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doi:10.2204/iodp.proc.346.104.2015

Physical properties

Physical properties measurements at Site U1423 were conducted to provide high-resolution data on the bulk physical properties and their downhole variations in Holes U1423A and U1423B. High-resolution scanning at 2.5 cm intervals on whole-round sections was immediately performed with both the WRMSL (Sections 1, 2, and 3) and Special Task Multisensor Logger (STMSL) (Sections 4, 5, 6, and 7) after the cores were sectioned on the catwalk. The WRMSL was used to measure gamma ray attenuation (GRA) bulk density, magnetic susceptibility, and P-wave velocity, whereas the STMSL measured GRA bulk density and magnetic susceptibility. The GRA bulk density and magnetic susceptibility from each section were combined into individual core data sets for stratigraphic correlation. P-wave velocities measured in Sections 1, 2, and 3 with the WRMSL were consolidated into individual core data sets as well.

After the sections reached thermal equilibrium with the ambient room temperature of ~20°C, thermal conductivity (one per core) and NGR measurements (eight per full section) completed the suite of whole-core measurements. Cores were then split parallel to the core axis. One half was reserved for archiving and one half was for analysis and sampling (working half). Shear stress measurements (one per core) were performed on the working halves of Hole U1423A and the lower part of Hole U1423B (below 205 m CSF-A) with more success. Moisture and density measurements were performed on discrete core samples (two per core) collected from the working halves of Hole U1423A and the lower part of Hole U1423B. Spectral diffuse reflectance (mostly at 1 cm intervals) and point magnetic susceptibility (mostly at 2 cm intervals) were measured using the SHMSL on the archive halves. Physical properties measurements are presented in Figures F33, F34, F35, and F36.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually near the middle of Section 4. Thermal conductivity values range from ~0.8 to ~1.1 W/(m·K). Values are largely scattered from 0 to 120 m CSF-A and nearly constant at ~0.9 W/(m·K) deeper than 120 m CSF-A. This highly variable feature in lithologic Subunits IA, IB, and IIA and the lack of an increasing trend downhole suggest a compositional control on this parameter.

Moisture and density

GRA wet bulk density was measured using the WRMSL and STMSL. Although measurement error exists in GRA bulk density data because of the presence of air between sediments and the core liner, in general, GRA bulk density tends to reflect the characteristic of each lithologic unit (Fig. F33; see “Lithostratigraphy”). Similar to Site U1422, GRA bulk density at Site U1423 is highly variable in the upper part (between 0 and 103 m CSF-A) with values ranging from 1.2 to 1.8 g/cm³. High variations of GRA bulk density in this interval are closely related to the alternating very dark brown to black organic-rich sediment and lighter olive and green hemipelagic sediment. This relationship is matched well with the high GRA bulk density values characteristic of the dark layer–organic rich intervals in Site U1422. After sharply decreasing at ~103 m CSF-A, coinciding with the lithologic Unit I/II boundary, GRA bulk density tends to decrease slightly with depth to 249 m CSF-A. GRA bulk density at Site U1423 correlates well with the density log acquired in open Hole U1423B (see Fig. F37 and “Downhole measurements”).

Although discrete wet bulk density and grain density are relatively constant for the entire interval, ranging from 1.2 to 1.6 g/cm³ and from 2.4 to 2.8 g/cm³, respectively, the primary trends agree well with GRA bulk density (Fig. F35). Porosity and water content show generally reversed trends when compared to density, ranging from 68.1% to 84.7% and from 45.0% and 70.5%, respectively. Discrete bulk density and grain density have small step increases between 0 and 20 m CSF-A, where porosity and water content of the sediment decrease inversely. The trend of increasing density downhole reverses at ~20 m CSF-A and then generally decreases to the bottom of the hole. Porosity and water content gradually increase below 20 m CSF-A and vary highly with depth. However, all discrete bulk density, grain density, porosity, and water content measurements remain relatively constant from 103 m CSF-A to the bottom of the hole, corresponding to lithologic Unit II. The largest scatter of discrete bulk density and porosity occurs between 55 and 75 m CSF-A.

Magnetic susceptibility

Magnetic susceptibility is the degree to which a material can be magnetized by an external magnetic field. Therefore, magnetic susceptibility provides information about sediment composition. Magnetic susceptibility at Site U1423 generally decreases with depth (Fig. F33). Although the mean values remain between 10 × 10^–5 and 20 × 10^–5 SI for this site, the highest magnetic susceptibility readings occur shallower than ~22 m CSF-A (average = 91.5 × 10^–5 SI) with high variation. A magnetic susceptibility maximum occurs between 20 and 22 m CSF-A, where values as high as 200 × 10^–5 SI were measured. This occurrence of high magnetic susceptibility in the uppermost part (as was also the case with Site U1422) is likely due to highly magnetic authigenic mineral formation because there is no apparent primary lithologic correspondence to this region (see “Lithostratigraphy”). This lack of correlation between magnetic susceptibility and decimeter-scale lithology also suggests a diagenetic influence is decreasing the signal between 0 and 22 m CSF-A near the SMT zone with the signal severely muted deeper. After a large step decrease of magnetic susceptibility between 22 and 25 m CSF-A, magnetic susceptibility tends to decrease slightly downhole, with the exception of spikes of low magnetic susceptibility from 110 to 123 m CSF-A. Magnetic susceptibility also shows a small decrease at ~70 m CSF-A.

Point magnetic susceptibility from the SHMSL agrees with the whole-core trends as well (Fig. F33), but values become significantly greater with depth than the whole-core equivalents. The increasing gap between two magnetic susceptibility values in the lower part of the hole may be related to fractures in the whole-core sediment and the inherent smoothing imposed by the magnetic susceptibility loop, resulting in reduced the whole-core intensity.

Natural gamma radiation

The variation patterns of NGR are conformable with GRA bulk density and correlate well with the total gamma ray log acquired in open Hole U1423B (see Fig. F37 and “Downhole measurements”). Between 0 and 20 m CSF-A, the total NGR counts show a large step increase from 15 to 40 cps and then slightly increase to 70 m CSF-A. Although the total NGR counts show strong cyclicity between 20 and 125 m CSF-A, the highest NGR counts (82 cps) occur at ~70 m CSF-A, corresponding to a maximum in GRA bulk density. NGR counts gradually decrease between 70 and 125 m CSF-A, which coincides with the lithologic Subunit IIA/IIB boundary and also approximates the depth at which the logging Subunit LIa/LIb boundary has been placed (see Fig. F38). The sharp decrease from 40 to 17 cps near 103 m CSF-A coincides with the Unit I/II boundary and the depth of decreasing GRA bulk density and magnetic susceptibility. As discussed at Site U1422, these variation patterns of NGR may be explained by increased U associated with organic-rich layers in Unit I. This interpretation is in agreement with the downhole spectral gamma ray measurements performed in Hole U1423B, evidencing a higher mean uranium content above ~103 mbsf compared to the rest of the hole (see Fig. F38). Subsequently, low NGR counts remain stable to 180 m CSF-A with relatively less scatter. NGR counts again slightly decrease between 180 and 200 CSF-A and then inversely increase to the bottom of the hole.

Compressional wave velocity

Compressional P-wave velocity was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1423A and U1423B. Although P-wave velocity is generally measured with the WRMSL after the sections reach thermal equilibrium with the ambient room temperature of ~20°C because P-wave velocity is significantly affected by temperature (Shumway, 1958), P-wave velocity at Site U1423 was measured before temperature equilibrium (~12°C) for quick stratigraphic correlation. Therefore, we remeasured P-wave velocity after temperature equilibrium (~20°C) in Cores 346-U1423B-3H, 7H, and 16H to compare with P-wave velocity measured before temperature equilibrium (~12°C). The two sets of P-wave velocity values matched well (Fig. F34) with average maximum differences of 5 m/s (i.e., Core 3H was 1509 m/s before and 1514 m/s after, Core 7H was 1517 m/s before and 1519 m/s after, and Core 16H was 1525 m/s before and 1526 m/s after). However, because of poor sediment-to-liner coupling or the influence of small cracks in the relatively stiff and brittle sediment, results from the WRMSL include significantly higher or lower values than a typical data set. These values were removed manually, and then the P-wave velocity was combined as one data set of values from each hole (Fig. F33).

P-wave velocity at Site U1423 varies from 1451 to 1570 m/s (average = 1524 m/s) and generally increases with depth. Although P-wave velocity also shows relatively higher variation above 103 m CSF-A relative to the lower part, the trend is not clear enough to reflect the lithologic changes.

Vane shear stress

Undrained shear strength of soft sediment in the working half of the core was measured using an analog vane shear device. Shear strength ranges from 5.5 to 126 kPa and generally increases with depth (Fig. F35). In spite of the small scatter in the values, the curve does not show general trends that can be divided into successive intervals, although shear strength values at some depths (e.g., ~56 and ~189 m CSF-A) reflect well the composition of sediment. Although the stratigraphic distance between two measurements obtained from a black organic layer (56.45 m CSF-A) and a paired light green hemipelagic layer (56.55 m CSF-A) of Core 346-U1423A-7H is only 10 cm, shear strength sharply increases from 39.6 to 50.1 kPa in this interval. Shear strength abruptly decreases at 189 m CSF-A, where a highly diatomaceous layer occurs. The high fluid content in the diatomaceous layer at this depth is likely responsible for this decrease in shear strength.

Diffuse reflectance spectroscopy

Color reflectance data measured on the split archive-half sections show high variation of color, especially between 0 and 100 m CSF-A (Fig. F36). The L*, a*, and b* values represent lightness, red-green ratio, and yellow-blue ratio, respectively. These variations in color reflectance are closely related to lithologic changes. For example, the lithologic characteristic of Unit I, which consists of alternating very dark brown to black organic-rich sediment and lighter olive and green hemipelagic sediment, is responsible for the high variation of color reflectance. As was the case at Site U1422, as the dark bands fade out downhole in Unit II the color reflectance of L* and a* shows little variation to the bottom of the hole.

Summary

Physical properties measured at Site U1423 generally show trends that follow lithostratigraphy. Magnetic susceptibility, bulk density, and NGR have higher values in lithologic Unit I than in Unit II, whereas porosity and water content show opposite trends. P-wave velocity and shear strength gradually increase with depth because of sediment compaction. Color reflectance shows higher variation in lithologic Unit I than in Unit II, and the variations are closely related to the lithology of Unit I, which consists of alternating very dark brown to black organic-rich bands and lighter olive to green hemipelagic sediment.

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