Previous   |    Next

doi:10.2204/iodp.proc.346.110.2015

Physical properties

Physical properties measurements at Site U1430 were conducted to provide high-resolution data on the bulk physical properties and their downhole variations in Holes U1430A–U1430C. 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 each split core was reserved for archiving and the other half was for analysis and sampling (working half). Shear stress measurements were performed (most commonly one per core) from 0 to 240 m CSF-A on the working halves of Hole U1430A. Moisture and density (MAD) measurements were performed on discrete core samples (most commonly one or two per core) collected from the working halves of Hole U1430A. Diffuse spectral reflectance (most commonly at 2 or 5 cm intervals) and point magnetic susceptibility (most commonly at 2 or 5 cm intervals) were measured using the SHMSL on the archive halves. Physical properties measurements are presented synthetically in Figures F43, F44, F45, F46, and F47.

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 0.82 to 1.11 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. In general, thermal conductivity values are high (0.97–1.11 W/[m·K]) in lithologic Unit I and Subunit IIA (shallower than 73.1 m CSF-A for Hole U1430A) and low (0.82–0.91 W/[m·K]) in lithologic Subunits IIB and IIIA (deeper than 73.1 m CSF-A for Hole U1430A). Some positive peaks with values of ~1.00 W/(m·K) occur in Subunit IIIA with similar values to lithologic Unit I and Subunit IIA.

Moisture and density

Although measurement errors exist in GRA bulk density data because of the presence of air between a core and a core liner, in general, GRA bulk density tends to reflect the characteristic of each lithology (Fig. F43; see “Lithostratigraphy”). GRA bulk density at Site U1430 is largely similar in pattern to Site U1425 for Subunits IA, IB, and IIA (Fig. F44). Such similarity is not kept for Subunits IIB and IIIA because of a potential hiatus within lithologic Subunit IIB at Site U1430 (see “Biostratigraphy”). GRA bulk density is highly variable in the uppermost part (between 0 and 57 m CSF-A) coinciding with lithologic Unit I, with values ranging from 1.2 to 1.7 g/cm³ (Fig. F43). As at previous sites, high GRA bulk density values and variability predominate in Unit I, reflecting alternating very dark brown to black organic-rich and lighter olive and green hemipelagic sediment. GRA bulk density values sharply drop at the lithologic Unit I/II boundary, and then increase gradually downhole in Subunit IIA, reaching maximum values (1.6 g/cm³) around ~75 m CSF-A before decreasing to ~1.3 g/cm³ within lithologic Subunit IIB. GRA bulk density values remain low and stable between 82 and 157 m CSF-A (Subunit IIB) with a slight overall increase downhole. After a small step decrease and increase between 157 and ~170 m CSF-A, GRA bulk density gradually decreases with depth to ~230 m CSF-A with relatively low scatter. These changes of GRA bulk density in Subunit IIIA are probably related to alternation between clay-rich and biogenic component (mainly nannofossil and diatom)–rich sediments (see “Lithostratigraphy”). Relatively lower bulk density values dominate in biogenic component–rich sediment layers. From 230 m CSF-A to the bottom of the hole, GRA bulk density increases abruptly, and the highest value (up to 1.8 g/cm³) of GRA bulk density occurs at ~250 m CSF-A. These abrupt changes of GRA bulk density between ~230 and ~250 m CSF-A may reflect the diagenetic transition from opal-A to opal-CT, which defines Subunit IIIA/IIIB boundary for Sites U1430 and U1425 (Fig. F44). This transition was also confirmed by XRD (see “Lithostratigraphy”). The GRA bulk density trends at Site U1430 correlate well with density log acquired in open Hole U1430B with the exception of the bottom part of the hole (see Fig. F49 and “Downhole measurements”).

Although discrete wet bulk density and grain density are relatively constant for the entire core interval, ranging from 1.2 to 1.7 g/cm³ and from 2.3 to 2.8 g/cm³, respectively, the primary trends agree well with GRA bulk density (Fig. F45). Porosity and water content show generally reversed trends when compared to density, ranging from 59.4% to 84.6% and from 35.5% to 70.0%, respectively. Discrete bulk density and grain density increase with depth in Subunit IA (decrease in porosity and water content) similar to GRA bulk density. Subsequently, discrete bulk density and grain density generally decrease with depth to ~230 m CSF-A, where porosity and water content of the sediment increase. These decreases in bulk density and increase in porosity and water content with depth, which is contrary to the typical trends in marine sediment, are closely related to the downward increase in diatom silica as discussed at Sites U1424 and U1425. At ~245 m CSF-A, coinciding with the Subunit IIIA/IIIB boundary, discrete bulk density, ranging between 1.2 and 1.5 g/cm³ in Subunit IIIA, increases rapidly to 1.7 g/cm³, whereas porosity and water content show large step decreases of 15.1% and 19.4%, respectively.

Magnetic susceptibility

Whole-core magnetic susceptibility values Site U1430 are consistently low downhole typically below 10 × 10^–5 SI, with the exception of several high magnetic susceptibility values in Unit I (Fig. F43). Although magnetic susceptibility at Site U1430 shows low variability, the downhole trend agrees well with GRA bulk density and varies with lithologic changes. Point magnetic susceptibility from the SHMSL closely tracks whole-core magnetic susceptibility, with mean values for the site between 0 and 15 × 10^–5 SI. High magnetic susceptibility peaks in Unit I agree well with the depth of tephra/ash layers (see “Lithostratigraphy”) due to highly magnetic authigenic minerals within tephra/ash layers. Low magnetic susceptibility remains stable in Units II and III, with relatively less scatter, and then also with large increases at the Subunit IIIA/IIIB boundary. This may be related to the diatom ooze/diatom-rich clay being replaced by siliceous claystone at the bottom of Subunit IIIA and the top of Subunit IIIB (see “Lithostratigraphy”).

Natural gamma radiation

NGR shows strong cyclicity that is similar to the GRA bulk density cyclicity (see also “Downhole measurements”). As these conformable variation patterns with GRA bulk density suggest that their controls are closely related, NGR also shows the trend pattern of lithologic changes, alternating between very dark brown to black organic-rich bands and lighter olive to green hemipelagic sediment. Between 0 and 8 m CSF-A, the total NGR counts show a large step increase from 20 to 55 cps and then a gradual increase to ~45 m CSF-A (Fig. F43). This coincides with the Subunit IA/IB boundary, with strong cyclicity. As discussed at previous sites, these variation patterns of NGR may be explained by high uranium content associated with organic-rich layers in Unit I. After sharply decreasing at 58 m CSF-A, which corresponds to the Unit I/II boundary, NGR counts increase and then decrease again between 58 and 82 m CSF-A. Deeper, NGR gradually increases to 157 m CSF-A. Subsequently, NGR counts generally decrease to ~230 m CSF-A and then increase from 10 to 70 cps between ~230 and ~250 m CSF-A.

Compressional wave velocity

Compressional P-wave velocity was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1430A–U1430C and then combined as one data set of values for this site. P-wave velocity at Site U1430 varies from 1488 to 1726 m/s (average = 1540 m/s) and generally increases with depth. The trend, however, is not clear enough to reflect the lithologic changes because of the lack of a data set for the entire site. In Unit I and Subunit IIA, high P-wave velocity is related to coarser grained tephra/ash layers.

Vane shear stress

Shear stress measurements were performed (generally one per core) from 0 to 240 m CSF-A on the working halves of Hole U1430A using an analog vane shear device. Shear strength ranges from 6.3 to 192.1 kPa and generally increases with depth (Fig. F45). Between 0 and ~50 m CSF-A, shear strength linearly increases from 6.3 to 38.2 kPa, and shear strength values show relatively high scatter with an overall increase to ~180 m CSF-A. Deeper, shear strength linearly decreases to 15 kPa and then largely increases to its maximum. This scattering shear strength in the lower part of the hole may be related to the highly diatomaceous layers or microfractures within cores.

Diffuse reflectance spectroscopy

Color reflectance data measured on the split archive-half sections at Site U1430 are little different from previously drilled Site U1425 (Fig. F46). Although there is high variability, reflecting the lithologic change alternating between very dark brown to black organic-rich bands and lighter olive to green hemipelagic sediment, occurring in the physical properties within Unit I, the cyclicity of color reflectance at Site U1430 is much weaker than at Site U1425. Below Subunit IIA, color reflectance remains stable to the bottom of the hole, whereas the color bands fade out downhole below Unit I. The trend of luminance (L*) in this interval is much lower than the trend at Site U1425. Because of this, although Site U1425 shows higher variability in Unit I, the mean value of L* at Site U1430 (average = 31.5) is lower than Site U1425 (average = 34.0) (Fig. F47). This indicates that Site U1430 consists of more dark colored sediment. Parameters a* and b* combined show the variable presence of primarily yellowish–blueish compounds.

Summary

Physical properties measured at Site U1430 generally show trends that are similar to Site U1425 for Subunits IA, IB, and IIA, following sediment lithology. Magnetic susceptibility, bulk density, and NGR have higher values in Unit I than in Unit II, whereas porosity and water content show opposite trends. Magnetic susceptibility and P-wave velocity also show high peak values, which agrees well with the depth of tephra/ash layers in Unit I. Shear strength generally increases to ~180 m CSF-A with depth because of sediment compaction, then decreases to ~230 m CSF-A, and finally shows a large increase between ~230 and ~240 m CSF-A. Color reflectance shows higher variation in physical properties in 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. All physical property values exhibit a large change at ~240 m CSF-A, which is assumed as the opal-A/opal-CT boundary transition zone.

Top of page   |    Previous   |    Next