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doi:10.2204/iodp.proc.320321.108.2010

Physical properties

Physical properties at Site U1336 were measured on whole cores, split cores, and discrete samples. WRMSL measurements (GRA bulk density, magnetic susceptibility, and P-wave velocity), thermal conductivity, and NGR measurements comprised the whole-core measurements. For Hole U1336A, 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-U1336A-1H through 35X. The SHMSL, configured with the Ocean Optics sensor, was used to measure spectral reflectance on archive-half sections. Resolution was reduced from 2.5 to 5 cm for Hole U1336A cores as a result of time constraints.

Hole U1336B cores were analyzed by the WRMSL and NGR detector during Expedition 320 and placed directly into cold storage unsplit. During Expedition 321, thermal conductivity measurements were made on whole-round sections. After the cores were split, MAD and velocity measurements were made on samples from every other section. Sediment reflectance was measured on all archive-half cores with the SHMSL at a resolution of 2.5 cm.

Density and porosity

Two methods were used to determine bulk sediment properties at Site U1336, GRA for the bulk density of whole-round sections (Fig. F20), and MAD analyses for wet bulk density, dry bulk density, grain density, water content, and porosity of discrete samples (Fig. F21; Table T19). MAD and GRA bulk density measurements display the same trends and are similar in absolute values through the entire section (Fig. F21B). Cross-plots of wet bulk density and dry bulk density versus interpolated GRA bulk density (Fig. F22) show excellent correlation between MAD and GRA density data.

Generally, wet bulk density corresponds with changes in lithology (Fig. F21B). Wet bulk density is lowest in lithologic Unit I (1.4–1.7 g/cm³) and reflects variations in the amount of minor lithologic components (clay, radiolarians, and diatoms) within the major lithology of nannofossil ooze. In Unit II, bulk density increases slightly and becomes more uniform (~1.7 g/cm³). In Unit III, which also corresponds to the XCB-cored interval, bulk density becomes highly irregular, ranging from lower density at the top of cores to higher density at the base of cores. This pattern reflects drilling disturbance at the top of XCB cores. The average wet bulk density for the XCB portion can be determined from the discrete MAD analyses that were cut directly from undisturbed intervals. Wet bulk density values in Unit III average 1.9 g/cm³, reflecting increased compaction of the sediments at depths >180 m CSF.

Variations in grain density (ρ_S) in Hole U1336A generally match changes in lithology (Fig. F21C). At the top of Hole U1336A grain density is 3.0 g/cm³ and decreases rapidly, reflecting clay-rich sediments that grade rapidly into nannofossil ooze. Grain density averages 2.7 g/cm³ for most of the section, varying from 2.6 to 2.9 g/cm³. This average and range reflect the dominance of carbonate material (calcite; ρ_S = 2.70 g/cm³) at Site U1336. Grain densities determined for Hole U1336B average ~0.1 g/cm³ less than grain densities determined for the same interval in Hole U1336A. The lower density of Hole U1336B material most likely reflects a larger sample volume and reduction of measurement error and error associated with mass and volume values derived from the assumed salt content of the pore fluid.

Porosity and water content vary inversely with wet bulk density (Fig. F21A). The highest porosities occur in lithologic Unit I, varying from 65% to 80%. At the boundary between Units I and II porosity begins to decrease gradually toward the base of Unit II, where values average from 55% to 60% at ~184 m CSF. A shift to lower porosity accompanies the shift to higher density in Unit III. Porosity in this interval ranges from 45% to 60%.

Magnetic susceptibility

Whole-round magnetic susceptibility measurements correlate well with the major differences in lithology at Site U1336 (Fig. F20). Magnetic susceptibility values are highest in lithologic Unit I and show high amplitude and high frequency variations from 5 × 10^–5 to 30 × 10^–5 SI. Across the boundary between Units I and II, susceptibility decreases from ~10 × 10^–5 to near 0 SI and remains low to the bottom of Hole U1336A. There is a slight increase in the amplitude and frequency of the variation of magnetic susceptibility in Unit III.

Compressional wave velocity

Shipboard results

Velocity measurements made with the P-wave logger (PWL) on whole-round sections and discrete measurements from split cores follow similar trends of increasing velocity with increasing depth as a result of increased compaction (Fig. F23). Discrete velocity measurements along the x-axis (contact probe) follow the trend of the PWL results in Hole U1336A (Fig. F23; Table T20) but exceed the PWL velocities on average by 20 m/s. An offset of 40 m/s is present in the first 25 m of the section in Hole U1336A. The cores in this interval appeared dry and fractured, which may account for the disparity. The x-axis velocities determined in Hole U1336B average 50 m/s higher than those over the comparable interval in Hole U1336A. Differences in the pressure applied in making contact between the sediment and transducers may account for this difference. Velocity measurements in the y- and z-axis directions (insertion probe) generally exceed the PWL velocity by 10 m/s. Differences between the whole-core and split-core measurements possibly reflect the presence of water in the space between the core liner and sediment in the whole cores, sediment fracturing as the y- and z-axis transducers penetrate the split core, and slight compaction of the sediment in the contact probe technique. No y- or z-axis velocity measurements were made below ~150 m CSF, as the XCB-cored sediment was hard and fractured easily with insertion of the transducers.

Velocities are between 1480 and 1500 m/s in lithologic Unit I and between 1480 and 1530 m/s in Unit II. At ~176 m CSF, velocities begin to increase rapidly into Unit III, from 1530 to ~1960 m/s at 208 m CSF. The remainder of Unit III shows high frequency and high amplitude variation, averaging 1910 m/s in this lithified and highly biscuited interval.

Postcruise correction

The x-direction velocities for Hole U1336A were determined using a liner thickness of 3.2 mm, the correction that was initially applied at Site U1334 (see "Physical properties" in the "Site U1334" chapter). This correction was not applied to velocities determined for Hole U1336B. These velocities were determined using a core liner thickness of 2.7 mm for the liner correction. During the analysis of Hole U1337A cores, it was determined that high x-direction velocities do not result from thicker than expected core liner but instead 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. During the analysis of Hole U1337A cores, it also was determined that consistently low PWL velocities required the addition of a constant value that would produce a reasonable velocity of water (~1495 m/s) for the quality assurance/quality control 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 in both holes at Site U1336 (Fig. F20). The highest counts per second are present at the seafloor (~56 cps). The NGR signal rapidly decreases with depth in lithologic Unit I, with small variations centered at 5 cps. At the boundary between Units I and II, NGR values decrease to ~2 cps. NGR is uniform at ~2 cps through Units II and III.

Thermal conductivity

Thermal conductivity was measured on the third section of each core from Hole U1336A and the fourth section of each core from Hole U1336B (Table T21). Thermal conductivity increases slightly with depth at the site (Fig. F24) and is inversely related to porosity (Fig. F25). Lower conductivity occurs with higher porosity as increased interstitial spacing attenuates the applied current from the probe. Overall, thermal conductivity decreases from 1.2 to 1 W/(m·K) through lithologic Unit I, with a minimum conductivity of 0.91 W/(m·K) at 31 m CSF. Through Units II and III, thermal conductivity increases from 1 to 1.4 W/(m·K), whereas porosity decreases over a similarly gradual trend of 75% to 50% (Fig. F21A). Below 183 m CSF, thermal conductivity decreases in XCB-cored sediment, possibly as a result of increased fractures and occurrence of sediment biscuits. Thermal conductivity values were not recorded deeper than 189 m CSF because the sediment was too hard to insert the probe.

Reflectance spectroscopy

Spectral reflectance was measured on the archive-half sections from Holes U1336A and U1336B using the SHMSL (Fig. F26). Variations in luminance (L*) correspond to pronounced lithologic changes. In lithologic Unit I, L* values are lowest (50–80) and display high-amplitude variations, reflecting changes in minor lithologic components. Values in Unit II are higher and more uniform, reflecting the relatively pure nannofossil ooze of this unit. Luminance values are slightly lower and more variable in Unit III. Variations in a* (blue–yellow) and b* (green–red) also reflect changes in lithology, with higher and more variable values in Unit I compared to Unit II. Both a* and b* exhibit a steplike decrease at 92 m CSF that is associated with a sharp change in the color of the nannofossil ooze from pale yellow to greenish gray.

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