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doi:10.2204/iodp.proc.323.108.2011

Physical properties

Site U1344 is located at a water depth of ~3185 m along the summit of a canyon interfluve ~10–15 km southeast of Pervenets Canyon, a large submarine canyon that deeply and widely incises the Beringian margin (Normark and Carlson, 2003). Pervenets Canyon, along with companion Zhemchug Canyon, adjacent to Site U1343, was discovered in the early 1960s by the Soviet fishing industry and named after one of the discovering trawlers. At times of glacially lowered sea level, the head of Pervenets Canyon is commonly presumed to have been one of the outfall locations for the Anadyr River, which presently drains the far Russian northeast and enters the Bering Sea at the Gulf of Anadyr.

Regional studies document that the stratigraphic sequence underlying Site U1344 is 8–10 km thick and overlies a deeply subsided basement of Mesozoic rock. In the vicinity of nearby Site U1343, the sequence acoustically displays a vertical series of bottom-simulating reflectors (BSRs); the shallowest reflector, at ~350 m, has been linked to the formation of interstitial methane gas hydrate, and a deeper one, at ~1 km, has been linked to the diagenetic transition of opal-A to opal-CT (Cooper et al., 1987). In Hole U1344A, the posited hydrate BSR is acoustically weak, but it can be traced laterally on site survey reflection Record Stk3-7 (Takahashi et al., 2009) to a sloping stack of bright seafloor subparalleling reflection, typical of gas-charged sediment, that crosses upward into the hydrate stability field. Hole U1344A penetrates the putative BSR at ~450 mbsf. The in situ temperature calculated at this depth from downhole thermal data is ~22°C (see "Downhole measurements"). The ambient temperature and pressure (360–370 bar) are consistent with the phase boundary separating methane hydrate and methane gas.

Five holes, U1344A–U1344E, were drilled at Site U1344. The deepest penetration, 745 mbsf, was in Hole U1344A. Only moisture and density (MAD) physical property measurements were made on discrete samples collected from APC and XCB cores recovered at this site (Table T19). A transition from APC to XCB coring occurred at Core 323-U1344A-28X at ~255 mbsf. Core-section disruption from expanding gas degraded "fast track" Special Task Multisensor Logger (STMSL) and Whole-Round Multisensor Logger (WRMSL) P-wave velocity and gamma ray attenuation (GRA) bulk density logging. P-wave velocity scanning was turned off at Core 323-U1344A-3H. Gas expansion continued to separate and crack core continuity to the bottom of Hole U1344A.

Similar to the sedimentary sequence cored at Site U1343, the sequence penetrated in Hole U1344A is relatively uniform in lithology and described as one lithologic unit, Unit I (see "Lithostratigraphy"). Unit I is texturally dominated by clayey silt with varying amounts of diatom frustules. Sand, ash, and other microfossils are lesser components.

GRA wet bulk density

In Hole U1344A, WRMSL GRA readings of wet bulk density exhibit a noisy record of rapid, low-value excursions interpreted to reflect gas-expansion gaps that, which affected all cores to the bottom of the hole at 745 mbsf. The downhole density profile for Hole U1344A is displayed in Figure F27A.

In Figure F27B, the downhole profile of bulk density readings by the triple combo logging tool (below ~100 m wireline log matched depth below seafloor [WMSF]) reveals interpretable details and evidence of breaks in trends and amplitude and wavelength of oscillations. Breaks in trend are most noticeable at ~250 m WMSF, coincident with a change from APC to XCB core recovery at ~450 m WMSF, where the BSR is recorded and associated with an upward shift in average density from ~1.72 to 1.75 g/cm³, and at ~620 mbsf, where values shift higher to ~1.78 g/cm³. Below ~440 m WMSF, the wavelength of oscillations broadens and the average amplitude of the swings is higher.

Magnetic susceptibility

Magnetic susceptibility, as tracked by the WRMSL, exhibits little change in average value and character with depth in Hole U1344A (Fig. F28). The average value is ~50 SI units, but the range in readings varies from 0, which probably reflects a gas-expansion air gap, to 250 SI units and a few higher excursions. The spacing between high-value excursions is ~20–40 m, but these are superimposed on a longer wavelength of ~80–100 m. No change was noted in the overall pattern at the transition from APC to XCB core recovery (~250 mbsf) or at the projected position of the BSR (~450 mbsf). Based on what was learned at previous sites, the rhythmic oscillations are presumed to be a function of lithologic composition and patterns of in situ sediment alteration.

P-wave velocity

Except in the first three cores from Hole U1344A, P-wave velocity readings for the sedimentary section penetrated in Hole U1344A were only collected by the sonic downhole logging tool (see "Downhole measurements"). Sonic P-wave velocity data shown in Figure F29 reveal a profile similar to that recorded in Hole U1343E in that the average velocity increases downsection in steplike sectors. The upper section records a velocity increase from a near-surface average of ~1550 m/s to ~1650 m/s at ~230 m WMSF. After a sharp drop to ~1600 m/s, P-wave velocity increases steadily from ~1640 m/s at 250 m WMSF to ~1750 m/s at 450 m WMSF, the depth of the acoustically weakly recorded BSR. At ~485 mbsf, P-wave velocity drops back to ~1700 m/s before resuming its steady increase downhole. A bottom step increase in velocity to ~1800 m/s occurs at ~620 mbsf, a depth of poor core recovery (Cores 323-U1344A-66X and 67X).

Natural gamma radiation

Except in the uppermost ~80–100 m, across which natural gamma ray (NGR) readings increase from a near-surface measurement of ~25 count/s to ~34 counts/s, NGR values oscillate around an average value of ~30 counts/s to the bottom of Hole U1344A at 745 mbsf (Fig. F30). The wavelength of the oscillations broadens below the depth of the APC to XCB coring transition (~250 mbsf), but no obvious change occurs at the weak BSR at ~450 mbsf. Variations in NGR presumably reflect downhole changes in clay and siliciclastic mineral content.

Thermal conductivity

Thermal conductivity was measured on cores recovered from Holes U1344A, U1344B, U1344D, and U1344E. In Hole U1344A, the downsection distribution of thermal conductivity measurements can be grouped into an upper and lower sequence. As shown in Figure F31, the upper vertical sequence has an estimated average reading of ~0.905 W/(m·K) and extends downhole from the near surface to ~260 mbsf, below which APC refusal caused a change to XCB coring and P-wave velocity shifts abruptly to higher readings (Fig. F29). Thermal conductivity values in the upper group range from readings as low as ~0.8 W/(m·K) to highs near ~1.105 W/(m·K). The lower group begins near ~280 mbsf with a shift to an estimated average of ~1.02 W/(m·K), a larger range in low to high excursions, and a longer wavelength of oscillations between them. The average thermal conductivity for the entire section is ~0.910 W/(m·K) (see "Downhole measurements").

MAD (discrete sample) wet bulk density

The downhole variability of MAD (discrete sample) wet bulk density is displayed in Figure F32 and listed in Table T19. It is instructive to compare Figure F32 with Figure F27B, which presents downhole density recorded by the Hostile Environment Litho-Density Sonde (HLDS) logging tool. Although the MAD and logging profiles are similar, with the exception of the uppermost ~100 m of section not recordable by the triple combo tool, some important details were only revealed by the density log (see above). A significant loss of resolution is evident in MAD data below the transition from APC to XCB coring operations at ~250 mbsf. Because the lithologic character changes little downhole in Hole U1344A, the rapid downhole increase in MAD-measured density from 1.45 g/cm³ in near-surface sediment to ~1.65 g/cm³ at ~100 mbsf is interpreted as a record of early sediment compaction and water expulsion.

MAD porosity and water content

The downhole distribution of sediment porosity and water content in Figure F33A follows an oscillating but progressive decrease to the bottom of Hole U1344A. Near-surface porosity averages ~75%, which is similar to the porosity of nearby Site U1343 and, as described in the "Site U1343" chapter, is distinctly lower than is typical of non-Beringian margin Sites U1339, U1340, U1341, and U1342, all of which are characterized by a sedimentary column richer in diatom content. In Hole U1344A, porosity decreases most rapidly in the upper part of the drilled section, falling to an average of ~60% at 80–100 mbsf. Below this depth, values remain fairly uniform.

In Figure F33B, porosity derived from the density log changes in average value and trend. An abrupt shift at ~250 m WMSF to porosity as high as 80% is due to a borehole washout and is not valid. Below this depth, porosity averages ~60% from ~300 to ~440 m WMSF, where it decreases to an average of ~56%. At greater depths, sediment porosity gradually increases to ~63% at 620 m WMSF. At this depth, the trend shifts abruptly to ~55% and slowly decreases to ~52% at the bottom of Hole U1344A. The lowermost shift in trend at ~620 m WMSF is coincident with poor core recovery and changes in value and trend of other physical properties (Figs. F27B, F29, F30).

The overall downhole decrease in porosity tracked by MAD and logging data is presumably a manifestation of compaction. Details and offsets in this general trend are linked to lithostratigraphic changes and the influence of diagenetic factors. Downhole rhythmic oscillations from lower to higher porosity and water content readings, which are detected by virtually all other physical properties, likely reflect lithostratigraphic variations.

Grain density

Except for a few anomalous excursions, grain density can be grouped into three sections of decreasing density with increasing depth (Fig. F34). Similar groupings were identified in Hole U1343E. In Hole U1344A, the average density from the surface to ~160 mbsf is ~2.70 g/cm³. The middle sequence, ~160–620 mbsf, has an average density of 2.65 g/cm³, and the underlying basal group has a density of ~2.62 g/cm³. In comparison, the average density groupings in Hole U1343E are, respectively and with increasing depth, ~2.68 g/cm³ (versus 2.70 g/cm³), ~2.65 g/cm³ (versus 2.65 g/cm³), and ~2.63 (versus 2.62 g/cm³). Presumably, an overall uphole increase in the deposition of denser siliciclastic mineral debris with decreasing age is recorded at both Beringian margin drilling sites.

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