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doi:10.2204/iodp.proc.317.106.2011

Physical properties

At Site U1354, gamma ray attenuation (GRA) densitometer bulk density, magnetic susceptibility (loop sensor; MSL), natural gamma radiation (NGR), and P-wave logger (PWL) velocity were measured on whole-round core sections from Holes U1354A–U1354C. Discrete P-wave velocity measured using the P-wave caliper (PWC) and P-wave bayonets (PWB), moisture and density (MAD), and sediment strength were measured on section halves from Holes U1354B and U1354C. Magnetic susceptibility (point sensor; MSP) and spectrophotometry and colorimetry were measured on cores from all three holes. Measurements were made on APC and XCB cores from Holes U1354A–U1354C to depths of 85.4 m (Section 317-U1354A-19H-5), 77.2 m (Section 317-U1354B-15H-5), and 384.2 m (Section 317-U1354C-36X-2). Unless otherwise specified, all depths in this section are reported in m CSF-A.

Gamma ray attenuation bulk density

GRA bulk density was measured at 2.5 cm intervals (measurement time = 3 s). The raw data range from –0.44 to 2.48 g/cm³ (Fig. F19). Variations in GRA density may reflect varying sand content in the cores.

A comparison of GRA densitometer data with MAD data from Hole U1354B highlights key similarities with Site U1353 (Fig. F20). In particular, MAD and GRA densitometer results show the same multicore trends, but GRA densitometer bulk density estimates are consistently ~2%–3% higher than MAD bulk density estimates. Although this is only slightly above the expected error of the MAD method, such an error should be randomly distributed about the GRA results; thus, a systematic error either in MAD measurements or in GRA densitometer calibration must be present. As discussed in "Physical properties" in the "Site U1353" chapter, the most likely problem is a calibration error with the GRA system.

Magnetic susceptibility

Magnetic susceptibility (MSL) was measured at 2.5 cm intervals (measurement time = 2 s), and magnetic susceptibility (MSP) was measured at 5 cm intervals. MSP measurements were made on all sections unless drilling or surface disruption precluded the collection of meaningful results (Fig. F19).

Raw MSL data range from 1.1 to 275.0 instrument units in Holes U1354A and U1354B and from 0.2 to 87.9 instrument units in Hole U1354C (Fig. F19). To help illustrate key trends, the signal was cleaned using a Gaussian low-pass filter (30 passes; Fig. F19).

MSP magnetic susceptibility data at Site U1354 correlate well with data obtained from MSL measurements made on whole-round core sections (Fig. F19). Both MSP and MSL magnetic susceptibility measurements show distinct variations in the uppermost ~170 m in all three holes, particularly the uppermost 80 m where core recovery was best.

The general cyclic pattern is very similar to that observed previously, particularly at Site U1351. Three similar intervals between ~12 and ~76 m are characterized by a decreasing downhole trend followed by an abrupt change to higher values at 26, 34, and 54 m (Hole U1354B). The abrupt changes at 34 and 54 m coincide with two changes in sulfate–methane abundances in Hole U1354A (see "Geochemistry and microbiology"). An interval of high magnetic susceptibility between ~146 and ~152 m is not yet understood. The upper end of this interval at ~146 m might be associated with the Subunit IA/IB boundary (see "Lithostratigraphy"). A slight change in magnetic susceptibility between ~68 and 70 m in Hole U1354A and between 64 and 65 m in Hole U1354B marks the Brunhes/Matuyama boundary and a hiatus that is well documented in paleomagnetic, lithologic, and biostratigraphic records (see "Lithostratigraphy," "Biostratigraphy," and "Paleomagnetism").

The overlapping of Sections 317-U1354A-11H-3 through 11H-5 with 12H-1 through 12H-3 (between 56 and 60 m) can be explained by drilling disturbances in the lower part of Core 11H (Fig. F21). The shell-hash interval in this core, as observed in the conspicuously low values of magnetic susceptibility and NGR for Hole U1354A, was not observed in Hole U1354B.

Natural gamma radiation

NGR was measured at 10 cm intervals on all core sections as deep as 240 m in Hole U1354B (Section 36X). The measured values range from near zero to >70 counts per second (cps), with higher values typically associated with muddy lithologies and lower values associated with sands (Fig. F19).

A large-scale sinusoidal pattern of average NGR signal is apparent downhole, with ~70 cps peaks at ~14 and 215 m that are separated first by a gentle decline to ~20 cps at ~75 m, followed by a gentle increase again from ~75 to 215 m. The change from declining to increasing NGR occurs near the 69.9 m (Hole U1354A) and 64.75 m (Hole U1354B) locations of the Brunhes/Matuyama boundary and also coincides with a marked reduction in the abundance and diversity of nannofossil assemblages (see "Biostratigraphy").

The larger scale sinusoidal NGR pattern is modulated by the same shorter, cyclic changes in NGR documented for the upper portions of Sites U1351–U1353. Many of these changes probably reflect lithologic changes related to sea level variations at 100 k.y. (back to 0.6 Ma) and 40 k.y. (earlier) Milankovitch cyclicity. Above the Brunhes/Matuyama boundary, the NGR record (and also the magnetic susceptibility record) reflects two major cycles that probably correspond to marine isotope Stages (MIS) 1–7, with significant sand units at 22–32 and 56–61 m that can be correlated tentatively with MIS Interglacials 5 and 7, respectively. Below the Brunhes/Matuyama boundary, probable 40 k.y. Milankovitch-scale variations in NGR and lithology continue, but no specific correlation can yet be made with the established oxygen isotope record.

P-wave velocities

P-wave velocities were recorded continuously in Holes U1354A–U1354C at 2.5 cm intervals using the PWL. The PWC and PWB were used to measure P-wave velocity in Holes U1354B and U1354C (Fig. F22B). P-wave measurements yielded good results in the uppermost 217.5 m. Comparably good results were observed only in cores from Site U1353. At both sites, the long records are a result of the absence of the sediment cracking caused by high gas content at Sites U1351 and U1352. With this data set, an excellent positive correlation was found between PWL estimates from both holes, and a good correlation was found between PWL, PWC, and PWB estimates in Holes U1354B and U1354C. Nevertheless, PWB P-wave velocities are generally slightly lower than velocities measured with the PWL and PWC.

P-wave velocities show little vertical trend and average ~1500–1600 m/s. One major step was observed at 20–21 m in Hole U1354A and at 19–20 m in Hole U1354B. An offset between ~68 and 70 m in Hole U1354A and between 64 and 65 m in Hole U1354B marks a hiatus that was also observed in magnetic susceptibility.

Spectrophotometry and colorimetry

Spectrophotometric measurements and associated colorimetric calculations were made on section halves at 5 cm intervals at the same positions as MSP measurements. Color data were recorded as L*, a*, and b* variations. Several pronounced changes in color occur at Site U1354, and these are particularly well expressed in the upper part of the site, where core recovery was greatest (Fig. F23). Decreases in L* are typically coeval with decreases in b* and increases in a*, a pattern also noted at Site U1353. An example occurs in the interval between 20 and 26 m in Hole U1354B (all sections of Core 7H). This shift between 20 and 26 m is associated with a decrease in both NGR and magnetic susceptibility (Fig. F23). Lithologically, this interval is associated with a sandy horizon. A sharp excursion to higher b* values occurs at ~50 m in Hole U1354B, which is also coeval with a decrease in NGR and magnetic susceptibility. A less pronounced and more protracted increase in L* values is associated with this shift, as well as a shift to lower a* values. (Fig. F23). This excursion is associated with a marl interval (see "Lithostratigraphy"). The fact that both of these intervals are associated with low NGR and magnetic susceptibility values highlights the utility of using color measurements to help distinguish between the causes of changes in NGR and magnetic susceptibility.

Moisture and density

MAD sampling of cores on the catwalk, followed by adjacent sampling on the sample table, was carried out in cores from Holes U1354B and U1354C (Cores 1X through 15X). MAD samples were taken prior to discrete P-wave analysis. This approach effectively removed any bias toward more water being in the sample table samples: there was excellent correlation between catwalk and sample table MAD results (Fig. F24). We recommend that future MAD sampling be performed in this manner and that catwalk samples are not required.

In the uppermost 10 m of Hole U1354B, porosity and void ratio decrease and bulk density increases. There is little change in the following 100 m, below which porosity decreases again (Fig. F25). These observations are consistent with the porosity trends seen at the other three sites. Grain density shows some scatter near the surface, reflecting the variable lithology in Hole U1354B (see "Lithostratigraphy"). These grain densities in the top of Hole U1354B are more variable than those at Sites U1352 and U1353 but are less variable than those at Site U1351, suggesting decreasing variability with distance from the shore.

Sediment strength

Sediment strength measurements were conducted on working section halves from Holes U1354B and U1354C using automated vane shear (AVS) and fall cone penetrometer (FCP) testing systems (Fig. F26).

A comparison of both measurement methods is shown in the cross-plot in Figure F26C. Shear strength indicates that sediments range from very soft (0–20 kN/m²) to very stiff (150–300 kN/m²). Vane shear and fall cone shear strength correlate well in very soft and soft sediments, but AVS values are about three times lower in firm to very stiff sediments (standard deviation = 22.8 kPa) than FCP values (standard deviation = 86.0 kN/m²). A similar pattern was observed at all other Expedition 317 sites. These findings suggest that the applicability of vane shear in firm to very stiff sediments is limited and that the vane shear test underestimates the strength of stiffer sediments. Overall, vane shear and fall cone strength data from Hole U1354B are positively correlated (Fig. F26). Between 0 and ~250 m, shear strength generally increases, indicating a change from very soft to firm sediments. The lower sediment strength below ~250 m was also observed in cores from Hole U1353C and might coincide with XCB drilling.

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