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doi:10.2204/iodp.proc.346.109.2015

Physical properties

Physical properties measurements at Sites U1428 and U1429 were conducted to provide high-resolution data on the bulk physical properties and their downhole variations in Holes U1428A, U1428B, U1429A, U1429B, and U1429C. 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 reserved for analysis and sampling (working half). Shear stress measurements were performed (most commonly one per core) on the working halves of Holes U1428A and U1429A. Moisture and density (MAD) measurements were performed on discrete core samples (most commonly one or two per core) collected from the working halves of Holes U1428A and U1429A. Diffuse spectral 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 F50, F51, F52, F53, F54, F55, and F56.

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 1.9 W/(m·K). Thermal conductivity follows porosity and GRA bulk density, and thus, in part, lithology. A slight increasing trend with depth shallower than ~180 m CSF-A is found in lithologic Unit A. Significant increasing of thermal conductivity values deeper than ~180 m CSF-A corresponds to the lithologic Unit A/B boundary.

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 the GRA bulk density at Sites U1428 and U1429 reflects the core’s lithologic characteristic (Figs. F50, F51; see “Lithostratigraphy”). GRA bulk density at Site U1428 ranges from 0.80 g/cm³ to values >2.10 g/cm³ and generally increases with depth (Fig. F50). A minimum value was measured at ~87 m CSF-A, and a maximum value was measured at ~170 m CSF-A. The values of GRA bulk density linearly increase from 0 to ~135 m CSF-A, generally coinciding with lithologic Unit A (see “Lithostratigraphy”). High peaks of GRA bulk density (up to 1.6 g/cm³) occur at 3, 16, 41, 55, and 114 m CSF-A. High values at 3, 16, and 41 m CSF-A are due to coarse-grained sediment in the bottom of tephra/ash layers, whereas fine-grained sand layers are responsible for other high GRA bulk density values at 55 and 114 m CSF-A. GRA bulk density largely decreases between 83 and 92 m CSF-A, where a ~9 m thick spongy or soupy tephra/ash layer exists. From ~135 m CSF-A, GRA bulk density increases abruptly, and the highest values of GRA bulk density occur to the bottom of the hole with high scatter. This interval (between 135 and 210 m CSF-A) of high GRA bulk density consists of almost sandy sediment and largely reflects lithologic Unit B. High scatter of GRA bulk density values in this interval is closely related to disturbed sandy sediment with limited recovery.

Although the detailed pattern is different, the primary trends of GRA bulk density at Site U1429 agree well with Site U1428 (Fig. F51). GRA bulk density at Site U1429 generally increases with depth, ranging from 1.02 to 2.09 g/cm³, high GRA bulk density values related to identical lithologic characteristics with Site U1428 occur at 3, 17, 48, 70, and 157 m CSF-A (see “Lithostratigraphy”). Site U1429 also shows low GRA bulk density values (<1.3 g/cm³) with high scatter between 115 and 122 m CSF-A because of spongy and soupy state of a thick tephra/ash layer. GRA bulk density largely increases from ~175 to the bottom of the hole, which generally reflects lithologic Unit B.

Discrete wet bulk density and derived parameters (i.e., porosity and water content) at Sites U1428 and U1429 agree well with the primary trends in GRA bulk density (Figs. F52, F53), varying with the lithology. At Site U1428, discrete wet bulk density increases with depth, ranging from 1.39 to 2.02 g/cm³, whereas porosity and water content show generally reversed trends when compared to density, ranging from 42.8% to 77.7% and from 21.8% to 57.4%, respectively. Grain density, however, is relatively constant for the entire core interval, ranging from 2.48 to 2.80 g/cm³. Discrete bulk density, porosity, and water content show large shifts close to the lithologic Unit A/B boundary, whereas grain density is not influenced. At the thick tephra/ash layer between 83 and 92 m CSF-A, discrete bulk density slightly increases, whereas grain density largely decreases with porosity and water content. This is related to the tephra/ash materials that have relatively low solid mass compared with solid volume. Because of this, although the tephra/ash layer consists of relatively coarse grains in this interval, grain density (calculated by mass and volume of solid) decreases with the increase of wet bulk density (primarily controlled by wet mass of sediment).

MAD data at Site U1429 also show a similar trend pattern with Site U1428 (Fig. F53). At ~180 m CSF-A close to the lithologic Unit A/B boundary, discrete bulk density, porosity, and water content show large changes. Although grain density is relatively constant downhole, low grain density values occur in tephra/ash layers at ~100 and ~115 m CSF-A, with decreases in porosity and water content.

Magnetic susceptibility

Whole-core magnetic susceptibility at Sites U1428 and U1429 shows consistently low values in lithologic Unit A, typically <15 × 10^–5 SI with the exception of several high magnetic susceptibility values (Figs. F50, F51). Point magnetic susceptibility from the SHMSL closely tracks whole-core magnetic susceptibility. Although the mean values stay between 5 × 10^–5 and 10 × 10^–5 SI for Unit A at Site U1428, high magnetic susceptibility maxima (up to 30 × 10^–5 SI) occur at 3, 43, 52, and 110 m CSF-A. These high magnetic susceptibility intervals are due to highly magnetic authigenic minerals within tephra/ash layers (see “Lithostratigraphy”). Magnetic susceptibility largely increases in the lithologic Unit A/B boundary, and a maximum occurs at ~165 m CSF-A where values up to 170 × 10^–5 SI were measured. Magnetic susceptibility at Site U1429 is also relatively constant between 5 × 10^–5 and 10 × 10^–5 SI for Unit A and largely increases from 178 m CSF-A to the bottom of the hole, coinciding with Unit B. High magnetic susceptibility values (up to 30 × 10^–5 SI) related to tephra/ash layers occur at 3, 52, 65, and 118 m CSF-A.

Natural gamma radiation

Total NGR counts at Sites U1428 and U1429 range from 13 to 59 cps and from 18 to 53 cps, respectively, and generally increase downhole with a closely similar trend pattern (Figs. F50, F51). Minimum values were determined in the top of the hole, and maximum values were measured in lithologic Unit B. NGR shows strong cyclicity, which is similar to the GRA bulk density cyclicity. Relatively high NGR counts occur at identical depths with high GRA bulk density. The upper ~130 m CSF-A show lower values compared to the rest of the hole, likely reflecting the abundance of nonradioactive elements within lithologic Unit A (calcareous nannofossils and other siliceous components, see “Lithostratigraphy”). The increased NGR counts deeper than ~130 m CSF-A may reflect the abundance of mica (and the common presence of other minerals such as K-feldspar) in the sands forming lithologic Unit B, although quartz (nonradioactive mineral) is the predominant mineral in this unit (see “Lithostratigraphy”).

Compressional wave velocity

Compressional P-wave velocity at Sites U1428 and U1429 was measured with the WRMSL in Sections 1, 2, and 3 of each core for Holes U1428A, U1428B, U1429A, U1429B, and U1429C and then combined as one data set of values for each site. Although compressional P-wave velocity was only measured in the upper ~50 m CSF-A at these two sites because of the poor sediment-to-liner coupling or the influence of small cracks in the relatively stiff and brittle sediment, meter-scale cyclicity is evident, following the cycles in GRA bulk density and NGR (Figs. F50, F51). The mean values of P-wave velocity at Sites U1428 and U1429 are 1527 and 1525 m/s, respectively. P-wave velocity largely increases up to 1600 m/s at ~3, ~16, and ~40 m CSF-A at Site U1428 and ~3, ~17, and ~50 m CSF-A at Site U1429. These depths of high P-wave velocity agree well with high GRA bulk density and NGR.

Vane shear stress

Shear stress measurements were performed (one per core) on the working halves of Holes U1428A and U1429A using an analog vane shear device. The shear strength at Site U1428 ranges from 5.8 to 65.7 kPa and linearly increases to ~103 m CSF-A (Fig. F52). Deeper than ~103 m CSF-A, shear strength shows little scatter and then largely decreases to ~6 kPa in lithologic Unit B because of disturbed sandy sediment. Site U1429 also shows a similar shear strength trend, with data ranging from 10 to 66 kPa (Fig. F53). After the linear increase to ~105 m CSF-A, shear strength shows relatively high scatter to the lithologic Unit A/B boundary.

Diffuse reflectance spectroscopy

Spectral reflectance data measured on the split archive-half sections at Sites U1428 and U1429 are distinctly different from the previously drilled sites. At previous sites, L*, a*, and b* show variability caused by the alternating dark organic-rich and greenish organic-poor lithologic packages or between clay-rich and biogenic component–rich sediment, whereas these two sites show a trend pattern mainly controlled by carbonate content, color of tephra/ash layers, and a sand layer (see “Lithostratigraphy”). Although the variability in the upper part of the sequence is lower than at previous sites, the trends of color reflectance at Sites U1428 and U1429 agree well with changes in the content of carbonate and other physical properties related to tephra/ash and sand layers (Figs. F54, F55). In particular, L* better reflects the lithologic characteristic of these two sites. This can be observed in the parameters L* and a* combined that is primarily variable in dark–light compounds (Fig. F56). L* of Sites U1428 and U1429 extends more widely between values of 23 and 58, compared to L* of Site U1427 extended between values of 24 and 43. This wide range of L* values is due to the high carbonate content, light or dark tephra/ash layers, and dark sand in Unit B. This high content of carbonate may also be responsible for the higher mean value of L* than found at IODP Sites U1425 and U1427. Comparison of L*-a* color spaces, which are plotted without the sand layer, indicates that the color of sand in Unit B dominates in compounds of high a* and low L*.

Summary

Physical properties measured at Sites U1428 and U1429 show similar trends and follow sediment lithology. Bulk density and NGR gradually increase downhole in lithologic Unit A, and the highest values occur in the sandy sediment of lithologic Unit B. Porosity and water content show the trend opposite to bulk density and NGR. Magnetic susceptibility shows high values in tephra/ash layers and also largely increases in lithologic Unit B. At these two sites, the downhole profiles of color reflectance agree well with changes in the carbonate content and other physical properties related to tephra/ash and sand layers. Comparison of L*-a* color spaces indicates that color changes at these two sites are mainly controlled by carbonate content, color of tephra/ash layers, and a sand layer.

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