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doi:10.2204/iodp.proc.346.108.2015

Physical properties

Physical properties measurements at Site U1427 were conducted to provide high-resolution data on the bulk physical properties and their downhole variations in Holes U1427A, U1427B, and U1427C. 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. One half of the split core was reserved for archiving and the other half was for analysis and sampling (working half). Shear stress measurements were performed (most commonly two per core) from 0 to 200 m CSF-A on the working halves of Hole U1427A. Moisture and density measurements were performed on discrete core samples (most commonly one or two per core) collected from the working halves of Hole U1427A. Color reflectance (most commonly at 5 cm intervals) and point magnetic susceptibility (most commonly at 5 cm intervals) were measured using the SHMSL on the archive halves. Physical properties measurements are presented synthetically in Figures F46, F47, F48, and F49.

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 ~1.2 W/(m·K) without any clear trend downhole. The relatively large scatter of data suggests a wide variety of sediment properties controlled by the relative abundances of detrital and biogenic materials.

Moisture and density

Although measurement errors exist in gamma ray attenuation (GRA) bulk density data because of the presence of air between a core and a core liner, in general the GRA bulk density reflects the core’s lithologic characteristic (Fig. F46; see “Lithostratigraphy”). GRA bulk density ranges from 1.11 g/cm³ to values >1.90 g/cm³ and generally increases with depth. A minimum value was determined at ~228 m CSF-A, and a maximum value was measured at ~478 m CSF-A. GRA bulk density at Site U1427 differs significantly from previous sites. The higher GRA bulk density values continue to the bottom of the hole with high variability. The mean GRA bulk densities at previous Sites U1422–U1426 were 1.37, 1.42, 1.40, 1.41, and 1.49 g/cm³, respectively, whereas the mean GRA bulk density at Site U1427 is 1.62 g/cm³. This mean higher bulk density may be related to relatively higher abundance of coarse terrigenous material supplied from the nearby continental shelf and upper continental slope. Although GRA bulk density at previous sites reflected well the lithologic unit boundaries, GRA bulk density at this site shows a strong relationship with the lithologic changes alternating between heavier clay-rich and lighter biogenic components (mainly nannofossil- and diatom-rich sediment) (see “Lithostratigraphy”). Although GRA bulk density generally increases with depth because of sediment compaction, higher bulk density values (ranging from 1.55 to 1.99 g/cm³) occur in intervals corresponding to clayey silt and silty clay sediment, whereas relatively lower bulk density values (ranging from 1.31 to 1.66 g/cm³) dominate in biogenic component–rich sediment layers. This relationship between GRA bulk density and lithologic change becomes more evident when comparing with color reflectance b* values (yellow-blue ratio, Fig. F47). Higher GRA bulk density values are matched with blueish compounds of b*, with high NGR values. In long term trend, GRA bulk density generally increases between 0 and ~220 m CSF-A. At ~228 m CSF-A a minimum value of GRA bulk density (1.11 g/cm³) appears to correlate with to a thick tephra/ash layer. GRA bulk density slightly decreases to ~400 m CSF-A and then increases again to ~480 m CSF-A. The low density values recorded deeper than ~482 m CSF-A may result from drilling disturbance caused by the change from the APC to XCB coring system. The above GRA bulk density trends at Site U1427 correlate well with density log acquired in open Hole U1427A (see Fig. F47 and “Downhole measurements”).

Although discrete wet bulk density and grain density are relatively constant for the entire core interval, ranging from 1.42 to 1.84 g/cm³ and from 2.50 to 2.77 g/cm³, respectively, the primary trends agree well with GRA bulk density (Fig. F48). Porosity and water content show generally reversed trends when compared to density, ranging from 51.4% to 73.8% and from 29.3% to 53.4%, respectively. Discrete bulk density and grain density generally increase with depth where porosity and water content of the sediment decrease. Although Site U1427 has relatively higher discrete bulk density and lower porosity (and derived water content) throughout its entire depth than at previous sites, the trends also reflect well the lithologic changes alternating between clay-rich and biogenic component–rich sediment.

Magnetic susceptibility

Whole-core magnetic susceptibility at Site U1427 shows consistently low values downhole, typically <15 × 10^–5 SI (Fig. F46). Point magnetic susceptibility from the SHMSL closely tracks whole-core magnetic susceptibility. Although the mean values stay between 10 × 10^–5 and 15 × 10^–5 SI for this site, high magnetic susceptibility maxima (up to 50 × 10^–5 SI) occur at 7, 205, 228, 291, and 458 m CSF-A. A magnetic susceptibility maximum occurs between 227 and 228 m CSF-A, where values as high as 370 × 10^–5 SI were measured. These occurrences of high magnetic susceptibility are due to highly magnetic authigenic minerals within tephra/ash layers (see “Lithostratigraphy”). Although magnetic susceptibility at Site U1427 shows low variability, the downhole trend agrees well with GRA bulk density and varies with lithologic changes alternating between clay-rich and biogenic component–rich sediment.

Natural gamma radiation

The NGR trends at Site U1427 correlate well with the gamma ray log acquired in open Hole U1427A (see Fig. F50 and “Downhole measurements”). Total NGR counts range from 7 to 65 cps and generally increase with depth (Fig. F46). A minimum value was determined at ~228 m CSF-A, coinciding with a thick tephra/ash layer, and a maximum value was measured at ~478 m CSF-A. NGR shows strong cyclicity similar to the GRA bulk density cyclicity. As these conformable variation patterns with GRA bulk density suggest that their controls are closely related, NGR also shows the trend pattern related to the lithologic changes alternating between clay-rich (higher NGR values) and biogenic component–rich sediment (lower NGR values) (see also “Downhole measurements”). Total NGR counts increase with blueish compounds of b* and decrease with yellowish compounds of b* (Fig. F47). This correlation between NGR and color reflectance b* is more clear than the relationship to GRA bulk density.

Compressional wave velocity

Compressional P-wave velocity was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1427A and U1427B. Although compressional P-wave velocities were only measured in the upper 6.7 m CSF-A at this site because of degassing cracks, meter-scale cyclicity is evident, with a range of velocities between 1493 and 1564 m/s following cycles in GRA bulk density (Fig. F46).

Vane shear stress

Shear stress measurements were performed (generally two per core) from 0 to 200 m CSF-A on the working halves of Hole U1427A using an analog vane shear device. Deeper than 200 m CSF-A, the high sediment strength did not allow measurements. Shear strength ranges from 12.8 to 115.4 kPa and generally increases with depth (Fig. F48). Between 0 and ~36 m CSF-A, shear strength gradually increases from 12.8 to 36.4 kPa, and shear strength values are highly scattered in the deeper sediment. This scattering shear strength may be related to the highly diatomaceous layers or degassing sediment fractures.

Diffuse reflectance spectroscopy

Color reflectance data measured on the split archive-half sections at Site U1427 is distinctly different from the previously drilled sites. At previous sites, L*, a*, and b* show high variability caused by the alternating dark organic-rich and greenish organic-poor lithologic packages in the upper part of the stratigraphic sequence, whereas this site shows relatively low variability of color reflectance except b* (Fig. F49). This can be observed in the range of L* that only extends between values of 24 and 43, compared to Site U1425 where L* in Unit I extended between values of 17 and 53 (Fig. F51). Parameters a* and b* combined indicate the variable presence of primarily yellowish-blueish compounds. In contrast, Site U1425 showed a larger dynamic range for a*, leading to a more diverse hue spectrum (Fig. F51). The lithologic changes alternating between clay-rich and biogenic component–rich sediment are responsible for this high variability of b*. Although color reflectance L* and a* show relatively low variation, the trends agree well with the lithologic change as well as with b*.

Summary

Physical properties measured at Site U1427 generally show trends that follow the sediment lithology alternating between dense clay-rich and less dense biogenic component–rich sediment (see also “Downhole measurements”). Bulk density, NGR, and magnetic susceptibility show higher values in the clay-rich sediment interval, whereas lower values occur in the biogenic component–rich sediment interval. Porosity and water content show a trend opposite to bulk density and NGR. Color reflectance also reflects well these lithologic changes. At this site, color reflectance b*, representing the yellow-blue ratio, is a good indicator to identify the clay-rich and biogenic component–rich sediment and correlates well with the trends of bulk density and NGR.

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