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doi:10.2204/iodp.proc.320321.105.2010

Physical properties

Physical properties at Site U1333 were measured on whole cores, split cores, and discrete samples. WRMSL (GRA bulk density, magnetic susceptibility, and P-wave velocity), 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-U1333A-1H through 20X. 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 U1333. GRA provided an estimate from whole cores (Fig. F20), and 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 T28). MAD and GRA bulk density measurements display the same trends and are also similar in absolute values through the entire section (Fig. F21B). Cross-plots of wet and dry bulk density versus interpolated GRA density (Fig. F22) show excellent correlation between MAD and GRA data.

Generally, wet bulk density corresponds with changes in lithology. Density is highest in Unit II, which also has high CaCO₃ content (see "Lithostratigraphy"). Wet bulk density is ~1.3 g/cm³ at the seafloor and increases sharply at the top of Unit II (~1.6 g/cm³). In the upper part of Unit II, wet bulk density varies between 1.6 g/cm³ and 1.2–1.3 g/cm³, which is consistent with the occurrence of radiolarian-rich intervals within the nannofossil ooze in the upper part of Unit II (see "Lithostratigraphy"). From 40 to 100 m CSF, wet bulk density values are less variable. At the top of Subunit IIIa, density decreases to values of 1.2 g/cm³, which coincide with the sudden drop in CaCO₃ (wt%) at the Eocene/Oligocene boundary (~116 m CSF). The transition from high to low density values at the Eocene/Oligocene boundary reveals a two-step transition that most likely covaries with CaCO₃ (wt%) (see "Stratigraphic correlation and composite section"). In Subunit IIIa, density varies from 1.2 to 1.6 g/cm³. Limited data for Unit IV indicate higher density values of ~1.6 to 1.8 g/cm³.

Variation in grain density in Hole U1333A generally matches changes in lithology (Fig. F21C). Grain density averages 2.7 g/cm³ in Units I and II in Hole U1331A, indicating the presence of carbonate-dominated lithologies (calcite = 2.7 g/cm³). Subunit IIIa shows increased variability and lower grain densities, consistent with a more radiolarian dominated lithology (see "Lithostratigraphy"). Subunit IIIb and Unit IV show a return to carbonate-dominated lithologies, with grain density averaging 2.7 g/cm³.

Porosity averages 65% in Unit II and varies around 75% in the other units. Porosity and water content vary inversely with wet bulk density (Fig. F21A).

Magnetic susceptibility

Whole-core magnetic susceptibility measurements correlate well with the major differences in lithology and changes in bulk physical properties (Fig. F20). Magnetic susceptibility values in Unit I are 25 x 10^–5 to 30 x 10^–5 SI. As with wet bulk density, magnetic susceptibility values show a variable pattern in the upper part of Unit II, which reflects intervals of nannofossil ooze with radiolarians. Magnetic susceptibility values become more uniform in the lower part of Unit II (below 40 m CSF). Magnetic susceptibility values increase abruptly at the top of Subunit IIIa, reflecting a greater concentration of ferromagnetic minerals. As with wet bulk density measurements, the transition from low magnetic susceptibility values to high values at the Eocene/Oligocene boundary reveals a two-step transition that most likely covaries with CaCO₃ content (see"Stratigraphic correlation and composite section"). Magnetic susceptibility is higher and more variable in Subunits IIIa and IIIb and Unit IV compared to Unit II.

Compressional wave velocity

Shipboard results

Whole-core P-wave logger (PWL) and discrete velocity measurements made on split cores follow similar trends, with key transitions occurring at lithologic boundaries (Fig. F23). Discrete velocity measurements along the y- and z-axes are in excellent agreement with PWL measurements, although x-axis velocities are ~100 m/s faster than PWL velocities (Table T29). Possibilities for this mismatch in absolute values are compression of the sediment during analysis with the P-wave x-axis caliper or an improper correction for the thickness of the core liner.

Slight downhole trends in velocity generally follow changes in lithology or bulk properties (Fig. F23). PWL velocity increases through Units I and II. In Subunit IIIa, the downhole increase in velocity becomes greater. Velocity measurements reach 1575 m/s in the lower part of Subunit IIIa.

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 U1333 (Fig. F20). The highest NGR values are present at the seafloor (~45 cps). NGR values decrease to the base of Unit I. NGR is uniform throughout Unit II and shows a slight increase across the lithologic transition into Subunit IIIa. NGR is slightly higher in Subunit IIIa.

Thermal conductivity

Thermal conductivity was measured on the third section of each core from Hole U1333A (Table T30). Thermal conductivity shows a strong dependence on porosity downhole through the succession (Figs. F24, F25). Decreased conductivity occurs with increasing porosity as increased interstitial spacing attenuates the applied current from the probe. Thermal conductivity is 0.8 W/(m·K) in Unit I and increases to a maximum value of 1.2–1.3 W/(m·K) in the middle of Unit II.

Reflectance spectroscopy

Spectral reflectance was measured on split archive-half sections from all three holes using the SHMSL (Fig. F26). The parameters L* (black–white), a* (green–red), and b* (blue–yellow) follow changes in lithology, with variations in L*, a*, and b* correlating very well to carbonate content, density, and magnetic susceptibility measurements (see Fig. F4). Carbonate-dominated sections, such as the interval of Unit II from 38 to 110 m CSF, are clearly recognized by an increase in L* values and a decrease in both a* and b* values, related to the paler color of these sediments. The boundary between Unit II and Subunit IIIa, marking the change from carbonate-dominated Unit II to radiolarian-dominated Unit III, is clearly marked by a sharp decrease in L* (from ~80 to 50). This boundary is also recognized in the a* and b* data as a peak, followed by a slight decrease in b* (from ~10 to ~6), whereas a* values remain fairly constant directly above and below this boundary peak (~4).

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