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doi:10.2204/iodp.proc.320321.103.2010

Physical properties

Physical properties at Site U1331 were measured on whole cores, split cores, and discrete samples. WRMSL (GRA bulk density, magnetic susceptibility, P-wave velocity, and electrical noncontact resistivity [NCR]), thermal conductivity, and NGR measurements comprised the whole-core measurements. Compressional wave velocity measurements on split cores and MAD analyses on discrete core samples were made at a frequency of one per undisturbed section in Cores 320-U1331A-1H through 22X. Compressional wave velocities were measured toward the bottom of sections. MAD analyses were located 10 cm downsection from carbonate analyses (see "Geochemistry"). Lastly, the Section Half Multisensor Logger (SHMSL) was used to measure spectral reflectance on archive-half sections.

Density and porosity

Two methods were used to evaluate wet bulk density at Site U1331. GRA provided an estimate from whole cores (Fig. F25). MAD samples gave a second, independent measure of wet bulk density, along with providing dry bulk density, grain density, water content, and porosity from discrete samples (Table T35). MAD and GRA bulk density measurements display the same trends and are also similar in absolute values through the entire section (Fig. F26B). In general, wet bulk density values are 0.2 g/cm³ higher than GRA bulk density, which has been attributed to the GRA coefficient used in data processing combined with the attenuation coefficient of the radiolarian-rich sediments (Lyle et al., 2002). This offset was confirmed by measuring water as a standard, which gave a value of 1.2 g/cm³. All GRA data have been corrected for this offset. Cross-plots of wet and dry bulk density versus interpolated GRA density (Fig. F27) show excellent correlation between MAD and GRA data.

Generally, wet bulk density corresponds to changes in lithology. Density is higher in Subunits IIa and IIc, which are relatively high in CaCO₃ (see "Lithostratigraphy"). Wet bulk density is ~1.25 g/cm³ from the seafloor to the base of Unit I. Values increase to 1.6 g/cm³ in Subunit IIa, which is characterized by high CaCO₃ weight percent. Bulk density decreases in Subunit IIb to values of 1.2 g/cm³. In Subunit IIc, where CaCO₃ content reaches 50 wt%, bulk density increases to ~1.35 g/cm³. Bulk density is 1.2 g/cm³ in Subunits IIIa–IIIc. These units are predominantly radiolarian ooze and show very little variation in bulk density values. Bulk density increases in a short interval at the base of the cored section that corresponds with the APC-cored sections that recovered carbonate-rich sediment.

Variation in grain density in Hole U1331A generally matches changes in lithology (Fig. F26C). Grain density averages 2.4 g/cm³ in Unit I in Hole U1331A. In Subunit IIa it increases to 2.7 g/cm³, coinciding with the increase in CaCO₃. In the upper section of Subunit IIb grain density decreases to 2.2 g/cm³ and becomes highly variable through the rest of the succession, ranging from 2.2 to 2.9 g/cm³. This variation is expected to be related to variations in radiolarian content (opal = 2.2 g/cm³) and that of carbonate material (calcite = 2.7g/cm³).

Porosity averages 83% through much of the succession. Porosity and water content vary inversely with wet bulk density (Fig. F26A). Porosity decreases to a minimum of 50% in the clay-rich upper Eocene section of Subunit IIa. The highest porosity (91%) is present in the radiolarian clay of Unit II at 65 m CSF.

Magnetic susceptibility

Whole-core magnetic susceptibility measurements correlate well with the major differences in lithology and changes in bulk physical properties (Fig. F25B). Magnetic susceptibility values in Unit I are relatively high. A significant decrease in susceptibility marks the top of Unit II at ~5 m CSF. Magnetic susceptibility values are low in Subunit IIa, averaging 10 x 10^–5 SI, with a gradual rise into Subunit IIb. The highest susceptibility in Subunit IIb (28.9 x 10^–5 SI) is reached at ~46 m CSF, after which values decrease toward the top of Subunit IIc. Through Subunit IIc susceptibility increases gradually from 6 x 10^–5 to 25 x 10^–5 SI. In Unit III magnetic susceptibility gradually decreases and then increases again, from ~20 x 10^–5 to 10 x 10^–5 SI, at the top of Unit IV where the record ends.

Variations in magnetic susceptibility are thought to correspond to alternations between nannofossil ooze (lower magnetic susceptibility) and clayey nannofossil ooze (higher magnetic susceptibility). The increase in susceptibility toward the base of Unit III is expected to relate to the presence of chert in the succession.

Compressional wave velocity

Shipboard results

Compressional wave velocity was measured by the P-wave logger (PWL) on all whole cores from Holes U1331A–U1331C and by the insertion and contact probe systems on split cores from Hole U1331A (Table T36), allowing determination of velocities in the y-, z-, and x-directions. Whole-core and split-core data follow similar trends, with key features occurring at similar locations (Fig. F28); however, values are offset with a difference of ~50 m/s between PWL and discrete z- and y-directions and a difference of ~100 m/s for the x-direction. This is thought to have resulted from excessive wetting of sediment to ensure a measurement could be taken on the split cores, a water-rich rind inside the core liner, and possible variations in pressure/probe contact between the PWL and discrete x-direction transducers.

Slight downhole trends in velocity generally follow changes in lithology and bulk properties (Fig. F28). Velocity (PWL) increases with depth in Unit I to the Eocene/Oligocene boundary. From 5 m CSF, near the base of Unit I, to 10 m CSF, near the top of Unit II, velocity decreases to 1440–1450 m/s. Overall, velocity gradually increases with depth in Unit II. Small internal variations in velocity may reflect alternations in lithology. Unit III is marked by fairly uniform velocities (~1470 m/s), again with internal variation most likely attributed to changes in lithology. Data are not available for the chert-rich lower part of Unit III. Limited data for Unit IV give the highest velocities seen in this section (~1485 m/s).

The lack of consistent downhole velocity trends in Hole U1331A is partly explained by the cross-plot of velocity and wet bulk density (Fig. F29). Unit I has increasing density with slightly decreasing velocity. Subunits IIa and IIc comprise radiolarian ooze and variable carbonate nannofossil components; these continue this trend of increasing density with decreasing velocity. Subunit IIb follows a similar but much weaker trend with much more scatter, whereas Subunit IIIa shows more limited variation of velocity with density; both have higher velocities than neighboring units.

Postcruise correction

During the initial sampling of Hole U1337A, it was observed that x-direction velocities are consistently higher than other velocities and that PWL velocities are consistently low for Hole U1337A and all holes drilled at Sites U1331–U1336. It was determined that the high x-directed velocities are the result of using an incorrect value for the system delay associated with the contact probe (see "Physical properties" in the "Site U1337" chapter). Critical parameters used in this correction are system delay = 19.811 µs, liner thickness = 2.7 mm, and liner delay = 1.26 µs. PWL velocities were corrected for Hole U1337A by adding a constant value that would produce a reasonable velocity of water (~1495 m/s) for the quality assurance/quality control (QA/QC) liner (see "Physical properties" in the "Site U1337" chapter). These corrections have not been applied to the velocity data presented in this chapter.

Natural gamma radiation

NGR was measured on all whole cores at Site U1331 (Fig. F25). The highest NGR values are present at the seafloor (~130 cps). NGR values remain high through Unit I and the top of Unit II, where they average 8.5 cps. Below 30 m CSF, values drop to ~5 cps throughout Units II–IV; this is relatively higher than the 1 cps values noted during Leg 199 and is expected to be the result of the use of a new, more sensitive NGR track that records at a higher resolution. A number of spikes in NGR occur in Units II–IV; these appear to correspond to sharp boundaries identified in the lithologic core descriptions. The steplike feature in Hole U1331A at ~78 m is an artifact resulting from a baseline shift of unknown origin that occurred during the analysis of cores from Hole U1331A.

Thermal conductivity

Thermal conductivity was measured on the third section of each core from Hole U1331A (Table T37). Thermal conductivity shows a strong dependence on porosity in Unit I (Fig. F30) but seems otherwise unaffected by porosity. Decreased conductivity occurs with increasing porosity as increased interstitial spacing attenuates the applied current from the probe. Thermal conductivity is almost constant through Units II–IV (~0.74 W/[m·K]), except for a notable increase to 0.82 W/(m·K) near the Unit II/III boundary.

Reflectance spectroscopy

Spectral reflectance was measured on split archive section halves from Holes U1331A–U1331C using the SHMSL (Fig. F31). The parameters L* (black–white), a* (green–red), and b* (blue–yellow) follow changes in lithology, with variations in L* and b* correlating very well to carbonate content, density, and magnetic susceptibility measurements. The parameter a* has a very limited relationship with lithologic variation downhole. Carbonate-rich sediments are found in Subunits IIa and IIc; these intervals are represented by a distinct increase in L* and b* values in these sections. Subunit IIa shows the highest L* and b* values (70 and 17, respectively), corresponding to the increased carbonate content (shown by geochemistry data) above the Eocene/Oligocene boundary. These increases in L*, a*, and b* clearly respond to lithology, where carbonate-rich sections are light brown-gray and radiolarian-rich sections are darker brown.

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