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doi:10.2204/iodp.proc.320321.109.2010

Physical properties

Physical properties at Site U1337 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. 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 Hole U1337A and in Cores 321-U1337B-22H through 26H. The SHMSL was used to measure spectral reflectance on all archive-half sections.

Density and porosity

Two methods were used to determine bulk sediment properties at Site U1337, GRA for the bulk density of whole-round sections (Fig. F27A) and MAD analyses for wet bulk density, dry bulk density, grain density, water content, and porosity of discrete samples (Fig. F28; Table T20). MAD and GRA bulk density measurements display the same trends at Site U1337 (Fig. F28B), although in lithologic Unit I the wet bulk density from the MAD analyses averages 0.05 g/cm³ less than the GRA bulk density. This difference most likely reflects the calibration of the GRA sensor with aluminum (density = 2.7 g/cm³) and the abundant biogenic silica (grain density = ~2.2 g/cm³) in Unit I. Cross-plots of wet bulk density and dry bulk density versus interpolated GRA bulk density (Fig. F29) show excellent correlation between the MAD and GRA density data for both APC- and XCB-cored intervals.

Variation in wet bulk density corresponds to lithologic changes at Site U1337. Wet bulk density is low (1.12–1.46 g/cm³) and variable in lithologic Unit I. Variations in the abundance of siliceous and calcareous components and clay are responsible for the differences in wet bulk density. Intervals rich in clay, diatoms, or radiolarians generally are less dense than the nannofossil-rich intervals. This pattern is reflected in a well-defined, direct relationship between carbonate content and wet bulk density at Site U1337 (Fig. F30). Wet bulk density increases slightly with depth in Unit II (Fig. F28B); however, this trend is reversed by a pronounced density minimum (1.2 g/cm³) associated with a diatom-rich interval at 180 m CSF. At the boundary between lithologic Units II and III, wet bulk density increases sharply to ~1.55 g/cm³, reflecting the increase in calcareous components within the sediment. Density is more uniform in Unit III as a result of the uniformly high carbonate content of the nannofossil oozes of this unit (see "Geochemistry"). Wet bulk density generally increases with depth in Unit III; however, this trend is interrupted by a density minimum (1.3 g/cm³) at 340 m CSF. Below this depth, the density gradient increases, accompanying the change from nannofossil ooze to nannofossil chalk. The bulk densities of the basal chalk at Site U1337 are the highest in the sediment column, ~1.90 to 1.95 g/cm³.

Variations in grain density at Site U1337 follow the changes in lithology (Fig. F28C). The lithologically diverse Units I and II are characterized by a wide range in grain density (2.17–2.85 g/cm³) and a low average grain density (2.51 g/cm³). The more uniform and more calcareous nannofossil ooze of Unit III is characterized by less variable grain density (2.30–2.82 g/cm³) and a higher average grain density (2.67 g/cm³). The uniform trend in grain density in Unit III is interrupted by a minimum (2.3 g/cm³) at 340 m CSF that is associated with a radiolarian-rich interval (see "Lithostratigraphy").

Porosity and water content vary inversely with wet bulk density (Fig. F28A). The highest porosities occur in Unit I, varying from 74% to 93%. Porosities decrease slightly with depth in Unit II but remain high (>80%). At the boundary between Units II and III porosity decreases sharply to 55% (Fig. F28A). Below this depth, the trend in decreasing porosity with depth is marked by a maximum (81%) in the radiolarian-rich interval at 340 m CSF and a change in gradient below this depth associated with the transition to nannofossil chalk. The porosity of the chalk immediately above the basement at Site U1337 is ~47%.

Magnetic susceptibility

Whole-core magnetic susceptibility measurements correlate well with the major differences in lithology at Site U1337 (Fig. F27B). Susceptibility is highest in lithologic Unit I and shows high-amplitude and high-frequency variations from 4 × 10^–5 to 18 × 10^–5 SI. These variations are characterized by low values in more calcareous intervals and higher values in more siliceous intervals. Across the boundary between Units I and II, susceptibility decreases from ~14 × 10^–5 to near 2 × 10^–5 SI and remains low for the remainder of the section. Magnetic susceptibility increases in Unit II from ~2 × 10^–5 to ~15 × 10^–5 SI at 180 m CSF associated with the diatom-rich interval at this depth. Below 180 m CSF, susceptibility drops back to 2 × 10^–5 SI for the remainder of Unit II. In Unit III, a similar increase from ~2 × 10^–5 to ~13 × 10^–5 SI at 340 m CSF is associated with a radiolarian-rich interval. An abrupt increase in magnetic susceptibility from ~2 × 10^–5 to ~10 × 10^–5 SI is present at the base of Unit III.

Compressional wave velocity

P-wave logger (PWL) velocity measurements from whole-round sections and discrete velocity measurements made on split cores follow similar trends at Site U1337. Velocity fluctuates around 1500 m/s for lithologic Units I and II. In Unit III, velocities begin to increase with depth relating to increased compaction (Fig. F31). A similar increase in velocity is present in the well log data (see "Downhole measurements").

During the initial sampling of Hole U1337A it was observed that x-directed velocities were consistently higher than the other velocity measurements. Measurements of the velocity of water using the contact probe (x-direction) were higher (~1540 m/s) than the known velocity of water at room temperature (1485–1490 m/s), probably as a result of incorrect calibration parameters (liner thickness = 3.2 mm; system delay = 20.61 µs; liner delay = 1.5 µs). After recalibrating the contact probe by measuring the velocity of water at room temperature and varying the distance between the x-axis transducers, new calibration parameters were determined (liner thickness = 2.7 mm; system delay = 19.811 µs; liner delay = 1.26 µs). These new parameters yield a reasonable velocity of water (~1495 m/s) at room temperature. The velocity of water obtained from the PWL was consistently low (~1450 m/s) for the quality assurance/quality control liner. Therefore, it was decided to add a constant shift of 40 m/s to the velocity derived from the PWL. After the correction, PWL results are in excellent agreement with discrete sample velocity measurements (Table T21; Fig. F31). Differences between 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 and the slight compaction of the sediment in the contact probe technique. No y- or z-axis measurements were taken below ~287 m CSF, as the XCB-cored sediment was hard and fractured easily with insertion of the transducers. Beginning with Core 321-U1337A-37X (340 m CSF), discrete samples were cut from the biscuits to measure velocity along the z-axis using the x-axis measurement systems. The effect of liner correction was removed to obtain the correct velocity.

Velocity at Site U1337 displays high-frequency fluctuations between 1490 to 1530 m/s in lithologic Unit I and between 1490 and 1510 m/s in Unit II. An increase in velocity from 1510 to 1560 m/s occurs at ~200 m CSF at the base of Unit II. This increase is probably related to the presence of a diatom mat interval. At ~340 m CSF, velocities begin to increase rapidly in Unit III, from 1510 to ~1800 m/s at the base of Unit III (Figs. F27C, F31). The velocity of the basalt basement was determined for one sample to be 6212 m/s.

Natural gamma radiation

NGR was measured on cores from all holes drilled at Site U1337. The highest counts per second are present in the upper 10 m of the sediment column, with values of ~65 cps near the seafloor (Fig. F27D). Below 10 m CSF, the NGR signal is characterized by small variations centered around 4 cps. From lithologic Unit II to Unit III, the strength of the NGR signal decreases further with most of the variation centered around 2 cps. Although NGR counts are low in Unit III, variations are associated with significant changes in sediment composition. Examples of these variations are shown in the ~8 cps increase in NGR centered on the chert rubble interval in Core 321-U1337D-28H (Fig. F32A) and the color change from greenish gray to pale yellow in Core 321-U1337A-44X (Fig. F32B). Both of these intervals display peaks in uranium abundance, as indicated in borehole spectral gamma logs (see "Downhole measurements"). In the chert rubble it is assumed that the uranium is associated with organic matter. The color change from green to yellow represents a redox boundary along which uranium is concentrated. Comparison of results of carbon analyses and NGR records for Hole U1337A suggests a correlation between the weight percent TOC and NGR counts for TOC values <0.4 wt%. (Fig. F33). Samples with TOC values >0.7 wt% do not fit the trend of the correlation. These samples include a near-surface sample with an NGR value of ~40 cps and a sample from 243.1 m CSF in Hole U1337A, close to the interval of the prominent chert layer in Unit III.

Thermal conductivity

Thermal conductivity was measured on the third section of each core from Hole U1337A that contained at least three sections (Table T22). Thermal conductivity increases gradually with depth at Site U1337 from a conductivity of 0.75 W/(m·K) at 9.25 m CSF to 1.64 W/(m·K) at 446.95 m CSF (Fig. F34). A local maxima in thermal conductivity occurs at 370.55 m CSF where the conductivity is 1.76 W/(m·K). The amplitude of conductivity variations increases below 220 m CSF, coinciding with the XCB-cored section of Hole U1337A, and possibly reflects differences among intact and disturbed intervals in the cores. Thermal conductivity at Site U1337 is inversely correlated to porosity (Fig. F35). Lower conductivity occurs with higher porosity, as increased interstitial spacing attenuates the applied current from the probe.

Reflectance spectroscopy

Spectral reflectance was measured on the archive-half sections from all holes at Site U1337 using the SHMSL. The three color parameters, L*, a*, and b*, show a systematic variation among the lithologic units (Fig. F36). Unit I is characterized by dark colors (low L*) dominated by a mix of yellow (positive a*) and red (positive b*). High-amplitude changes in the color parameters reflect the mix of lithologic components within the unit. At the boundary between lithologic Units I and II, a* and b* decrease sharply indicating an increase in blue and green components as the sediment takes on the greenish gray coloring of the nannofossil ooze. Luminance (L*) increases toward the base of Unit I, and overall Unit II is characterized by a higher luminance than that of Unit I. Values of a* and b* are similar for lithologic Units II and III. The main difference in the color of the two units is the lighter color (more positive L*) of Unit III. Below 410 m CSF in Unit III, a* and b* increase sharply as the nannofossil ooze changes from greenish gray to pale yellow. Near the base of the sediment column at Site U1337, the increase in a* values and the decrease in luminance represent the change from pale yellow to pale brown nannofossil ooze.

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