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doi:10.2204/iodp.proc.323.108.2011

Lithostratigraphy

Five holes were cored at Site U1344, reaching a maximum depth of 746.59 meters below seafloor (mbsf) in Hole U1344A. Sediments at this site are primarily composed of silt with varying amounts of sand, clay, and diatoms and minor amounts of ash, foraminifers, nannofossils, and sponge spicules. Authigenic carbonates occur frequently at this site in all holes below 50 mbsf. The sediments are predominantly dark/very dark greenish gray to dark/very dark gray. One lithologic unit spanning the early Pleistocene to the Holocene was defined at this site.

Description of unit

Unit I

• Intervals: Sections 323-U1344A-1H-1, 0 cm, through 79X-CC, 31 cm; 323-U1344D-1H-1, 0 cm, through 32H-CC, 17 cm; and 323-U1344E-1H-1, 0 cm, through 23H-CC, 29 cm

• Depths: Hole U1344A, 0–746.59 mbsf; Hole U1344D, 0–286.27 mbsf; and Hole U1344E, 0–211.24 mbsf

• Age: Holocene to Pleistocene

Unit I is characterized by two alternating lithologies determined by the proportions of siliciclastic and biogenic grains: (1) diatom-rich or diatom-bearing silts and clays with <40% biogenic components and (2) sediments with >40% biogenic components that include mixed lithologies of silt/clay and diatoms, diatom ooze, and, rarely, laminated diatom ooze with varying abundances of foraminifers, nannofossils, and sponge spicules (see "Site U1344 smear slides" in "Core descriptions"). These two lithologies alternate on a decimeter to meter scale (Figs. F6, F7, F8). The siliciclastic sediments are mostly diatom-rich clayey silt, silty clay, and silt, whereas mixed siliciclastic-biogenic sediments are generally diatom silt and occasionally diatom clayey silt. Siliciclastic and mixed sediments vary in color from very dark greenish gray (10Y 3/1) and dark greenish gray (10Y 4/1, 5GY 4/1, and 10GY 4/1) to dark gray (5Y 4/1 and 4/N) and very dark gray (5Y 3/1 and 3/N). Diatom oozes are generally olive (5Y 4/3) to olive-gray (5Y 4/2) and dark greenish gray (10Y 4/1 and 5GY 4/1). The boundaries between lithologic changes are usually gradational, and color and texture changes are very subtle. However, there are also several sharp boundaries, which are often associated with the lower boundaries of ash or sand layers. No visible soft-sediment deformation was observed at this site.

Similar to Site U1343, foraminifers, nannofossils, and sponge spicules in the main lithologies are generally rare (<5%). Sponge spicules occur as sponge spicule aggregates throughout the cores and are cylindrical with a central cavity (often filled with sediment; Fig.F9). Sponge spicule aggregates are concentrated at four depth intervals in Hole U1344A: between ~40 and 80, ~180 and 260, ~400 and 480, and 560 and 680 mbsf.

The siliciclastic fraction is composed of quartz, feldspar, rock fragments (commonly polycrystalline quartz and basalt), mica, and clay minerals. The amount of clay minerals identified in Site U1344 smear slides is similar to that identified at Site U1343 (see "Site U1344 smear slides" in "Core descriptions").

Numerous bivalve shells and shell fragments were found at this site, but these were generally less abundant than at Site U1343. However bivalve shell preservation was excellent in one case, where a complete bivalve shell was found lying open, ventral side up in the core, with the hinges touching (Fig. F10).

Only one diatom-rich laminated interval (Fig. F11) occurs at this site: in Hole U1344A at 5.3 mbsf, in Hole U1344D at 3.5 mbsf, and in Hole U1344E at 5.0 mbsf. The laminations are defined by faint color changes. The laminated interval has gradational boundaries with the surrounding lithologies. Other intervals described as laminated in the barrel sheets (Figs. F6, F7, F8; 117.6, 140.7, and 577.5 mbsf) are laminated sand layers that compare with similar layers described at Site U1343. Laminations in Hole U1344A at 177.6 and 140.7 mbsf are thin and parallel, and the interval has gradational boundaries with other lithologies. The laminated sand interval in Hole U1344A, 577.5 mbsf, shows thick, wavy laminations and a sharp base.

Sediments at Site U1344 are soft in Hole U1344A down to 255 mbsf, stiff between 255 and 280 mbsf, and soft again down to 622 mbsf. Farther downhole they vary between soft and stiff. Other minor variations in lithification, which can be linked to drilling disturbances created by XCB coring (e.g., biscuits of stiff sediment with soft to soupy material in between), were also noted. Notably, APC refusal at Site U1344 was reached at ~270 mbsf in an interval of stiff sediments.

Bioturbation is slight to moderate throughout the cores apart from the laminated intervals, where bioturbation is absent. Note that in XCB-cored sediments with faint color variations it is generally difficult to distinguish the degree of bioturbation. Mottling of sediment with different colors or textures is common throughout Site U1344. Many of the dark gray lithologies contain black streaky mottles that may be related to pyrite content, and dark green mottles likely contain glauconite (Section 323-U1344A-70X-7) (see XRD in "Supplementary material"). Site U1344 also has numerous sandy patches and layers (Fig. F12), which sometimes contain a higher ratio of polycrystalline quartz than is otherwise observed in the sediment. Both sandy patches and layers are concentrated mainly at four depth intervals in Hole U1344A: between ~45 and 80, ~190 and 255, ~400 and 485, and 675 and 715 m core composite depth below seafloor (CCSF-A). The three uppermost intervals overlap intervals rich in sponge spicule aggregates.

Many accessory sediment components were described at Site U1344. Subrounded to well-rounded clasts occur frequently and are usually granule to pebble sized. The clasts are often black and fine grained and composed of either basalt or quartzite. Notably large clasts include a 4 cm rounded plutonic pebble and an 8 cm plutonic cobble (both diorite) found in Cores 323-U1344A-65X and 69X, respectively (see "Site U1344 thin sections" in "Core descriptions;" Fig. F13). The large clast recovered in the core catcher of Core 323-U1344A-69X may have been the cause of no recovery in Cores 323-U1344A-66X and 67X. Although some intervals have higher clast abundances than others, clasts occur throughout the sedimentary record at Site U1344 (Figs. F6, F7, F8).

Ash is less common at this site than at sites farther south. However, several thin ash layers as well as ash-filled mottles were found in all holes at this site. The ash is usually fine-grained and either gray or light olive in color (Fig. F14). Black ash is less common.

Authigenic carbonates occur frequently at Site U1344. Rhombs, acicular crystals, and globular crystals with extreme birefringence were observed in many samples, similar to minerals observed at Site U1343. The sediment containing authigenic carbonates is often slightly lighter or more yellowish than the surrounding sediments (Fig. F15). The shallowest appearance of authigenic carbonate in Hole U1344A is at 52 mbsf (Core 323-U1344A-6H).

As at most previous Expedition 323 sites, the cored sediments were very gassy. The presence of gas in the sediments caused several types of coring disturbance, mostly cracks as wide as several centimeters. In some cases, this affected the stratigraphic integrity of the sediment, as at Sites U1339 and U1343. The sediment at the top of Cores 323-U1344A-7H through 27H was ejected out of the core barrel by gas expansion, and 20–150 cm of sediment was extruded onto the deck. This sediment was pushed back into a core liner; however, some sediment pieces may be out of order or upside down. Sediment from either the top or bottom of most cores between Cores 323-U1344A-29X and 77X was similarly extruded. Punctures were made in all cores after Core 323-U1344A-2H, causing a potentially significant loss of sediment from extrusion through the punctures. However, punctures were not always noted in core descriptions because they were not always visible on the cut surfaces of the cores. Similar coring disturbances occurred in sediments from all holes at this site. In conclusion, gas expansion features and sediment biscuiting from XCB coring led to a significant reduction in sediment recovery and core quality, respectively, below 270 mbsf in Hole U1344A (Figs. F6, F7, F8).

Discussion

Unit I at Site U1344 spans the Holocene to early Pleistocene and is comparable to Unit I defined at other Expedition 323 sites. It is very similar in lithology to Unit I defined at the other Bering Sea margin Sites U1339, U1343, and U1345. However, Site U1344 is distinct because it has an even higher proportion of siliciclastic components and a higher occurrence of sand-sized grains than Site U1343 and, especially, Site U1339. This is probably related to Site U1344's location on the continental slope and its relative proximity to sources of terrigenous sediments from the continental margin, whereas both Sites U1339 and U1343 are situated on submarine highs separated from the main continental slope. The siliciclastic layers were probably deposited during glacial sea level lowstands, when large portions of the Bering Sea shelf were subaerial, allowing rivers to transport terrigenous material closer to the shelf edge. This terrigenous material could have then been remobilized farther down the continental slope and redeposited at Site U1344.

Changes in the proportion of siliciclastic and mixed siliciclastic-biogenic sediments are probably related to changes in primary productivity, sea level, the proximity and flux of terrigenous sediments from rivers, and variable delivery of ice-rafted debris (IRD) by ice through glacial–interglacial cycles. Sandy lithologies are concentrated in four relatively distinct intervals in Hole U1344A (Cores 323-U1344A-5H through 8H, 21H through 25H, 48X through 50X, and 74X; Fig. F6). They can be correlated not only between the holes at Site U1344, but also to Site U1343, where four distinctly sandy intervals occur in Cores 323-U1343A-7H, 18H through 23H, 36H through 52X, and 77X through 83X. A clear correlation between sand-rich lithologies and isolated clasts (dropstones) is not evident, which suggests that they were not exclusively deposited by the same mechanism. However, isolated clasts may be undersampled in visual core descriptions, whereas continuous sandy layers are easier to recognize.

Isolated clasts (fine pebbles and pebbles) occur throughout all holes at Site U1344 and were probably transported to this site by ice. Although ice can transport clay-, silt-, and sand-sized grains, other mechanisms such as mass gravity flows may also be significant for the transport of sand-sized and smaller grains. The latter mechanism is consistent with the sharp bases and normally graded structure of some of the sand beds. Because the sandy lithologies appear to be clustered in distinct depth intervals at both Sites U1343 and U1344 and include normally graded sandy layers, they may document periods of increased siliciclastic input by mass movements that occurred along large parts of the northwest Bering Sea shelf break. Such depositional events may have been triggered by the melting of land ice, leading to strong river erosion and runoff during deglaciation periods or at the start of the following interglacial. Furthermore, according to VanLaningham et al. (2009), the siliciclastic flux to this site may have been influenced by the trajectory of the Yukon River, the largest source of sediment to the modern Bering Sea. Yukon-source sediment appears to have been transported southward through the Kamchatka Strait during glacial periods and northward through the Bering Strait during interglacials (VanLaningham et al., 2009). The path of the Yukon across the shelf during glacials is unknown, and sites close to the mouth of the Yukon River would presumably have had much greater sediment accumulation than locations more distal to the sediment source.

The lithologies dominated by diatoms are associated with changes in color reflectance (i.e., higher b* values) (Figs. F6, F7, F8). This is analogous to the changes in diatom abundance and color reflectance parameter b* at other sites (see the "Site U1339," "Site U1341," "Site U1342," and "Site U1343" chapters); however, it is subtler here because of the overall higher abundances of siliciclastic detritus, that is, a stronger dilution of the biogenic signal. The higher abundance of diatoms may reflect high diatom flux during interglacials, as previously observed in the Bering Sea (Okazaki et al., 2005). Alternatively, the input of detrital material may have been reduced during interglacials because of the higher sea level and more distal coastline. However, in contrast to previous sites, no laminated intervals dominated by biogenic components were found at Site U1344. This finding is probably related to the position of Site U1344 below the depth at which the OMZ impinges on the northwest Bering Sea slope. Therefore, we expect that there was well-oxygenated bottom water throughout the Pleistocene. It is thus very intriguing that thin laminated intervals occur at Site U1344. Future study will hopefully illuminate how these laminae were preserved.

Lithologies composed only of diatom ooze were described in a few instances, mostly in the uppermost part of the sediment record at this site. A possible explanation for the relatively low number of diatoms in the sediment could be Site U1344's location at the foot of the continental slope, which likely receives a large proportion of detrital material that dilutes the biogenic component (see discussion above). Alternatively, this site could have been influenced by lower surface productivity that resulted in a lower export of biogenic opal to the seafloor. The greater water depth of Site U1344 could explain why the abundance of biogenic carbonate is generally lower than that at Site U1343, especially in the lower part of the record, because Site U1344 may be affected by stronger calcite-undersaturated, and hence more corrosive, bottom waters. However, lower export productivity may also have contributed to the lower abundance of biogenic carbonate at Site U1344.

The lack of diatom ooze may also explain why APC refusal at Site U1344 was met in Core 323-U1344A-27H, about 100 m shallower than the depth of refusal at Site U1343. Also, overall lower porosity (see "Physical properties") could be related to the lack of high-porosity diatom oozes.

Interestingly, despite fewer biogenic remains in Site U1344 sediments and a very clear dominance of siliciclastic material, whitish sponge spicule aggregates were still found throughout large parts of the record. The abundance of these aggregates, which are probably remains of agglutinated benthic foraminifers, could hint at different paleoenvironmental conditions, such as bottom water redox conditions, than those at Site U1343. However, this possibility requires further investigation.

As at Site U1343, volcaniclastic material is a minor component of sediment at Site U1344 because this site is more distant from the Aleutian arc. Thin ash layers (5 cm) occur mostly in the upper part of each hole (above Cores 323-U1344A-25H and 323-U1344D-28H), as do ash-filled mottles, which probably represent bioturbated traces of ash layers. As at Site U1343, the ash is typically lighter in color than ash at the Bowers Ridge or Umnak sites, which is consistent with an explosive rhyolitic source capable of ejecting ash as far as this site.

Similar to Site U1343, many of the isolated clasts found in Site U1344 cores are basalt and may have originated from the Aleutian Islands or older volcanic rocks exposed on continental Alaska. Other pebble compositions include plutonic rocks (e.g., diorite and granodiorite) and quartzite. These pebbles likely originated from the Alaskan landmass and are tentatively classified as dropstones because they are isolated and matrix supported. Almost all dropstones are well rounded, indicating a period of reworking prior to incorporation in the ice. Because clasts associated with continental ice sheets tend to be derived from bedrock via freeze-thaw and ice wedging in periglacial regions or plucking from beneath the ice sheet, they are typically angular or striated rather than well rounded. Rounding therefore favors a coastal provenance and sea ice rafting rather than icebergs (Lisitzin, 2003). The rocks from which these clasts are derived may have been located farther inland, and the clasts could have been transported to the coast via rivers.

There is a shallow sulfate–methane transition zone (SMTZ) and abundant methane in the sediment column at Site U1344, as at the other slope Sites U1339 and U1343 (see "Geochemistry and microbiology"). This is probably a function of high-export production along the Bering slope Green Belt, where there is upwelling of nutrient-rich water due to the interaction of tides and the bottom topography (Springer et al., 1996). Authigenic dolomite, high-magnesium calcite, and aragonite can form from elevated alkalinity in the SMTZ where the vertical methane flux is high (Peckmann and Thiel, 2004). However, distinct and indurated authigenic carbonate layers are absent at Site U1344. Only slightly lighter colored intervals several centimeters thick occasionally occur throughout the sedimentary record. A similar situation with very few lithified dolostone layers was observed at Site U1343, suggesting a potential correlation between high sediment accumulation rates and a lack of distinct, (semi-)lithified authigenic carbonate horizons. In the sediment sections below Core 323-U1344A-24H, finely disseminated authigenic carbonate precipitates of (sub)millimeter size are frequent. In thin sections they are acicular, rhomboid, or square and thus may represent different carbonate minerals and/or different stages of formation. This implies that active diagenetic alteration of the sedimentary record related to organic matter degradation is taking place, but signs of nonsteady-state diagenesis are less pronounced than at Sites U1339, U1340, U1341, and U1342 (see "Geochemistry and microbiology"). Low sedimentation rates never prevailed long enough to fix the SMTZ at a certain depth and precipitate indurated authigenic fronts.

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