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doi:10.2204/iodp.proc.323.107.2011

Physical properties

Site U1343 is positioned on a large canyon interfluve bordering the western side of the lower course of the exceptionally large Zhemchug submarine canyon that deeply incises the Beringian continental margin. Hole U1343A was spudded at a water depth of 1964 m (APC corrected).

Sections of cores recovered from Holes U1343A–U1343E were scanned by the "fast track" Special Task Multisensor Logger (STMSL) to record whole-round magnetic susceptibility and GRA bulk density. After warming to ambient laboratory temperature, whole-round sections were placed on the Whole-Round Multisensor Logger (WRMSL) to record GRA bulk density, magnetic susceptibility, and P-wave velocity. Sediment samples were collected from sections recovered from Holes U1343A and U1343E and analyzed using moisture and density (MAD) procedures to determine moisture, bulk density, grain density, and porosity properties. MAD data from Hole U1343A (0–202 mbsf) were merged vertically with the lower part of Hole U1343E (0–744 mbsf). As shown on all MAD figures, the merge or splice depth of Holes U1343A and U1343E was selected at 200 mbsf (~220 m CCSF-D). The collection of overlapping MAD data in Hole U1343E began at Core 323-U1343E-33H, the top of which is at ~183 mbsf.

Gas-expansion voids and cracks throughout Site U1343 cores disrupted core-sediment integrity and created gaps and breaks that degraded the accuracy and in situ applicability of STMSL and WRMSL recordings. The transition from APC to XCB coring began at ~354 mbsf in Hole U1343E (Core 323-U1343E-42X), a depth that closely corresponds to that of an acoustically bright band of reflection horizons at 0.46 s two-way traveltime. Based on its cross-cutting relations to dipping beds and pressure-temperature setting, the band was identified earlier as a gas hydrate BSR by Cooper et al. (1987). XCB coring below the band mechanically disrupted and injected fluidized mud into the core sediment, which further denigrated physical property measurements with onboard magnetic susceptibility logging systems and MAD discrete sampling.

The sedimentary column cored at Site U1343 is relatively uniform in general lithology, and only one lithologic unit (Unit I) was defined (see "Lithostratigraphy"). Unit I varies in composition and texture from siliciclastic silt and silty clay beds to sequences of mixed siliciclastic and biogenic sediment (diatom ooze). Bulk density and P-wave velocity data from core measurements and logs suggest that the cored section may be more lithologically variable than one unit.

GRA wet bulk density

WRMSL GRA readings of wet bulk density for Hole U1343E are noisy and spiky because of expanding gas disruption of recovered cores and XCB coring disturbances below ~360 mbsf (Fig. F24A). Below ~10 mbsf, wet bulk density increases slightly from an average of ~1.60 g/cm³ to ~1.65 g/cm³ at ~100 mbsf. The average value below this depth, although oscillatory, does not seem to change until ~360 mbsf, where it decreases to ~1.60 g/cm³. This decrease is coincident with the change from APC to XCB coring and the consequent recovery of drilling-disturbed core sections. The transition to XCB coring also corresponds to the calculated depth (~360 mbsf) of the methane hydrate BSR identified by Cooper et al. (1987).

In contrast, the downhole triple combo logging tool string records a shift in average bulk density below the posited BSR from ~1.65 to ~1.72 g/cm³ (Fig. F24B; see "Downhole measurements"). These observations imply that sediment strengthening, which necessitated a shift to XCB coring, occurs at and extends below the strong reflectors. Additionally, onboard measurements of physical properties on mechanically disturbed and mud-slurry-injected core sediment produce generally lower readings than in situ values measured by the density log. At ~520–530 mbsf both the WRMSL and the downhole logging density tool record a slight shift to higher average density, from ~1.65 to 1.75 g/cm³ for whole-round GRA and from 1.72 to 1.75 g/cm³ for the downhole logging tool. This change to higher readings coincides with an increase in the V_P log (Fig. F25).

Despite gas expansion and coring degradation of sediment integrity, the downhole profile of average values recorded by the WRMSL GRA densitometer indicates cyclicity, with a dominant wavelength ranging between ~25 and ~50 m. Cyclic variations in wet bulk density are more clearly recorded by the density log (Fig. F24B). These oscillations likely reflect lithologic variations. If so, the downhole profile of logging-tool density implies the existence of at least two and possibly three physically distinguishable lithologic units.

Magnetic susceptibility

Figure F26 shows that magnetic susceptibility measured on Hole U1343E cores exhibits cyclicity, from averages of ~20–25 SI units to readings of >200 SI units. Peak readings, which are roughly separated by 30–50 m, are prominent to ~360 mbsf. Below this depth, which is coincident with the conjectured gas hydrate BSR, wavelength broadens and average values decrease. The bright band of the BSR is underlain by a deeper zone (>2 s two-way traveltime) of laterally disrupted reflection horizons, an observation commonly ascribed to acoustic scattering by interstitial gas bubbles. Diagenetic alteration of the deeper stratigraphic section through which the hydrate BSR has presumably vertically migrated may have degraded the magnetic susceptibility properties of deeper sediment.

P-wave velocity

P-wave velocity was measured in only the first core of Hole U1343A. Because of abundant, wide cracks and breaks in core-sediment continuity caused by gas expansion, P-wave velocity could not be measured with any useful meaning with respect to in situ values. P-wave and shear wave velocities are, however, well recorded by the downhole FMS-sonic logging tool string (see "Downhole measurements"). The log of P-wave velocity readings displayed in Figure F25 documents an increasing average velocity with depth from a near-surface value of ~1550 m/s to ~1840 m/s at the bottom of Hole U1343E at ~744 mbsf. Three gradients of increasing P-wave velocity are recognizable: Gradient 1 (0 to ~360 mbsf; increasing at ~110 m/s/km), Gradient 2 (360–520 mbsf; increasing at ~550 m/s/km), and Gradient 3 (~530–744 mbsf; increasing at ~890 m/s/km). The transition between Gradients 1 and 2 is coincident with the depth of the methane hydrate BSR identified by Cooper et al. (1987) along this sector of the Beringian margin and the shift from APC to XCB coring. The steepened lower gradient presumably reflects lithification, perhaps contributed to by the ascent of the BSR through the underlying sediment. The conspicuous ramp of Gradient 3 cannot be linked to a described change in dominant lithology or solely to compaction effects, although average bulk density does increase slightly on the downhole profiles of WRMSL GRA and MAD measurements (Fig. F24). An explorable explanation for the steepness of the lower gradient is the increased abundance of authigenic carbonate minerals and shell debris observed in cores recovered in Hole U1343E below ~520 mbsf (see "Lithostratigraphy").

Thermal conductivity

Thermal conductivity was determined on whole-round core sections, typically on Sections 1 or 2 of each core recovered from Holes U1343A, U1343C, U1343D, and U1343E. Thermal conductivity readings in Hole U1343E (Fig. F27) vary widely between ~1.2 and ~0.6 W/(m·K) about a mean value of ~0.95 W/(m·K). Exceptionally low excursions are most likely the consequence of gas expansion and core disruption. In general, cores collected above the posited methane hydrate BSR at ~360 mbsf exhibit higher variability and are also the most gas-disrupted sections measured. Cores collected below the transition to higher carbonate content at ~520 mbsf display the highest range of thermal conductivity values (Figs. F24, F25).

Natural gamma radiation

Similar to WRMSL GRA measurements of wet bulk density, NGR readings on the same core sections from Hole U1343E also track a downhole trend of increasing values from near-surface readings of ~25 counts/s to ~32 counts/s at ~100 mbsf (Fig. F28). Below this depth, NGR oscillates greatly and decreases slightly to ~25 counts/s at ~320 mbsf, where a significant increase to ~40 counts/s is recorded. This depth is effectively the same as that of the posited hydrate BSR and the shift to XCB coring. Deeper in Hole U1343E, average NGR decreases gradually to ~25–26 counts/s. At ~520 mbsf, readings increase to ~30 counts/s. The shift to higher NGR corresponds to the top of trends of higher readings of wet bulk density and downhole-measured P-wave velocities (Figs. F24, F25).

NGR generally tracks the abundance of clay minerals and their absorbed radioactive nuclei. Apparently, higher bulk density sediments in Hole U1343E are also richer in clay and other siliciclastic minerals. Diatoms and other siliceous microfossils that resist compaction and sediment consolidation do not make up the dominant component constructing the stratigraphic section sampled at Site U1343. Perhaps because of this circumstance, NGR readings track compaction-driven densification of clay-rich beds, an observation consistent with the progressive downhole increase in P-wave velocities (Fig. F25; see "Downhole measurements").

MAD (discrete sample) wet bulk density

Similar to downhole distribution of wet bulk density tracked by the WRMSL GRA densitometer, average values of MAD or discrete sample density increase downhole in Hole U1343A from a near-surface value of ~1.50 g/cm³ to an average near 1.70 g/cm³ at ~100 mbsf (Fig. F29; Table T17). The average MAD bulk density changes little below this depth, including across the transition to samples collected from Hole U1343E below 200 mbsf and at the BSR at ~360 mbsf. Significant excursions above and below this mean are probably spurious measurements. To understand the accuracy and usefulness of MAD data measured on gas- and coring-disturbed cores, it is helpful to compare MAD bulk density measurements to in situ bulk density measurements recorded by the downhole triple combo logging tool string (Fig. F24B; see "GRA wet bulk density" and "Downhole measurements").

MAD porosity and water content

Figure F30 displays closely similar profiles of water (moisture) content and sediment porosity recorded in core samples recovered from Holes U1343A and U1343E. Near-surface porosity is ~70%, noticeably lower than that measured at Sites U1339 (~80%), U1340 (~75%), U1341 (~78%), and U1342 (~80%). This circumstance can be ascribed to the lower overall content of siliceous microfossils composing the sedimentary section cored at Site U1343. Porosity (and water content) decreases sharply downhole to ~60% at ~80 mbsf, below which it only gradually lessens to ~56% at ~744 m CCSF-D. Porosity or gradient changes are not recorded across the presumed hydrate BSR at ~360 mbsf (390 m CCSF-D) or below ~520 mbsf (547 m CCSF-D), where shifts to higher in situ P-wave velocity and bulk density readings occur (Figs. F24B, F25). The gentle downhole reduction in porosity and water content can best be ascribed to compactive dewatering, which is most prominently exhibited in the uppermost ~80 m of section.

Grain density

Stripped of rapid excursions to single-point high and low values, average grain density seems to show three density-fluctuating (±0.2–0.3 g/cm³) (Fig. F31) groupings: an upper group from the seafloor to ~100 mbsf with an average density of ~2.68 g/cm³; a middle group between ~100 and 540 mbsf with an average density of ~2.65 g/cm³; and a basal group with a lower density of ~2.55 g/cm³ at ~540 mbsf that increases downhole to 2.70 g/cm³ at 744 mbsf. Only the lower group displays a clear increase with depth that matches both the downhole increase in the bulk density and P-wave velocity logs and the occurrence of authigenic carbonate and shell debris (Figs. F24B, F25). Within the groups, fluctuation in grain density appears to increase downhole. The range for the bottom group, 2.35–2.82 g/cm³, is quite high, a pattern that matches the wide, oscillating swings of bulk density measured by the density log (Fig. F24B). The wavelength of oscillations broadens below ~320–340 mbsf, the approximated depth of the seismically identified methane hydrate BSR.

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