that were equilibrated to room temperature (Fig. F34), confirming that sediment temperature does not affect this bulk parameter at this site. Similarly, point magnetic susceptibility (SHMSL) tracks whole-core trends well (Fig. F34), but values are significantly higher than the whole-core equivalents because of the inherent smoothing imposed by the magnetic susceptibility loop, which lowers whole-core intensities. Although mean values stay between 10 × 10^–5 and 20 × 10^–5 SI for the site, the highest magnetic susceptibility readings occur above ~20 m CSF-A but below the oxic zone at the mudline (i.e., the red–dark brown zone containing iron oxyhydroxides at ~0–50 cm CSF-A) (e.g., Lyle, 1983). A magnetic susceptibility maximum occurs between 2 and 6 m CSF-A where values up to 200 × 10^–5 SI were measured. Because no apparent primary lithology corresponds to this region, it is likely that the strong intensities are due to highly magnetic authigenic mineral formation. As no visible signs of monosulfides are evident in freshly split cores, ferrimagnetic greigite is probably not the likely cause, but formation and survival of authigenic magnetite grains beyond the oxic zone may be an alternative explanation. Furthermore, there is no evident correlation between magnetic susceptibility and decimeter-scale lithology, which also suggests diagenetic influence in lowering of the signal between ~6 and 20 m CSF-A near the SMT zone with the signal severely muted after that. However, there is an increase in mean and upper range values in the turbidite-rich sequence below ~140 m CSF-A.

Natural gamma radiation

Total NGR counts show strong cyclicity in Unit I that imparts a decreasing trend in counts with depth (Fig. F34). NGR counts range from 20 to 80 cps in Unit I and fall to between 20 and 40 cps in Unit II. Inspection of the NGR spectra shows that U-series radionuclides are likely to be the largest contributors to the total NGR counts, as has been observed elsewhere with this same instrument (Vasiliev et al., 2011). Therefore, the variability in NGR may be explained by increased U associated to massive organic-rich layers in Unit I (Fig. F36). The multimeter-scale NGR cyclicity observed in Unit I closely follows the cycles in GRA bulk density. This correlation between NGR and GRA bulk density adds some confidence in interpreting the NGR data, which is collected at an interval greater than the thickness of the dark layers. With the disappearance of dark layers in Unit II, NGR counts decrease to background levels typical for clay or even lower because of the abundance of the biosiliceous component.

Compressional wave velocity

The WRMSL was used to measure compressional wave velocity and an attempt was made to collect equivalent P-wave velocities on split cores (Fig. F34). Because of poor sediment to liner coupling, results from the WRMSL could only be obtained for the upper ~60 m CSF-A (Subunit IA). Even here some velocities are 100 m/s lower than the average, also suggesting the influence of small cracks in the relatively brittle sediment. Good data in this interval of P-wave velocity fluctuate at ~1500 m/s. A clear maximum occurs only in the soft, wet sediment of the uppermost ~2 m. Few good measurements could be collected with the gantry. The formation of small cracks at the numerous and sharp lithologic changes and within lithologic packets themselves might have negatively affected signal propagation through the sediment. Where collected, minor but clear differences between whole-core and split-core measurements possibly reflect the presence of water in the space between the core liner and sediment in the whole cores and the slight compaction of the sediment in the contact probe technique.

Vane shear stress

Undrained shear strength generally increases with depth through all lithologic units, reaching a maximum value of 88.6 kPa (Fig. F35). Measured values, however, are erratic due to small cracks in sediment throughout.

Diffuse reflectance spectroscopy

Color reflectance data measured on the split archive-half sections show high variability, especially in Unit I, reflecting the variegated colors of the decimeter- to centimeter-scale lithologic packages (Fig. F37). Unit I is generally characterized by high variability in lightness (L*), red-green ratio (a*), and yellow-blue ratio (b*), as very dark brown to black organic-rich bands occur here often and alternate with lighter olive and green hemipelagic sediment. Subunit IA can be clearly distinguished from Subunit IB by b* variability expressing a change in frequency of occurrence of the dark layers. As the dark bands disappear altogether in Unit II, L* and a* vary less to the bottom of the hole. However, the variability in color between different types of turbidites as well as hemipelagic packages in the lower part of Unit II keeps b* highly variable. The presence of dark green bands throughout the site suggests that Fe²⁺ is present in the mineral phase (clay?) accumulating in green fronts (Giosan, 2001). Shore-based analyses will further examine this hypothesis.

Summary

Physical properties at Site U1422 are highly variable, reflecting various lithologies and accompanying diagenetic processes. In Unit I, cyclical variability in density and NGR appears to be driven by the occurrence of thick, massive organic-rich dark layers, whereas in Unit II the more subdued variability is largely due to terrigenous clastics from turbidites and biogenic silica from diatoms. Magnetic susceptibility is strongly influenced by redox processes with severe muting of the signal below the SMT zone (see “Geochemistry”). P-wave and shear stress data collection was strongly affected by degassing, leading to microfractures in the core. Reflectance data quantify the variegated colors of the diverse lithologic packages at this site and provide the opportunity for the decimeter- to centimeter-scale correlation for Unit I between this and future sites.

Top of page   |    Previous   |    Next