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doi:10.2204/iodp.proc.346.107.2015

Physical properties

Site U1426 is a redrill of Site 798 (Ingle, Suyehiro, von Breymann, et al., 1990) on the Oki Ridge. Compared to the previously drilled Sites U1422–U1425, Site U1426 is much shallower and is protected from the input of coarse sediment because of its ridge location. As such, Site U1426 presents some unique physical properties that contrast with the pattern seen at the previous deeper sites, but it also has enough similarities to provide context for the overall regional interpretation. Better preservation of calcium carbonate together with increased organic carbon (see “Geochemistry”) and terrigenous clay input results in a more homogeneous sedimentary sequence in terms of physical properties compared to previously drilled sites. Dissolution of silica may affect physical properties in the lower part of the sequence (deeper than 300 m CSF-A). The same suite of whole- and split-core logging as well as discrete sample properties that was measured at previous sites was also measured at Site U1426; the results are presented in Figures F43, F44, F45, F46, F47, and F48. We adopt the lithostratigraphic unit definitions in the discussion because the physical properties are largely an expression of lithology.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually near the middle of Section 4. Overall, thermal conductivity values range from 0.6 to 1.7 W/(m·K) without a clear increasing trend with depth. However, thermal conductivity follows porosity and gamma ray attenuation (GRA) bulk density, and thus, in part, lithology, with broad peaks over ~1 W/(m·K) in the denser layers at the bottom of the site (i.e., the uppermost at 320 m CSF-A and the second at 400 m CSF-A).

Moisture and density

GRA bulk sediment density at Site U1426 is largely similar in pattern to the equivalent age sediment in Site 798 but differs significantly from Sites U1422–U1425. Unit I displays strong high-frequency variability (i.e., decimeter to multimeter scale in Subunit IA and largely multimeter scale in Subunit IB) within a general range between 1.2 and 1.6 g/cm³ (Fig. F43). The upper limit of this range is significantly lower than at previous deeper sites despite an increase in carbonate in this unit at Site U1426. Variability in Unit I derives from alternating light colored clay-rich layers with dark colored organic carbon–rich layers (see “Lithostratigraphy”). Further downhole in Unit II, instead of being attenuated as at deeper water sites, variability continues and is expressed as multimeter-long cycles driven by alternating heavy clay-rich and lighter colored diatom-rich sediment. Successive increases in GRA bulk density with muted shorter scale variability possibly linked to lithification processes are also evident in Unit II and are separated by a broad local minimum in GRA bulk density at ~350 m CSF-A.

Discrete wet bulk density and derived parameters (i.e., porosity and water content) agree well with the primary trends in GRA bulk density (Figs. F44, F45), varying with lithology. Although porosity (and derived water content) decreases (increases) downhole, wet bulk and grain density display no clear trend. The relatively high carbonate content in the upper part of Unit I and the largely decreased carbonate content deeper than 170 m CSF-A (see “Lithostratigraphy”) also have no dramatic influence on the trend of discrete wet bulk and grain density. Comparing with XRD opal-A counts, discrete wet bulk and grain density show a contrary relationship with the opal-A counts trend (Fig. F45). Higher density values occur in low opal-A counts, whereas lower density values dominate in high opal-A counts. Therefore, although high carbonate occurs in Unit I, lithologic change alternating between clay-rich and biogenic component–rich (diatom, siliceous material, and nannofossil) sediment may be more responsible for these variations of density at Site U1426.

A comparison of high-resolution moisture and density (MAD) data in Hole U1426B collected for studies of pore water diffusion (see “Geochemistry”) with low-resolution MAD data in Hole U1426A (Fig. F46) indicates that much of the fine structure in physical properties cannot be observed without higher frequency sampling. This underscores the need for shore-based studies to derive physical properties at high resolution by calibration of track to MAD data.

Magnetic susceptibility

Whole-core magnetic susceptibility as well as point magnetic susceptibility (SHMSL) show consistently low values downhole, typically <10 × 10^–5 SI (Fig. F43). Although strong diagenetic processes at this site could affect magnetic minerals (see “Geochemistry”), a relatively good paleomagnetic signal is recorded (see “Paleomagnetism”). This suggests robust contributions of less diagenetically active terrigenous magnetic carriers.

Natural gamma radiation

NGR shows strong cyclicity (Figs. F43, F45) that parallels the GRA bulk density cyclicity downhole (see above), suggesting that their controls are closely related. NGR in Unit I is less driven by U associated with organic matter as at the deeper sites (see “Lithostratigraphy”). NGR counts are significantly lower than at previous deeper sites, ranging from 10 to 60 cps.

Compressional wave velocity

Compressional P-wave velocity was only measurable in the upper 12 m CSF-A at this site because of degassing (Fig. F43). Meter-scale cyclicity is evident, following cycles in GRA bulk density with a velocity range between 1475 and 1550 m/s.

Vane shear stress

Undrained shear strength shows a variable but steady increase from the seafloor to ~300 m CSF-A through all lithologic units. Values reach a maximum of ~120 kPa (Fig. F44), deeper than ~300 m CSF-A sediment becomes too stiff for shear strength measurements.

Diffuse reflectance spectroscopy

Color reflectance data measured on the split archive-half sections at Site U1426 are distinctly different from the previously drilled sites, especially in Unit I (Fig. F47). Although L*, a*, and b* show the highest variability in Unit I as at previous sites, this variability is not only caused by the alternating dark organic-rich and greenish organic-poor lithologic packages but also by a significant increase in calcium carbonate and possibly a different diagenetic state for the organic matter. This can be observed in the range of L* that extends primarily into the lighter domain relative to the average site value, compared to previous sites where L* in the time-equivalent Unit I extended primarily into the darker domain (Fig. F48). Parameters a* and b* combined indicate the variable presence of primarily yellowish compounds, particularly in dark organic-richer layers from the seafloor all the way to the boundary of Unit II, where they become cyclical at ~10–20 m scale. In contrast, the previous sites showed a larger dynamic range for a*, leading to a more diverse hue spectrum.

Summary

Physical properties at Site U1426 are significantly different from previous sites but preserve a similar time-correlative Unit I that records higher variability in lithologic composition and thus physical properties. In this unit, cyclical physical properties appear to be driven not only by the binary mixture of organic matter–hemipelagic sediment, but also by the addition of carbonate (see “Geochemistry”) and possibly a decline in authigenic pyrite. This new type of mixture drives the density as well as color characteristics at the site. Compared to previous sites, clay present in a higher proportion appears to overtake organic matter in the main role in influencing NGR variability. Magnetic susceptibility is minimal, but magnetic carriers appear sturdy enough to preserve a good paleomagnetic signal (see “Paleomagnetism”).

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