Previous   |    Next

doi:10.2204/iodp.proc.346.105.2015

Physical properties

Site U1424 is a redrill of Site 794 (Tamaki, Pisciotto, Allan, et al., 1990), and therefore there are similarities between the two sites, although some differences do occur as described herein. Site U1424 is also largely analogous to the two previous Expedition 346 sites (U1422 and U1423) with a highly variable lithology in Unit I, with alternating organic-rich and hemipelagic sediment, and a more homogeneous hemipelagic Unit II (see “Lithostratigraphy”). Physical properties primarily reflect the lithology of the sediment and only secondarily reflect diagenetic or high-pressure/high-temperature processes, and therefore there was no need to define units independent of the lithologic units for this site. The same suite of whole-round and split-core logging as well as discrete sample properties that were measured at Site U1423 were also performed at Site U1424 and are presented in Figures F38, F39, and F40.

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 0.7 to 1.0 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 lows of ~0.8 W/(m·K) near the seafloor (i.e., uppermost 20 m CSF-A), in Subunit IIA, and at the bottom of Subunit IIB.

Moisture and density

GRA bulk sediment density at Site U1424 is largely similar in pattern to Site 794 as well as Sites U1422 and U1423. Unit I displays strong high-frequency variability (i.e., decimeter- to multimeter-scale) within a general range between 1.2 and 1.8 g/cm³ (Fig. F38). This variability is gradually suppressed in Subunit IIA and reaches a minimum in Subunit IIB. Subunit IIB is also characterized by a drop in density associated with a clear increasing trend with depth. As at previous sites, variability in Unit I appears to come from the massive very dark layers that exhibit lower bulk density than the rest of the sediment. However, an attempt to confirm this via discrete measurements was inconclusive (Fig. F39). Higher density values typical for Subunits IB and IIA are probably due to an increase of terrigenous materials from turbidites and a general increase in terrigenous fraction in hemipelagics (see “Lithostratigraphy”). The generally lower density observed in Subunit IIB likely reflects the increasing diatom content within this lithologic unit. Unlike the previously drilled sites, a sharp reduction in density that is noticeable in P-wave data occurs at ~138 m CSF-A in Hole U1424A and ~133 m CSF-A in Hole U1424B and suggests an abrupt change in sedimentary conditions. This shift correlates with a centimeter-scale tephra layer that represents a significant boundary between dark gray bioturbated sediment (above) and brown sediment (below), observed at 138.08 m CSF-A in Hole U1424A (Sample 346-U1424A-15H-6, ~28 cm) and at 133.24 m CSF-A in Hole U1424B (Sample 346-U1424B-15H-5, ~102 cm) (see “Lithostratigraphy”). This may or may not reflect a depositional hiatus (see “Lithostratigraphy”), as the age model may be too coarse to detect it (see “Biostratigraphy”). Discrete wet bulk density and derived parameters (i.e., porosity and water content) agree well with the primary trends in GRA bulk density (Fig. F39). Overall, bulk sediment density decreases with depth, which is contrary to the typical trend seen in marine sediment when subject to increased compaction and dewatering with depth. The decrease in density with depth can be explained by the downhole increase in diatom content. Diatoms have relatively high intraparticle porosity and pack well without breaking during compaction (Tada and Iijima, 1983; Tamaki, Pisciotto, Allan, et al., 1990). The downhole decrease in grain density is in agreement with this petrophysical model.

Magnetic susceptibility

Whole-core magnetic susceptibility closely tracks point magnetic susceptibility (SHMSL) with mean values for the site between 10 × 10^–5 and 20 × 10^–5 SI. Several additional horizons downhole reach to and above 100 × 10^–5 SI. In contrast to previous Sites U1422 and U1423, the SMT zone is not prominent at Site U1424, suggesting a weaker diagenetic influence on formation and destruction of magnetic minerals (in agreement with results from “Geochemistry”), leading to better preservation of the primary magnetic carriers (see “Paleomagnetism”).

Natural gamma radiation

NGR shows strong high-amplitude cyclicity in Subunit IA and gradually muted variability from Subunit IB to the bottom of the hole (Fig. F38). NGR counts range from 20 to 80 cps in Unit I and decrease to between 20 and 40 cps in Unit II. This was also recorded at the previous drilled sites. This variability in NGR may be explained by high uranium content associated with massive organic-rich layers in Unit I whereas Unit II is enriched in nonradioactive elements (diatoms and other siliceous components, see “Lithostratigraphy”). The meter-scale pattern of the total NGR counts is very similar to the ones at Sites U1422 and U1423. At the bottom of Subunit IIB at Site U1424, very low NGR counts are encountered and the transition to this state is very abrupt from the sediment immediately shallower, which could be suggestive of a hiatus that was not encountered at previous sites (see also discussion about GRA density above).

Compressional wave velocity

Compressional P-wave velocity was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1424A, U1424B, and U1424C following the same strategy as at Site U1423. The combined P-wave velocity data set (Fig. F38) shows meter-scale cyclicity on a clear increasing trend with depth. Velocity ranges between 1470 and 1570 m/s, and the general influence of the changing lithology is less prominent when compared to other physical properties. In the lowermost 20 m of the drilled hole, P-wave velocity is consistently and uniformly higher than in sediment, suggesting a possible hiatus (see above).

Vane shear stress

Undrained shear strength shows a stable increase from the seafloor to ~110 m CSF-A in Hole U1424 through all lithologic units, reaching a maximum value of ~100 kPa (Fig. F39). Shear strength abruptly decreases between 115 and 130 CSF-A, where a clay-rich interval occurs in the otherwise diatomaceous oozes of Subunit IIB.

Diffuse reflectance spectroscopy

Similar to the previous drilled sites, spectral reflectance data measured on the split archive-half sections at Site U1424 show high variability, especially in Unit I, reflecting the variegated colors of the decimeter- to centimeter-scale lithologic packages (Fig. F40). Subunit IA is generally characterized by high variability in luminance (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. An ~50 cm thick section of red and black sediment that occurs below the mudline (sediment/bottom water interface), and inferred to contain Fe to Mn (oxy)hydroxides, is clearly indicated by the a* parameter (see also “Geochemistry”). In Subunits IB and IIA, the decrease in the number of the dark layers can be seen in both L* and b*. Subunit IIB shows suppressed variability in all three colorimetric parameters, as the dark bands disappear altogether.

Summary

Physical properties at Site U1424 are largely similar to previous drilled sites with the exception of a possible hiatus at the bottom of the hole (see also “Lithostratigraphy”). Unit I, with cyclical physical properties driven by the occurrence of thick, massive organic-rich dark layers, transitions into Unit II, with more subdued variability in physical properties that is largely due to alternating hemipelagic and biogenic silica–rich sediment. Magnetic susceptibility is less influenced by redox processes than at previous sites, whereas P-wave and shear stress data show consistent increasing trends downhole because of less degassing. Reflectance data best capture the lithologic and diagenetic variability at this site at the decimeter- to centimeter-scale.

Top of page   |    Previous   |    Next