Previous   |   Next

doi:10.2204/iodp.proc.323.103.2011

Physical properties

Cores recovered from Holes U1339A–U1339D from Site U1339, located on the northern end of Umnak Plateau, were placed on the Special Task Multisensor Logger (STMSL) "fast track" to record magnetic susceptibility and GRA bulk density values. Core sections were then allowed to warm to ambient laboratory temperature (19°–20°C) before being placed on the Whole-Round Multisensor Logger (WRMSL) to measure magnetic susceptibility, GRA bulk density, P-wave velocity, and noncontact resistivity. Cores 323-U1339B-1H through 4H were mostly consumed for microbiology sampling after STMSL processing, so no WRMSL data are available for these cores. By the third core (323-U1339A-3H) it was apparent that allowing the cores to warm to ambient temperature was also allowing gas expansion, probably due to the disassociation of intergranular particles of methane gas hydrate. The freed water and expanding gas (expansion ratio on the order of 1:165) significantly disrupted the depositional and structural fabric of the core sediment and presented a significant hazard for core handling. To minimize this disturbance, small holes were drilled through the core liner of the sections of Hole U1339A that were already racked and housed in the core laboratory. Subsequently, for Holes U1339B–U1339D, holes were drilled through the core liners while the cores were on the catwalk. To further mitigate core disruption, whole-round sections were scanned on the WRMSL as soon as possible after recovery. Next, natural gamma radiation (NGR) logging and thermal conductivity measurements were conducted routinely. Because of operational problems and time constraints, P-wave velocity and sediment shear strength measurements were not determined on selected sections of the working halves after core splitting.

Magnetic susceptibility

Magnetic susceptibility values vary widely, reflecting downhole changes in relative concentrations of terrigenous and tephra debris over biogenic material. These variations were expected because, during episodes of glacially lowered sea level, Alaskan drainage of the Yukon and Kuskokwim rivers presumably emptied near the shelf edge in the vicinity of the nearby Pribilof Islands, 200–250 km to the north (VanLaningham et al., 2009). Many of the recovered ash layers were probably launched by the basaltic centers of these islands and the more siliceous edifices of the Aleutian Islands, located an equal distance to the south.

Magnetic susceptibility data exhibit a downhole pattern of cyclic excursions between high and low values. These data are presented and described in "Lithostratigraphy" and "Stratigraphic correlation."

GRA wet bulk density

WRMSL GRA bulk density readings also oscillate downhole from relatively low values reflecting higher concentrations of biogenic (mainly diatom) debris to high values reflecting denser sediment richer in terrigenous components. In the "Lithostratigraphy" and "Stratigraphic correlation" sections, GRA data are displayed and compared with downhole changes in lithologic characteristics. Bulk density extracted from discrete sediment samples collected from the working halves is described below.

Natural gamma radiation

NGR readings vary generally rhythmically with depth from highs of 25–40 counts/s to lows of ~10 counts/s. High NGR likely tracks clay mineral–bearing sediment, whereas low NGR tracks less radiogenic deposits richer in biogenic debris. Peaks in NGR are 10–25 m apart, reflecting sections of more abundant deposition of terrigenous material over Umnak Plateau. The downhole NGR profile for Hole U1339D, which is similar to those for combined Holes U1339A and U1339B as well as Hole U1339C, is displayed in Figure F21.

P-wave velocity

WRMSL P-wave velocity was measured on the four cores taken from Hole U1339A. Because readings for Cores 323-U1339A-3H and 4H were seriously degraded by core-cracking and gapping caused by gas expansion, the P-wave logger was turned off for all core sections deeper than ~33 mbsf. The noncontact resistivity scanner was also turned off because of noisy readings. Velocity readings acquired in the uppermost ~15 m of Holes U1339A and U1339D range from ~1.54 km/s near the seafloor to <1.5 km/s at ~4 mbsf to ~1.57 km/s at the maximum depth of measurement. Because of the presence of gas bubbles and cracks, most of the recorded values below a depth of a few meters are questionable. Reliable in situ downhole velocity measurements are reported in "Downhole measurements."

MAD (discrete sample) wet bulk density

To measure moisture and density (MAD) properties, discrete samples of core sediment were taken from the working halves of the split sections. For Cores 323-U1339A-1H and 2H, discrete samples of ~10 cm³ were collected across 2 cm wide segments of each core section, typically at 50–51 and 99–100 cm from the section top. To speed data gathering and processing for deeper cores, sediment samples from Holes U1339A and U1339B were taken across just one segment of core section, typically at 29–31 cm. Below Core 14H, discrete sampling was typically limited to odd-numbered sections (i.e., 1, 3, 5, and 7), depending on core recovery. The sediment sampling tool was a 2 cm inner diameter plastic syringe that matched the throat opening of a calibrated and numbered sample vial into which the sample was extruded.

The depth distribution of wet bulk density reveals a cyclicity of high values averaging ~1.6 g/cm³ and low values of ~1.4 g/cm³ (Fig. F22). Peaks and troughs of bulk density excursions are commonly separated by 25–40 m. The succession of highs and lows presumably reflects relatively terrigenous-rich and biogenic-rich sediment. Average bulk density increases gradually from a near-seafloor value of 1.35 g/cm³ to 1.55 g/cm³ at the bottom of the hole at ~200 mbsf. This gradient of ~0.1 g/cm³/100 m of depth most likely expresses sediment compaction.

MAD porosity and water content

Porosity (percent pore space of wet sediment volume) measured on core samples exhibits an oscillating cyclicity with depth similar but of opposite polarity to the depth distribution of wet bulk density (Table T17). Water content, which is directly proportional to porosity, tracks its downhole distribution. Both curves are shown in Figure F23. Variations in porosity and water content dominantly reflect the changing concentration of biogenic debris—particularly highly porous diatom frustules—with respect to more terrigenous and less porous sediment. Average porosity decreases downhole from a near-surface value of ~85% to ~65% at ~200 mbsf, presumably reflecting compaction (Fig. F23).

Grain density

Grain (mineral) density versus depth oscillates from ~2.95 to ~2.3 g/cm³ (Fig. F24; Table T17). One-point excursions to extreme high and low values are probably spurious readings. Peaks are separated by 15–30 m. Grain density generally decreases downhole from a near-surface value of 2.65 g/cm³ to 2.55 g/cm³ at ~200 mbsf. Grain density variations presumably reflect interbedded diatom-rich and terrigenous-rich units and possibly a downhole overall trend of increasing diatom frustules and debris. This trend is similar to the downhole decrease in thermal conductivity (Fig. F23), which may also track increasing diatom content.

Thermal conductivity

Thermal conductivity was routinely measured toward the middle (~70–80 cm) of Section 3 and commonly also in Section 6 of each core from Holes U1339A–U1339D. Values range widely from a low of ~0.5 W/(m·K) to >1 W/(m·K). The smallest range of values (~0.96–0.66 W/[m·K]) was recorded in Hole U1339D (Fig. F25). Lower values presumably reflect water-rich diatomaceous sections, whereas higher values reflect sections with more abundant terrigenous debris. In Hole U1339D, thermal conductivity generally decreases downhole from 0.8 W/(m·K) near the surface to 0.7 W/(m·K) at ~200 mbsf. This downhole decreasing trend is less clear in thermal conductivity data from Hole U1339C sections and is not exhibited by thermal conductivity data from combined Holes U1339A and U1339B.

Formation factor

To determine formation factor, electrical conductivity (see "Physical properties" in the "Methods" chapter) was measured every 10 cm in the working half of the first core of Hole U1339A and every 20 cm in the working halves of Cores 323-U1339A-2H and 3H. Sediment conductivity ranges from 1.37 to 3.70 µS/cm. The highest values were recorded in ash layers, and formation factor generally increases downhole.

Top of page   |   Previous   |   Next