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doi:10.2204/iodp.proc.323.105.2011

Physical properties

Site U1341 was spudded at a water depth of ~2193 m on the broad bathymetric terrace that rings the west-facing inside curve of Bowers Ridge. Seismic reflection data imply that the drilled section is a blanket of pelagic sediment draped over undulating basement relief. Cores recovered from Holes U1341A–U1341C were sectioned on the catwalk and placed on the Special Task Multisensor Logger (STMSL) "fast track" to record whole-round magnetic susceptibility and GRA bulk density readings. Core sections were then allowed to warm to ambient core laboratory temperature (~4 h) before being placed on the Whole-Round Multisensor Logger (WRMSL) for GRA, magnetic susceptibility, and P-wave scanning. Noncontact resistivity was not recorded with the WRMSL, and P-wave velocity and sediment shear strength were not measured on working-half sections.

Physical properties measured on cores recovered from Hole U1341B are described below with respect to the principal lithologic units (see "Lithostratigraphy" and "Stratigraphic correlation"). Core recovery was virtually continuous to a bottom-hole depth of ~605 mbsf. Examination of the recovered cores identified two dominant lithologic units over which physical properties were measured. The major lithologic divisions shown in the figures accompanying this chapter are Unit I (diatom silt; surface to ~203 mbsf) and Unit II (nannofossil, sponge spicule–rich diatom ooze; ~203 to 605 mbsf).

GRA wet bulk density

The WRMSL GRA sensor recorded a trend of slightly decreasing values of wet bulk density from a near-surface reading of ~1.4 g/cm³ to ~1.32 g/cm³ at the bottom of Hole U1341B at ~605 mbsf. At ~220 mbsf, which is below the transition from Unit I to Unit II, a discernible but small shift to lower density near ~1.25 g/cm³ is evident (Fig. F24B). This shift is more prominently revealed in the downhole distribution of moisture and density (MAD) discrete sample bulk density values discussed below and shown in Figure F24A. The entire vertical profile of bulk density undulates broadly from an average of ~1.30 g/cm³ to a maximum of ~1.45 g/cm³. The wavelength of fluctuations narrows downhole. Oscillations from low to high values presumably reflect changing relative concentrations of terrigenous or siliciclastic detritus and siliceous microfossils.

The small overall downhole decrease in average bulk density likely reflects an increase in low-density siliceous microfossils, in particular diatom frustules, which counteract the density-increasing effects of compaction.

Magnetic susceptibility

The uppermost ~210 m of sediment recovered from Hole U1341B exhibits rapid excursions to readings of >200–400 SI units (Fig. F25). Below this depth, magnetic susceptibility readings are subdued and rapid deflections to >100 SI units are uncommon except for a broad band of high susceptibility that occurs between 565 and 575 mbsf. The rapidly varying high values of the upper section are coincident with lithologic Unit I. Although ash layers occur in this unit, they are equally if not more abundant in underlying Unit II, which displays only background variations in readings. The contrast in the profiles of magnetic susceptibility readings between Units I and II perhaps reflects the diagenetic degradation of magnetic susceptibility properties.

Natural gamma radiation

Figure F26 records downhole NGR readings, which spike to highs of >40 counts/s above an undulating and generally decreasing trend from a near-surface average of ~15 counts/s to less than ~5 counts/s at the bottom of Hole U1341B. A shift to lower values occurs near the Unit I/II boundary at ~203 mbsf (Fig. F26). The overall decreasing trend and broad superimposed oscillations suggest that clay mineral content decreases irregularly downhole, which is consistent with the decreasing terrigenous content of the drilled section, from the diatom silt of Unit I to the dominantly siliceous ooze of Unit II. Within this unit, a broad swing to higher readings above ~25 counts/s occurs between ~380 and 400 mbsf, presumably recording higher siliciclastic content (see "Stratigraphic correlation").

P-wave velocity

WRMSL P-wave velocity increases downhole from ~1.51 km/s for near-surface sediment to ~1.56 km/s at the bottom of the hole at 605 mbsf (Fig. F27). The spacing of velocity measurements was changed from 30 cm in the uppermost ~180 m of Hole U1341B to 5 cm between ~180 and 260 mbsf and 15 cm below 260 mbsf to the bottom of the hole at ~605 mbsf. A slight shift to lower readings (0.02 km/s) appears to occur across the transition from lithologic Unit I to Unit II. The low overall gradient in downhole velocity, which is estimated at only 0.008 km/s/100 m, is a manifestation of the ability of diatomaceous sediment to resist compaction (see "MAD porosity and water content" below).

MAD (discrete sample) wet bulk density

To measure physical properties with the MAD technique, sediment samples of ~10 cm³ were routinely collected from Sections 1, 3, and 5 of each core recovered from Hole U1341B. Wet sediment bulk density, one of the principal physical properties measured, can be compared with GRA wet bulk density readings recorded with the STMSL and WRMSL scanning systems.

Figure F24A presents the downhole distribution of MAD bulk density, which can be compared with the GRA bulk density profile in Figure F24B. MAD density plotted at a higher exaggeration clearly reveals the contrasting density characteristics of the diatom silt of lithologic Unit I and the siliceous microfossil ooze of Unit II. Unit I sediment density fluctuates between 1.62 and 1.2 g/cm³ in a slight but perceptible increase with depth to the Unit I/II boundary. Density decreases at this boundary to ~1.27 g/cm³, a value effectively the same as that recorded by the WRMSL GRA scanner (Fig. F24B). Density in Unit II varies less than in Unit I and increases to a slightly higher average of ~1.36 g/cm³ at ~380 mbsf. Below this depth, bulk density decreases slightly and irregularly to the bottom of Hole U1341B. The lower overall bulk density in Unit II apparently reflects a higher concentration of low-density siliceous microfossils than that found in the diatom silt of overlying Unit I.

MAD porosity and water content

Just beneath the surface, porosity values average ~80%, with a corresponding water content of ~60% (Fig. F28; Table T16). At the bottom of Hole U1341B, water content decreases slightly to ~56% and porosity correspondingly decreases to ~75%. Similar to the depth profiles of most other physical properties, the downhole distribution of sediment porosity exhibits undulations or excursions to higher and lower values. These oscillations are more pronounced within the diatom silt beds of lithologic Unit I than they are within the siliceous ooze of underlying Unit II. Across the transition from Unit I to Unit II, a prominent break to higher readings is evident in both the water content and porosity profiles. This change reflects the high porosity of diatom ooze and its resistance to compaction-driven consolidation and densification, as demonstrated by the overall downhole decrease in bulk density (Fig. F24).

Grain density

The downhole variation in grain density is prominently offset, from an average density of ~2.39 g/cm³ to 2.23 g/cm³ at the Unit I/II boundary (Fig. F29; Table T16). Average density also decreases with depth from a near-surface value of ~2.50 g/cm³ to a low of 2.10 g/cm³ at the bottom of Hole U1341B. These trends are interpreted as tracking the downhole increase in the relative abundance of low-density siliceous microfossils, particularly diatoms. Superimposed on this overall gradient are excursions to higher and lower readings similar to those exhibited by the profiles of porosity and water content (Fig. F28), wet bulk density (Fig. F24), and NGR (Fig. F26).

Thermal conductivity

Thermal conductivity measurements were typically made on Section 3 of each core recovered from Hole U1341B. Thermal conductivity decreases overall with depth from a near-surface value of 0.85 W/(m·K) to 0.80 W/(m·K) at the bottom of Hole U1341B (Fig. F30). This profile thus parallels the downhole decreasing values of most other physical properties measured on cores recovered from Hole U1341B. A break or change in the thermal conductivity profile is not obvious at the 203 mbsf Unit I/II boundary. However, with respect to the density of measurement points of other MAD properties, thermal conductivity determinations are more widely spaced and are least capable of tracking a change in thermal conductivity at the boundary. As was conjectured for sediment recovered at Sites U1339 and U1340, the decrease in thermal conductivity readings with depth is ascribed to the increased abundance of siliceous microfossils, which also lowers the readings of NGR, porosity, water content, bulk density, and grain density (Figs. F24, F26, F28, F29).

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