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doi:10.2204/iodp.proc.320321.110.2010

Physical properties

Physical properties at Site U1338 were measured on whole cores, split cores, and discrete samples. WRMSL measurements (GRA bulk density, magnetic susceptibility, and P-wave velocity) (Fig. F28A–F28C), thermal conductivity, and NGR measurements (Fig. F28D) 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 U1338A and Cores 321-U1338B-27H through 31H, 34H through 38H, 40H, and 41H. 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 U1338, GRA for bulk density of whole-round sections (Fig. F28B) and MAD analyses for wet bulk density, dry bulk density, grain density, water content, and porosity of discrete samples (Fig. F29; Table T20). MAD and GRA bulk densities display excellent agreement at Site U1338 (Fig. F29B). Cross-plots of wet bulk density and dry bulk density versus interpolated GRA bulk density (Fig. F30) show that MAD and GRA density data are well correlated for both APC- and XCB-cored intervals.

Variation in wet bulk density reflects the variation in biosiliceous and calcareous components at Site U1338. As at Site U1337, a direct relationship exists between carbonate content and wet bulk density (Fig. F31). Wet bulk density in lithologic Unit I ranges from 1.19 to 1.46 g/cm³ and averages 1.29 g/cm³. Overall density decreases with depth in Unit I from ~1.4 g/cm³ near the seafloor to 1.2 g/cm³ at the base of Unit I (~50 m CSF) as the abundance of radiolarians and diatoms increases in the nannofossil ooze (see "Lithostratigraphy"). Within this depth interval density variation is characterized by small-scale fluctuations of ~0.10 to 0.20 g/cm³ with roughly 1 m spacing. The small fluctuations are superimposed on a larger scale variation with a 10 to 20 m spacing. Below the density minimum that marks the boundary between Units I and II, density increases with depth from ~1.3 g/cm³ at 50 m CSF to 1.5 g/cm³ at 175 m CSF. The interval from 175 m CSF to the boundary between Units II and III (243.94 m CSF in Hole U1338A) is characterized by a broad density low that accompanies an increase in radiolarians and diatoms in the sediment (see "Lithostratigraphy"). In this interval, wet bulk density averages 1.40 g/cm³. From the top of Unit III to the base of the sediment section, bulk density increases more regularly as the sediment becomes a more uniform nannofossil ooze and eventually chalk. Carbonate contents are uniformly high in this interval (see "Geochemistry"). Wet bulk density increases from ~1.5 g/cm³ at the top of Unit III to 1.7 g/cm³ near the base of Hole U1338A. One sample from immediately above the basement (Sample 321-U1338A-44X-3, 129–131 cm) has a density of 1.89 g/cm³. Within Unit III the variation of wet bulk density is less compared to the other units at the site, with roughly meter-scale fluctuations of 0.1 g/cm³ superimposed on 0.1–0.2 g/cm³ fluctuations with a spacing of ~10 m.

Variations in grain density at Site U1338 also reflect the relative abundances of biosiliceous and calcareous components in the sediment. In Hole U1338A, from near the seafloor to 175 m CSF, grain density averages 2.59 g/cm³ and varies within a 0.15 to 0.20 g/cm³ wide band (Fig. F29C). The lowest grain densities occur in the interval between 175 m CSF and the base of Unit II. In this interval grain density ranges from 2.29 to 2.66 g/cm³ and averages 2.55 g/cm³. Grain density is higher and more uniform in Unit III, averaging 2.64 g/cm³ and generally varying within a 0.10 g/cm³ wide band. Grain density at the base of the sediment section in Hole U1338A is 2.75 g/cm³.

Porosity and water content vary inversely with wet bulk density (Fig. F29A). Porosity near the surface at Site U1338 is 80%. At the base of Unit I in Hole U1338A, porosity is 88%. In the upper part of Unit II, porosity decreases with depth with the same cyclical pattern as that displayed by wet bulk density. Porosity is at a minimum (69%) at 175 m CSF in Hole U1338A. Below this depth and extending to the base of Unit II, porosity is higher, averaging 76%. Porosity decreases more uniformly in Unit III, going from ~70% near the top of the unit to 56% near the base of the unit. Sample 321-U1338A-44X-3, 129–131 cm, at 407.10 m CSF in Hole U1338A, has a porosity of 50%.

Magnetic susceptibility

Whole-core magnetic susceptibility values are closely correlated with lithology at Site U1338 (Fig. F28B). Susceptibility in lithologic Unit I shows high-amplitude variations from 0 to 24 × 10^–5 SI. Higher susceptibility intervals are associated with dark-colored, siliceous microfossil–rich sediments that have lower bulk density. The profiles of magnetic susceptibility and GRA bulk density display an inverse relationship in Unit I that coincides with changes in sediment color (Fig. F32). At the boundary between Units I and II, susceptibility decreases from 5 × 10^–5 SI to near zero and remains low for the upper part of the Unit II (~50–178 m CSF). Susceptibility in the interval from 178 to 214 m CSF fluctuates with variation ranging from 4 × 10^–5 to 10 × 10^–5 SI. Higher susceptibility values are associated with diatom-rich intervals that display lower GRA bulk density, similar to Unit I. In the lowermost part of Unit II, from 214 to 245 m CSF, the susceptibility record shows 2 to ~4 m–scale fluctuations from 0 to 15 × 10^–5 SI that do not correspond to changes in GRA bulk density. Across the boundary between Units II and III, susceptibility drops steadily to roughly zero. Below 295 m CSF, susceptibility increases from 0 to ~4 × 10^–5 SI, with frequent susceptibility peaks of 5 × 10^–5 to 10 × 10^–5 SI that are associated with siliceous microfossil–rich intervals (see "Lithostratigraphy"). An increase in susceptibility and high susceptibility peaks, from ~5 × 10^–5 to ~24 × 10^–5 SI, are present below 395 m CSF and extend to the base of Unit III.

Compressional wave velocity

P-wave logger (PWL) measurements from whole-round sections and discrete velocity measurements made on split cores follow similar trends for Site U1338. 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. F33). A similar increase in velocity is observed in the borehole data (see "Downhole measurements").

The velocity of water obtained from the PWL was consistently low, ~1423 m/s, for the quality assurance/quality control liner. Therefore, it was decided to add a constant shift of 67 m/s to the measured velocity so that the water velocity equaled 1490 m/s at room temperature (20°C). The velocity of water was also measured with the P-wave gantry at regular intervals of 12 h, and it was observed that the velocity measured along the x-axis was higher (~1500 m/s) with a variability of ~10 m/s. Therefore, a shift was applied to the measured velocity along the x-axis based on the observed water velocity, such that the water velocity remained constant (1490 m/s) for all the cores. No correction was required for the velocities measured along the y- and z-axis. After applying proper corrections, discrete velocity measurements are in good agreement with the PWL results (Table T21; Fig. F33). The velocity along the x-axis is lower than that of the PWL in the biosiliceous-rich interval around 180 to 240 m CSF (see "Lithostratigraphy"). This difference is attributed to the fracturing induced during the contact probe measurement. Minor 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 reliable y- or z-axis measurements were obtained below ~140 m CSF, as the sediment was hard and easily fractured with insertion of the probes. Beginning with Core 321-U1338A-37X (337 m CSF), discrete samples removed from the core were used to measure the velocity along the x-axis. Liner correction was not applied to these measurements.

In Unit I, velocity increases linearly with depth from 1515 to 1545 m/s as a result of the increase in abundance of siliceous microfossils. Velocities fluctuate between 1495 and 1540 m/s in Unit I and between 1500 and 1520 m/s in Unit II. In Unit III, a linear increase in velocity from 1520 m/s to 1570 m/s occurs between 290 and 380 m CSF. At ~380 m CSF in Unit III, split core velocities begin to increase rapidly with depth from 1610 to ~1820 m/s at the base of Unit III.

The cross-plot of GRA bulk density with PWL velocity suggests that velocity is negatively correlated with bulk density in Unit I (Fig. F34) because of the increase in abundance of siliceous microfossils and a generally higher velocity for intervals rich in radiolarians (Busch et al., 2006) For constant porosity, an increase in abundance of biosiliceous material results in the decrease in bulk density and increase in P-wave velocity. In Unit II, bulk density decreases as a result of the increase in abundance of biosiliceous components; however, velocity remains constant because of the increase in porosity. In Unit III, which is predominantly nannofossil ooze, density and velocity are positively correlated.

Comparison between PWL velocity and borehole log velocity is excellent, if PWL velocities are offset (Fig. F35). A slight shift in the depth scale is observed as the depth of PWL velocity is recorded in CSF, and the depth of the logs is in the WMSF domain. It should be noted that PWL and log velocities are plotted in different scales, as PWL measures the velocity at standard pressure and temperature conditions, whereas the log measures the velocity at in situ condition. The excellent agreement between PWL- and log-derived velocities suggests that the transformation of laboratory-measured velocity to in situ velocity is approximately linear.

Natural gamma radiation

NGR was measured on cores from all holes drilled at Site U1338. The highest counts per second (cps) are present in the upper 5 m of the sediment column, with values of ~45 cps near the seafloor (Fig. F28D). Below 5 m CSF, the NGR signal is characterized by small variations centered around 4 cps. This pattern extends through lithologic Units I and II and represents the varying abundance of biosiliceous and calcareous components. Silica-rich intervals are generally characterized by darker color, presumably higher organic contents, and higher NGR. However, a consistent relationship between organic carbon and NGR values does not exist (Fig. F36). In Unit III, variations in the NGR signal are centered around 2 cps, reflecting the higher and more uniform carbonate content of the unit (see "Lithostratigraphy"). In the lower part of Unit III, a small NGR peak of 4 cps at ~385 m CSF coincides with the color change from greenish gray to pale yellow. At the base of Unit III, several small spikes in the NGR signal are associated with manganese oxide fragments present in the sediment.

The peak in NGR near the seafloor is a common feature at all Expedition 320/321 drill sites. Although prominent at Site U1338, the NGR peak is the lowest of the NGR peaks recorded during Expedition 320/321. The Site U1338 NGR record also is unusual in that a pair of peaks is present within 2 m of the seafloor. A series of closely spaced samples for carbon analysis were taken in the upper 2.4 m in Hole U1338A. The analyses show that several peaks in TOC are present in the upper 2 m of the sediment column; however, these peaks do not strictly coincide with the NGR peaks (Fig. F37). The NGR peaks near the seafloor do coincide with lower GRA bulk density, higher magnetic susceptibility, low carbonate concentration, and dark sediment color (Fig. F37). The explanation of the origin of the near-surface NGR peaks remains a subject for further study.

Thermal conductivity

Thermal conductivity was measured on the third section of each core from Hole U1338A that contained at least three sections and the first or second section from all other Hole U1338A cores (Table T22). Thermal conductivity increases gradually with depth at Site U1338 from 0.92 W/(m·K) at 2.25 m CSF to 1.24 W/(m·K) at 405.10 m CSF (Fig. F38). This trend in conductivity is nearly uniform throughout the section in Hole U1338A. Conductivity in the upper part of the hole, above ~185 m CSF, displays slightly larger amplitude variations than in the lower part of the hole. Thermal conductivity at Site U1338 is inversely correlated to porosity (Fig. F39). 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 split archive-half sections from all holes at Site U1338 using the SHMSL. The three light reflectance parameters (L*, a*, and b*) display a lithology-dependent variation that does not strictly follow the lithologic units identified for the site (Fig. F40). Unit I overall is characterized by darker, browner colors, those with low L* and higher a* and b* values. At ~25 m CSF, a* and b* decrease sharply as the sediment color changes to one dominated more by blue (low b*) and green (low a*). The sediment also becomes slightly darker (lower L*) at this depth. At the boundary between Unit I and II, b* decreases as blue colors become more dominant, a* remains relatively unchanged, and L* increases as the sediment lightens in color. From the top of Unit II (~50 m CSF) to ~192 m CSF, b* displays high-amplitude variations with a wavelength of ~50 cm. Values of a* display a variation similar in wavelength to that of b*, but lower in amplitude. The luminance record displays similar length cycles superimposed on a larger scale, a roughly 20 m wavelength variation. This variation in L* is apparent to ~300 m CSF, below which L* becomes more uniform. Below ~192 m CSF, the amplitude of the variation in b* and a* decreases, and the 20 m–scale color cycles become more apparent in these parameters. Between ~328 and 386 m CSF, the sediment color progressively becomes more yellow (higher b*) and more green (lower a*). Luminance remains relatively unchanged in this interval. The transition from greenish gray to pale yellow occurs at 386 m CSF and is marked by sharp increases in both a* and b*. The first appearance of brown-colored chalk, at the top of Core 321-U1338C-44H, is marked by a decrease in a*, b*, and L*.

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