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doi:10.2204/iodp.proc.346.106.2015

Physical properties

Physical properties measurements at Site U1425 were conducted to provide high-resolution data on the bulk physical properties and their downhole variations in Holes U1425B and U1425D using the same plan as at previous Sites U1423 and U1424. P-wave velocity that was measured in Sections 1, 2, and 3 of each core with the WRMSL was consolidated as one data set using measurements from Holes U1425B and U1425D.

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 300 m CSF-A on the working halves of Hole U1425B. Moisture and density (MAD) measurements were performed on discrete core samples (most commonly 1 or 2 per core) collected from the working halves of Hole U1425B and the lower part of Hole U1425D. We also collected MAD samples and accompanying shear stress data from pairs of organic-rich and hemipelagic sediment in Unit I in order to compare the physical properties of two types of sediment layers. Diffuse spectral reflectance (most commonly at 1 cm intervals) and point magnetic susceptibility (most commonly at 2 cm intervals) were measured using the SHMSL on the archive halves. Physical properties measurements are presented in Figures F55, F56, F57, and F58.

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) and decrease from 0 to ~150 m CSF-A before increasing to the bottom of the recovered section. This broad minimum at ~150 m CSF-A corresponds to lithologic Unit II with low gamma ray attenuation (GRA) bulk density suggesting a compositional control on this parameter.

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 lithologic unit (Fig. F55; see “Lithostratigraphy”). Between 0 and ~200 m CSF-A, GRA bulk density at Site U1425 is largely similar in pattern to previous Sites U1422–U1424. GRA bulk density is highly variable in the uppermost part (between 0 and 69 m CSF-A), roughly coinciding with lithologic Unit I, with values ranging from 1.2 to 1.7 g/cm³ (Fig. F55). As at previous sites, high variations of GRA bulk density dominate in Unit I with alternating very dark brown to black organic-rich sediment and lighter olive and green hemipelagic sediment. Low-value GRA bulk density remains stable between ~90 and ~180 m CSF-A, with relatively less scatter. Deeper than ~180 m CSF-A, GRA bulk density gradually increases with depth to ~330 m CSF-A, which is approximately consistent with the lithologic Subunit IIIA/IIIB boundary. From ~330 to ~340 m CSF-A, GRA bulk density increases abruptly, and the highest values of GRA bulk density occur from ~340 m CSF-A to the bottom of the hole with high scatter. This high scatter is closely related to biscuited and possibly disturbed sediment between ~340 m CSF-A and the bottom, which had limited recovery. The above GRA bulk density trends at Site U1425 correlate well with the density log acquired in open Hole U1425B (see Fig. F59 and “Downhole measurements”).

Although discrete wet bulk density and grain density are relatively constant for the entire core interval, ranging from 1.2 to 1.8 g/cm³ and from 2.3 to 3.1 g/cm³, respectively, the primary trends agree well with GRA bulk density (Fig. F56). Porosity and water content show generally reversed trends when compared to density, ranging from 53.1% to 87.4% and from 31.0% and 75.9%, respectively. The higher resolution MAD measurements performed in the uppermost part (between 0 and 50 m CSF-A) in organic-rich versus organic-poor hemipelagic sediment (Figs. F56, F57) show the large range of variability for discrete wet bulk density, grain density, porosity, and water content in Subunit IA. Discrete bulk density and grain density of organic-rich sediment (average = 1.38 and 2.61 g/cm³, respectively) are lower than hemipelagic sediment (average = 1.46 and 2.76 g/cm³, respectively), whereas porosity and water content (average = 77.6% and 57.7%, respectively) are higher than hemipelagic sediment (average = 74.8% and 52.6%, respectively). These differences in physical properties of two sediment layers are probably related to sediment texture because grain density between two sediment layers shows the largest difference.

Subsequently, discrete bulk density and grain density generally decrease with depth to ~180 m CSF-A, where porosity and water content of the sediment increase, except in an interval of relatively high bulk density and low porosity and water content between 78 and 82 m CSF-A. The decrease in bulk density and increase in porosity and water content with depth, which is contrary to the typical trends in marine sediment, is closely related to the downhole increase in diatom silica as discussed at Site U1424. Between ~180 and ~330 m CSF-A, discrete bulk density increases, whereas porosity and water content decrease with depth. At ~330 m CSF-A coinciding with the lithologic Subunit IIIA/IIIB boundary, discrete bulk density, ranging between 1.2 and 1.5 g/cm³ shallower than ~330 m CSF-A, increases rapidly to an average of 1.7 g/cm³, whereas porosity and water content show large step decreases averaging 14.5% and 16.7%, respectively.

These abrupt changes of GRA and discrete bulk density, porosity, and water content at ~330 m CSF-A strongly suggest that the lithologic Subunit IIIA/IIIB boundary corresponds to the diagenetic transition from opal-A to opal-CT. This transition is confirmed by both XRD and biogenic silica dissolution as seen in smear slides (see “Lithostratigraphy” and “Geochemistry”).

Magnetic susceptibility

Magnetic susceptibility at Site U1425 shows a different pattern compared to previous Sites U1422–U1424 (Fig. F55). Low magnetic susceptibility is typical for the entire drilled sequence without a high magnetic susceptibility peak in the uppermost part. The initial lack of magnetic carriers or perhaps their diagenetic dissolution may explain the poor paleomagnetic records at this site (see “Paleomagnetism”). Point magnetic susceptibility from the SHMSL closely tracks whole-core magnetic susceptibility with mean values for the site between 0 and 23 × 10^–5 SI. Magnetic susceptibility linearly decreases with depth from 0 to 134 m CSF-A except for a small step decrease near ~85 m CSF-A. Magnetic susceptibility only appears to be highly variable between ~134 and ~322 m CSF-A because the values are so low. Relatively high magnetic susceptibility, ranging from 8 × 10^–5 to 23 × 10^–5 SI, occurs from 338 m CSF-A to the bottom of the hole because the diatom ooze/diatom-rich clay is replaced by siliceous claystone at the bottom of lithologic Subunit IIIA and the top of Subunit IIIB (see “Lithostratigraphy”).

Natural gamma radiation

The variation patterns of NGR are conformable with GRA bulk density (Fig. F55) and correlate well with the total gamma ray log acquired in open Hole U1425B (Fig. F59). Between 0 and 10 m CSF-A, the total NGR counts show a large step increase from 10 to 57 cps and then gradually increase to 50 m CSF-A (Fig. F55), coinciding with the lithologic Subunit IA/IB boundary, with strong cyclicity. As discussed at previous sites, these variation patterns of NGR may be explained by increased uranium content associated with organic-rich layers in Unit I. This is supported by the downhole spectral gamma ray measurements, which indicate, despite the through-pipe attenuation of the signal, a higher mean uranium content shallower than ~50 mbsf compared to the deeper interval (~50–80 mbsf) (Fig. F60). After sharply decreasing at 50 m CSF-A, NGR counts decrease to 94.3 m CSF-A, which corresponds to the lithologic Unit I/II boundary, and then low NGR counts remain stable to 187 m CSF-A with relatively less scatter. These meter-scale patterns of the total NGR counts in Unit I are very similar to the ones at previous Sites U1422–U1424, exhibiting a one-to-one correlation. Subsequently, NGR counts increase between 187 and 253 m CSF-A and then decrease to 322 m CSF-A. Deeper than ~322 m CSF-A, NGR counts largely increase and show the highest values, ranging between 45 and 65 cps, in the opal-A–opal-CT transition zone.

Compressional wave velocity

Compressional P-wave velocity was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1425B and U1425D following the same strategy as at Site U1423. 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 velocities were combined as one data set of values from each hole (Fig. F55). P-wave velocity at Site U1425 varies from 1470 to 1740 m/s (average = 1545 m/s) and generally increases with depth. P-wave velocity shows a similar trend pattern with GRA bulk density in lithologic Unit I, inversely decreasing to 68 m CSF-A after increasing between 0 and 50 m CSF-A. Subsequently, although P-wave velocity gradually increases to 322 m CSF-A with a little variation, the trend is not clear enough to reflect the lithologic changes. P-wave velocity, however, largely increases from 1560 to 1625 m/s between 322 and 341 m CSF-A, which coincides with the lithologic Subunit IIIA/IIIB boundary. The highest value P-wave velocities occur with high scatter deeper than 340 m CSF-A, in the opal-A–opal-CT transition zone and the lowest part of Subunit IIIB.

Vane shear stress

Shear stress measurements were performed (generally one per core) from 0 to 300 m CSF-A on the working halves of Hole U1425B using an analog vane shear device. Deeper than 300 m CSF-A, high sediment strength did not allow measurements. Although shear strength was generally measured one per core, higher resolution measurements were performed between 0 and 50 m CSF-A to compare the shear strength of organic-rich and hemipelagic sediment. Shear strength ranges from 4.4 to 118.9 kPa and generally increases with depth (Fig. F56). Between 0 and 50 m CSF-A, shear strength linearly increases from 4.5 to 31.2 kPa in lighter olive and green hemipelagic sediment, whereas shear strength measured on very dark brown to black organic-rich sediment shows higher average values (~5.2 kPa). This higher shear strength encountered in organic-rich sediment may be explained by sediment texture and possible differential compaction. Deeper than ~80 m CSF-A, shear strength is highly scattered and does not reflect the lithologic units. This scattering shear strength may be related to the highly diatomaceous layers composing lithologic Unit II. As discussed at previous sites, the highly diatomaceous layers often show low shear strength values.

Diffuse reflectance spectroscopy

Similar to previous drilled sites, color reflectance data measured on the split archive-half sections at Site U1425 show high variability, especially in Subunit IA, reflecting the variegated colors of the decimeter- to centimeter-scale lithologic packages (Fig. F58). 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 sediment occurs here and alternates with lighter olive and green hemipelagic sediment. Although the dark bands fade out downhole below Subunit IA, the color reflectance shows little variation to the bottom of the hole.

Summary

Physical properties measured at Site U1425 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 an opposite trend. P-wave velocity and shear strength generally increase with depth because of sediment compaction, although shear strength data are highly scattered in the highly diatomaceous layers deeper than ~80 m CSF-A. Color reflectance shows higher variation 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 were largely changed at the opal-A–opal-CT transition zone, when entering in Subunit IIIB.

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