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doi:10.2204/iodp.proc.323.104.2011

Physical properties

Holes U1340A–U1340C were spudded at ~1297 m water depth in a sediment-filled structural depression on the upper eastern flank of Bowers Ridge. Core sections brought into the core laboratory from all holes were placed on the Special Task Multisensor Logger (STMSL) "fast track" and scanned for magnetic susceptibility and GRA bulk density. Core sediment was not gassy, as it was at Site U1339 on Umnak Plateau, and sections were allowed to warm to ambient laboratory temperature before being placed on the Whole-Round Multisensor Logger (WRMSL) for GRA, magnetic susceptibility, and P-wave scanning. Because of noisy data, noncontact resistivity values were not recorded. P-wave velocity and sediment shear strength measurements were not determined on working-half sections.

Magnetic susceptibility

Magnetic susceptibility values spike irregularly downhole in the uppermost 250 m of Hole U1340A, which was drilled to ~604 mbsf. The readings range from 500 to 1300 SI units. Excursions to higher values are thought to track layers richer in siliciclastic debris, in particular within the diatom silt of lithologic Unit I, which extends from the seafloor to ~275 mbsf (see "Lithostratigraphy"). Swings to high values are uncommon below ~250 mbsf, but at 525 mbsf, readings increase sharply to >1000 SI units, possibly registering a thick tephra unit. Magnetic susceptibility data are described in greater detail in "Lithostratigraphy" and "Stratigraphic correlation."

GRA wet bulk density

Bulk density determinations recorded by the WRMSL GRA sensor reveal high excursions and an apparent rhythmic pattern of higher values alternating with lower ones. The average reading in the tephra-bearing diatom silt of Unit I (surface to ~274.5 mbsf) is ~1.4 g/cm³. Within Unit I, bulk density decreases with depth to ~1.38 g/cm³ at ~360 mbsf. Below ~384 mbsf and a shift to XCB coring, values further decrease to ~1.32 g/cm³. In this section of diatom ooze, which defines lithologic Unit II, bulk density increases downhole from a low of ~1.32 g/cm³ to ~1.4 g/cm³ at the bottom of Hole U1340A at ~604 mbsf. GRA data are shown, more fully discussed, and compared with downhole changes in lithologic character in "Lithostratigraphy" and "Stratigraphic correlation." See also the below discussion in "MAD (discrete sample) wet bulk density."

Natural gamma radiation

In Hole U1340A, natural gamma radiation (NGR), which principally reflects clay mineral abundance, decreases with depth from near-surface readings averaging ~20 counts/s to ~10 counts/s at ~380 mbsf (Fig. F29). Below this depth, average NGR remains low, but significant excursions occur at levels of 30–40 counts/s and higher. A downhole oscillation from generally low to high values is discernible with a wavelength of ~80–120 m. The downhole decrease in NGR reflects the increasing relative abundance of siliceous microfossils of lithologic Unit II.

P-wave velocity

WRMSL P-wave velocity was measured on all core sections recovered at Site U1340. The recorded velocity profile for Hole U1340A, which reached the greatest subsurface depth (~604 mbsf) at Site U1340, is presented in Figure F30. Scattered values from ~1.45 to ~1.7 km/s increase overall from a near-surface velocity of ~1.52 km/s to ~1.55 km/s at ~280 mbsf. This section corresponds to lithologic Unit I (see "Lithostratigraphy"). The modest downsection increase in P-wave velocity presumably reflects slight compaction of the tephra-bearing diatomaceous silt of Unit I. In the underlying Unit II, particularly below the transition from APC to XCB coring at ~384 mbsf, average P-wave readings subtly increase from ~1.55 km/s to ~1.56 km/s at the bottom of Hole U1340A at ~604 mbsf. The diatomaceous ooze section and tephra beds of Unit II are likely mechanically stronger and less yielding to compaction than the overlying diatomaceous silt beds of lithologic Unit I. In general, the strength of diatomaceous deposits was also recorded in earlier DSDP holes drilled in the Bering Sea.

MAD (discrete sample) wet bulk density

Moisture and density (MAD) properties were measured on discrete samples (~10 cm³) of sediment taken from the working halves of core sections.

The depth distribution of MAD wet bulk density is closely similar to that traced by the WRMSL GRA sensor. MAD and GRA downhole profiles are shown and compared in Figure F31. The MAD profile documents a slight downhole trend of decreasing density from ~1.44 g/cm³ near the seafloor to ~1.38 g/cm³ at ~360 mbsf. Below this depth, which marks a switch to XCB drilling and a ~25 m thick section of poor core recovery (~360–384 mbsf), a low average density of ~1.32 g/cm³ was recorded. However, bulk density increases downhole to ~1.4 g/cm³ at the bottom of Hole U1340A at ~604 mbsf (Fig. F31). A water-rich and partially load bearing section signaled by the recovery of gravelly and sandy beds in the upper part of lithologic Unit II may separate the upper decreasing and lower increasing trends in bulk density. This conjecture is also suggested by the downhole break to higher sediment porosity discussed below in "MAD porosity and water content."

MAD porosity and water content

Figure F32 displays porosity and water content with depth for sediments recovered from Hole U1340A. The two curves are closely similar, following three separate shifts in trend downhole. For porosity, the upper trend from the seafloor to ~350 mbsf displays an average value near 74% that remains virtually constant with depth. The middle trend, which begins below the transition from APC to XCB drilling at ~384 mbsf and the zone of poor recovery from ~360 to 384 mbsf, documents a shift to a higher average porosity value near 80%, below which the average decreases progressively to ~66% at ~550 mbsf. Porosity then increases to ~75% at the bottom of Hole U1340A at ~604 mbsf (Fig. F32). Hydraulically, the middle porosity sections appear to be separated from the upper section by a permeability barrier in the zone of poor core recovery between ~360 and 384 mbsf. This surmised barrier occurs at a gravel-bearing sequence within the diatom ooze section of lithologic Unit II (see "Lithostratigraphy"). The isolation of the middle trend from the basal one is coincident with lithologic Subunit IIIA, a coarse-ashy layer overlying the sponge spicule–rich diatom ooze of Subunit IIIB.

Grain density

Average grain density decreases downsection from near-surface values of ~2.6 g/cm³ to ~2.3 g/cm³ at the bottom of Hole U1340A at ~604 mbsf (Fig. F33; Table T15). Wide excursions, some of which are so low (<1.5 g/cm³) or high (>2.9 g/cm³) that measuring error is suspected, occur about the mean (~2.45 g/cm³). The downsection decreasing values appear to reflect an increase in biogenic silica, chiefly diatom frustules, with respect to terrigenous mineral debris and tephra.

Thermal conductivity

Thermal conductivity was measured on one section, typically Section 1, of each core collected from Holes U1340A–U1340D; for some cores, thermal conductivity was also recorded on a second section. Thermal conductivity decreases downhole from 0.88 W/(m·K) near the surface to ~0.77 W/(m·K) at ~580 mbsf, just above the bottom of Hole U1340A (Fig. F34). Excursions to high readings between 1.07 and 1.08 W/(m·K) occur above ~275 mbsf. These excursions are not obviously linked to the occurrence of ash layers revealed by magnetic susceptibility measurements (see "Lithostratigraphy"). A similar downhole trend in decreasing thermal conductivity readings was observed at Site U1339 on Umnak Plateau. The downsection decrease in thermal conductivity presumably reflects increasing content of siliceous biogenic components with respect to terrigenous material and, particularly in Hole U1340A, sediment older than ~2.5 Ma, roughly the onset of NHG.

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