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doi:10.2204/iodp.proc.323.107.2011

Lithostratigraphy

Five holes were cored at Site U1343, reaching a maximum depth of 744.0 meters below seafloor (mbsf) in Hole U1343E. The sediments at this site are primarily composed of silt with varying amounts of clay and diatoms and minor amounts of sand, ash, foraminifers, calcareous nannofossils, and sponge spicules. Authigenic carbonates appear in all holes deeper than 35 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-U1343A-1H-1, 0 cm, through 22H-CC, 58 cm; 323-U1343B-1H-1, 0 cm, through 4H-CC, 26 cm; 323-U1343C-1H-1, 0 cm, through 26H-CC, 31 cm; 323-U1343D-1H-1, 0 cm, through 1H-CC, 14 cm; and 323-U1343E-1H-1, 0 cm, through 83X-CC, 40 cm

• Depths: Hole U1343A, 0–201.76 mbsf; Hole U1343B, 0–35.26 mbsf; Hole U1343C, 0–234.72 mbsf; Hole U1343D, 0–8.53 mbsf; and Hole U1343E, 0–744.00 mbsf

• Age: early Pleistocene to Holocene

Two mainly siliciclastic and mixed siliciclastic-biogenic lithologies alternate on a decimeter to meter scale. The siliciclastic sediments (<40% biogenic components) are mostly diatom-rich clayey silt/silt/silty clay. The mixed siliciclastic-biogenic sediments (>40% biogenic components) are mixed lithologies of silt/clay and diatoms, diatom ooze, and laminated diatom ooze with varying abundances of foraminifers, calcareous nannofossils, and sponge spicules. Siliciclastic and mixed sediments vary in color from very dark greenish gray (10Y 3/1) and dark greenish gray (10Y 4/1) to dark gray (4/N) and very dark gray (5Y 3/1). Diatom oozes are generally olive-gray (5Y 4/2) and dark greenish gray (10Y 4/1). The boundaries between changes in lithologies are usually gradational, and the color and texture changes are very subtle. There is no visible soft-sediment deformation at this site.

In the main lithologies, foraminifers, calcareous nannofossils, and sponge spicules are generally rare (<5%). Sponge spicules occur frequently as sponge spicule aggregates throughout the cores and may be the remains of agglutinated benthic foraminifers. The siliciclastic fraction is composed of quartz, feldspar, rock fragments (commonly polycrystalline quartz), micas, and clay minerals. The occurrence of clay minerals at Site U1343 is generally higher than at the Bowers Ridge sites. Many millimeter-sized pyrite-rich specks occur throughout the cores. Pyrite is also abundant in the smear slides (average = 4%) (see "Site U1343 smear slides" in "Core descriptions"). Numerous bivalve shells and shell fragments were found at this site, especially in Hole U1343E below 500 mbsf.

Six laminated intervals occur at this site, and these can be correlated between Holes U1343A, U1343C, U1343D, and U1343E based on lithologic, reflectance, and magnetic susceptibility data (Figs. F6, F7, F8). In two cases the laminated intervals are correlated to bioturbated diatom ooze or diatom silt intervals in Hole U1343E. In two other cases they are laterally discontinuous and correlate to gaps between cores (Fig. F9). In Hole U1343A the intervals are at 1.1, 2.9, 35.9, 44.2, and 109.2 mbsf. In Hole U1343C the bases of the intervals are at 1.1, 2.7, 36.7, 108.9, and 122.7 mbsf. In Hole U1343D the base of the interval is at 1.6 mbsf. In Hole U1343E the bases of the intervals are at 30.9, 41.9, 104.1, and 117.6 mbsf. Smear slide observations show that these laminations can be foraminifer rich (as much as 40%). However, they are poor in silicoflagellates (1%–2%) and calcareous nannofossils (as much as 2%–3%). This contrasts with previous sites, where calcareous nannofossils are abundant in laminated intervals. Laminated intervals have bioturbated gradational top boundaries and either sharp or gradational lower boundaries. In the latter case, the laminations themselves are slightly bioturbated, and some laminae are consequently wavy or discontinuous. Observations that laminated sediments in one hole correlate to a bioturbated diatom ooze or diatom silt in another hole indicate spatial heterogeneity in the preservation of laminations, as observed at other sites (see the "Site U1342" chapter).

The induration of sediments at Site U1343 is soft throughout Holes U1343A–U1343D and the upper part of Hole U1343E. The sediments are stiff in Hole U1343E below 456 mbsf (Core 323-U1343E-54X). In this hole, two notable cores (323-U1343E-41H and 52X) contain stiff sediment, although the surrounding cores are soft. The sediments in Core 323-U1343E-52X are very stiff but abruptly change in the middle of Section 323-U1343E-53X-1 (447.0 mbsf) to soupy, water-rich material and then change back to soft sediment in the middle of Section 53X-4 (452.0 mbsf). XRD analysis did not reveal any evidence of silica diagenesis in these cores (see XRD in "Supplementary material"). Logging data from this interval (see "Downhole measurements") show that the strata have unusually low density values, so the waterlogged sediment is a real feature that was probably disturbed during the coring process.

The sediments in Core 323-U1343E-41H are so hard that the core liner shattered. This core (346.8–354.4 mbsf) coincides with the depth of the BSR (see "Physical properties"). The sediments in Cores 323-U1343E-43X through 45X (360.4–388.4 mbsf) are distinct from the rest of the site. These sediments contain no diatoms, and the siliciclastic sediments are clays with a sticky texture that differ from the typically silty sediments at this site. The bulk density of these sediments is 1.6 g/cm³ on average, in sharp contrast to Core 323-U1343E-41X, which reaches a maximum density of 2.0 g/cm³, supporting the observed change in lithology (see "Physical properties").

Bioturbation is slight to moderate throughout the cores, with the exception of the laminated intervals that do not show obvious bioturbation. The transitions between sediments with different colors or textures sometimes have centimeter-scale mottling and sometimes are gradational. In general, centimeter-scale mottles contain silty clay, sandy silt, sand, ash, and/or pyrite. There are numerous sandy patches or layers at Site U1343. Sandy lithologies are concentrated in four intervals in Hole U1343E (~58–64, ~140–180, ~300–450, and ~675–740 mbsf). In Sections 323-U1343A-1H-2 (1.56–3.00 mbsf) and 323-U1343C-1H-2 (1.83–2.02 mbsf), pervasive millimeter-scale mottling occurs at the transitions from olive-gray to dark greenish gray diatom-rich and diatom-poor lithologies, respectively. Presumably, these mottles would become less distinct with increasing burial depth and compaction and would appear as a gradual color change between the two lithologies.

Subrounded to well-rounded granule- to pebble-sized clasts occur frequently at Site U1343. The clasts are often black and fine grained and are likely of volcanic origin. An 8 cm black rounded basalt cobble was found in Section 323-U1343E-34H-CC (Fig. F10). There are no obvious changes in the frequency or clustering of clasts through the holes. The sediments at Site U1343 are noticeably sandier than at the other sites. The lithologies with >25% sand-sized siliciclastic grains are predominantly purely siliciclastic lithologies rather than mixed siliciclastic-biogenic ones.

Ash is less common at this site than at sites farther south. However, there are numerous thin ash layers in all holes in addition to ash-filled mottles. The ash is usually fine grained and gray in color, whereas black ash is less common.

Authigenic carbonates occur frequently at Site U1343. Authigenic euhedral crystal shapes such as rhombs, acicular crystals, and globular crystals with extreme birefringence were observed in many smear slide samples. The crystals are usually 4–10 µm long, but some are as long as 50 µm (see "Site U1343 smear slides" in "Core descriptions"). The sediment containing authigenic carbonates tends to be slightly lighter or more yellowish in color than the surrounding sediment. Occasionally, the authigenic carbonate–rich layers are semilithified and much paler in color and are referred to as dolostone. Smear slide samples taken from around the dolostones typically are rich in rhombic carbonate crystals, and an X-ray diffraction (XRD) sample from one such layer shows the presence of dolomite (Fig. F11C–F11D); another shows the presence of high-magnesium calcite (Fig. F11A–F11B) (see XRD in "Supplementary material"). Authigenic carbonates are more common in Hole U1343E below 550 mbsf, appearing in 18 of 25 cores at this interval.

Mollusk shells were found in all holes at Site U1343. The frequency of shell occurrence increases by an order of magnitude in Hole U1343E below 500 mbsf, where shells were observed every 3.6 m on average. They were observed every 34 m on average above 500 mbsf in Hole U1343E. Shells were sometimes associated with authigenic carbonates. In one case, in Section 323-U1343E-82X-6, 82 cm, a 3 cm long shell was bisected in core splitting, and the sediment within and immediately around the shell was rich in authigenic carbonates.

The presence of gas in the sediments caused several types of coring disturbance that affected the stratigraphic integrity of the sediment. Below Cores 323-U1343A-6H, 323-U1343C-5H, and 323-U1343E-8H, the sediment at the top of the sections 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. In all holes, punctures were made in all cores from Core 3H and down, potentially causing significant loss of sediment from extrusion. However, punctures were not always noted in core descriptions because punctures are not always visible on the cut surfaces of the cores. Below 25–30 mbsf in all holes there are numerous cracks and voids from gas expansion, which affected physical property (gamma ray attenuation [GRA] bulk density, in particular) and color reflectance measurements. In addition, wafers of sediment between wide voids were broken and jumbled during core splitting. The other common coring disturbance was biscuiting, which was common in cores recovered with the XCB from Hole U1343E (Fig. F12).

Discussion

Unit I at Site U1343 encompasses a slightly longer time period than Unit I at all other sites. However, Site U1343 is distinct in having a higher proportion of siliciclastic versus biogenic grains than the Bowers Ridge sites, which is probably related to its location adjacent to the continental slope and its relative proximity to the source of terrigenous sediments. Also, at this site a higher occurrence of sand-sized grains were observed in smear slides. During glacial sea level lowstands, particularly during early stages of deglaciations, significant amounts of coarser grained detrital material may have been mobilized from the exposed Bering Sea shelf and redeposited farther down the continental slope.

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 delivery by ice through glacial–interglacial cycles. The siliciclastic flux to this site may be influenced by the trajectory of the sediment load of the Yukon River, which is the largest shelf source of sediment in the Bering Sea and whose path across the shelf during times of lower sea level is unknown. Yukon-source sediment is observed to be advected southward through the Kamchatka Strait or northward through the Bering Strait as a function of glacial–interglacial changes in sea level and circulation (VanLaningham et al., 2009). The four intervals of sandy lithologies can be tentatively correlated to similar sand-rich intervals at Site U1344 (Cores 323-U1344A-5H through 8H, 21H through 25H, 48X through 50X, and 74X) (see "Lithostratigraphy" in the "Site U1344" chapter), a deeper site that is also just south of the continental shelf. The same deposition mechanism is likely responsible for the sandy intervals in cores from both holes. Clasts were probably transported to this site by ice, although ice can transport clay-, silt-, and sand-sized grains as well. There is not a clear correlation between sand-rich lithologies and clasts, but both kinds of grains were probably undersampled in the smear slides and visual core descriptions. Other mechanisms such as mass gravity flows can transport sand-sized and smaller grains; however, no intervals that appeared to be the result of such a mass movement were observed.

Diatom ooze and mixed diatom-siliciclastic lithologies are generally associated with low natural gamma radiation (NGR), low bulk density, and low magnetic susceptibility. More siliciclastic lithologies are associated with higher NGR, higher bulk density, and higher magnetic susceptibility. The maxima and minima of these three physical properties do not always coincide, showing that the proportion of biogenic grains is not the only parameter affecting them. For example, as the lithology varies between silt- and clay-sized grains, NGR may reflect mineralogical changes in potassium-rich feldspar and potassium clay-mineral abundance, independent of the absolute dilution of siliciclastic grains by biogenic flux. The general trends in lithologic variations with depth are similar among the holes; however, small-scale lithologic variations are harder to correlate, probably because the lithologic changes were undersampled by smear slides and the different lithologies were very similar in appearance.

The lithologies with a higher abundance of diatoms, particularly laminated intervals, are associated with higher values of color reflectance parameter b* (Figs. F6, F7, F8). This is analogous to the changes in diatom abundance and b* at other sites (see the "Site U1339," "Site U1341," and "Site U1342" chapters). The uppermost (Holocene) sediments at this site are diatom rich and have the highest vales of b* observed at this site. Biogenic silica flux has been observed to be higher during interglacials than glacials in the Bering Sea (Okazaki et al., 2005), so these intervals of higher biogenic fraction and higher b* are interpreted as interglacials. The diatom ooze and diatom-siliciclastic mixed lithologies often correlate with higher abundances of foraminifers and calcareous nannofossils (see "Biostratigraphy").

Laminations were only observed in the uppermost 130 m of sediment at this site. The laminated intervals are small, and the laminations themselves are thin (submillimeter to millimeter in scale). It is possible that laminated sediments occurred earlier in time but are no longer easily visible in the sediment because of compaction. The laminated and biogenic-rich interval in Core 323-U1343A-12H was sampled more closely for the assemblage of microfossils by the biostratigraphy group and was tentatively identified as marine isotope Stage (MIS) 11 (see "Biostratigraphy" for discussion).

The pattern of laminations in Sections 323-U1343A-1H-1 and 1H-2 and 323-U1343C-1H-1 and 1H-2 is similar to that observed in sediments deposited during the last deglaciation in the Bering Sea (Cook et al., 2005) and around the Pacific margin (Van Geen et al., 2003), where the Bølling-Allerød and early Holocene are laminated. The timing of the onset of laminations depends on the position and intensity of the OMZ, local export production, and the water depth of the coring site. The laminated intervals at 1.00–1.10 mbsf in Hole U1343A and 0.97–1.07 mbsf in Hole U1343C are tentatively identified as the early Holocene, and the laminated intervals at 2.17–2.87 mbsf in Hole U1343A, 1.97–2.75 mbsf in Hole U1343C, and 1.03–1.60 mbsf in Hole U1343D are tentatively identified as the Bølling-Allerød. There is a small ash layer below the Bølling-Allerød laminae in Holes U1343A, U1343C, and U1343D. Neither the ash layer nor the laminations occur in Hole U1343E, suggesting that at least 3 m of sediment is missing from the top of that hole relative to the others.

In contrast to other sites, volcaniclastic material is a minor component of sediment at Site U1343 because this site is more distant from the Aleutian arc. Thin ash layers in addition to ash-filled mottles in each hole probably represent bioturbated traces of ash layers. The ash at this site is typically lighter in color than at the Bowers Ridge or Umnak sites, which is consistent with their source being explosive rhyolitic eruptions that would be carried as far as this site.

Many of the clasts deposited at Site U1343 are volcanic. They may have originated from the Aleutian Islands and been transported by ice or they may have been from older volcanic rocks exposed on continental Alaska. Delivery of ice-rafted debris (IRD) to this site may be influenced by the strength of the BSC, which today flows from Umnak Plateau to the northwest. An increase in the occurrence of clasts in Hole U1343E may reflect an increase in ice, an increase in BSC strength, or both.

There is a shallow sulfate–methane transition zone (SMTZ) and abundant methane in the sediment column at Site U1343, as at the other slope Sites U1339 and U1345 (see "Geochemistry and microbiology"). This is probably a function of high export production along the Bering Slope Green Belt, where there is upwelling 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).

Benthic and planktonic foraminifers encrusted with yellow minerals are common at this site (see "Biostratigraphy") and often coincide with samples that contain authigenic carbonates. The association of authigenic carbonates and apparent overgrowths implies that authigenic carbonates nucleated on the foraminifer tests. In this case, the stable isotope records from this site are potentially contaminated by the isotopic signature of the overgrowths, which would have anomalously low δ¹³C and high δ¹⁸O.

The occurrences of mollusk shells and authigenic carbonates significantly increase below 500 mbsf, so it is possible the two are related, assuming the shells can be identified as belonging to a taxon associated with cold seeps. Interestingly, the same pattern in shells and authigenic carbonates is not observed at Site U1339 (see the "Site U1339" chapter)

Distinct, strongly indurated authigenic carbonate layers are largely missing at Site U1343. This implies that nonsteady-state diagenesis did not fix the SMTZ at a certain sediment depth for as long as it did at other sites, where authigenic fronts may have persisted for longer periods of time because of very low sedimentation rates. Nodules of authigenic carbonates in an SMTZ are estimated to form on a timescale of thousands of years (Ussler and Paull, 2008). The high sediment accumulation rates at Site U1343 may have precluded concretions of authigenic carbonates from forming here.

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