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doi:10.2204/iodp.proc.323.109.2011

Lithostratigraphy

Five holes were cored at Site U1345, reaching a maximum depth of 150.89 mbsf in Hole U1345E. The sediments at this site are primarily siliciclastic with varying amounts of diatoms and minor amounts of foraminifers and calcareous nannofossils. Authigenic carbonates occur frequently at this site in all holes deeper than 30 mbsf. The sediments are predominantly dark/very dark gray and dark/very dark greenish gray. One lithologic unit spanning the middle Pleistocene to the Holocene was defined at this site.

Description of unit

Unit I

• Intervals: Sections 323-U1345A-1H-1, 0 cm, through 16H-CC, 58 cm; 323-U1345B-1H-1, 0 cm, through 4H-CC, 24 cm; 323-U1345C-1H-1, 0 cm, through 16H-CC, 34 cm; 323-U1345D-1H-1, 0 cm, through 16H-CC, 42 cm; and 323-U1345E-1H-1, 0 cm, through 16H-CC, 55 cm

• Depths: Hole U1345A, 0–147.43 mbsf; Hole U1345B, 0–35.26 mbsf; Hole U1345C, 0–148.53 mbsf; Hole U1345D, 0–150.51 mbsf; and Hole U1345E, 0–150.89 mbsf

• Age: mid-Pleistocene to Holocene

The lithologies at this site are mostly diatom-rich and diatom-bearing siliciclastic sediments, with frequent thinly laminated to thinly bedded 5–40 cm thick intervals that characterize ~10% of the sediments collected at this site (Figs. F6, F7, F8, F9). The siliciclastic sediments are mostly diatom-rich clayey silt, diatom-bearing sandy silt/silty clay/silt and silt/sandy silt/silty clay (see "Site U1345 smear slides" in "Core descriptions"). The sediments range in color from very dark greenish gray (10Y 3/1 and 5GY 3/1) and dark greenish gray (5GY 4/1) to dark gray (4/N) and very dark gray (3/N). The laminated/thinly bedded sediments typically are composed of alternating lithologies and layers that contain a higher fraction of diatoms. In general, the laminated/thinly bedded intervals have higher abundances of foraminifers and calcareous nannofossils than nonlaminated sediments. The lithologies consisting of laminations/thin beds are commonly diatom-bearing silty clay/clayey silt, diatom-rich silty/clayey silt, and diatom ooze.

In the main lithologies, the siliciclastic fraction is primarily composed of quartz, feldspar, rock fragments, and clay minerals (see "Site U1345 smear slides" in "Core descriptions"). Millimeter- to centimeter-scale pyrite mottles and specks are frequent, particularly in sediments with high proportions of biogenic grains. In smear slides, pyrite often appears as framboids within and among diatom frustules. The abundance of foraminifers and calcareous nannofossils is generally <10%. There are numerous sandy patches and layers at Site U1345.

The transitions between lithologies are gradational, and the color changes are subtle. The boundaries between laminated and nonlaminated sediment are often gradational, although sharp boundaries at the base of laminated intervals were also observed. The contacts between individual thin laminae are mostly sharp, but contacts in thin-bedded lithologies often have gradational boundaries.

In the laminated portions there are three small intervals of soft-sediment deformation characterized by folded or truncated laminations: intervals 323-U1345A-13H-7, 115–121 cm; 323-U1345D-6H-2, 85–96 cm; and 323-U1345D-1H-3, 99–105 cm (Fig. F10). Note that possible soft-sediment deformation in massive, homogeneous sediment could not be easily detected because of the lack of structures.

The induration of sediments at Site U1345 is soft throughout, with the exception of one hard dolostone layer in interval 323-U1345A-5H-5, 130–137 cm. The original sediment fabric, consisting of thin laminations, is preserved in the dolostone. The sediment above and below the dolostone is thinly laminated as well.

The laminated and thinly bedded intervals that occur at this site can be correlated between Holes U1345A, U1345C, U1345D, and U1345E based on visual observations, reflectance, and magnetic susceptibility data (Fig. F11). Most laminations and thin bedding include diatom-rich lithologies; however, several laminated intervals include only alternations of siliciclastic lithologies. The former intervals are often associated with lower magnetic susceptibility, lower density, lower natural gamma radiation (NGR), and higher values of color reflectance parameter b*. The siliciclastic-rich laminations are also associated with lower magnetic susceptibility but not with other coherent changes in physical properties.

Bioturbation is slight to moderate throughout the cores, except for some laminated intervals in which bioturbation is absent. The transitions between sediments with different colors or textures sometimes have centimeter-scale mottling and are occasionally gradational. In general, millimeter- to centimeter-scale mottles composed of pyrite or sand occur throughout the site and probably represent filled bioturbation burrows.

Subrounded to well-rounded granule- to pebble-sized clasts occur frequently at Site U1345. There are no patterns in the frequency or clustering of clasts in any of the holes. The sediments at Site U1345 contain the highest proportion of sand observed during Expedition 323.

Ash is less common at this site than at sites farther south. However there are thin ash layers in Holes U1345A, U1345C, and U1345D, in addition to ash-filled mottles. A 2 cm × 6 cm subvertical ash layer at interval 323-U1345A-1H-2, 0–10 cm, can be correlated to a 2 cm × 2 cm ash plug at the top of interval 323-U1345C-1H-2, 0–10 cm. In the latter, the majority of the tilted ash layer is in the working half of the split core. This ash is not present in Hole U1345D because coring began deeper than the ash layer. Core 323-U1345E-1H was not split in order to preserve it for shore-based whole-round analysis. The ash is denser and firmer than the surrounding very soft sediment, which probably accounts for its tilting during splitting or coring. Two other thin, light gray to white, disseminated, bioturbated ash layers occur. One is found only in Core 323-U1345C-12H; the other is found in Cores 323-U1345D-14H and 323-U1345E-14H.

Finely disseminated authigenic carbonates occur frequently at Site U1345 below 30 mbsf. Euhedral crystal shapes such as rhombs, acicular crystals, and globular crystals with extreme birefringence are usually 4–10 µm long but reach 50 µm in some of samples. The sediment containing authigenic carbonates tends to be slightly lighter or more yellowish in color than the surrounding sediment. Authigenic carbonates occasionally occur in laminated sediments.

A mold of a tube worm colony was found in interval 323-U1345C-16H-3, 105–118 cm, surrounded by authigenic carbonate–rich sediments (Fig. F12). The tube walls appear to have originally been flexible, as evidenced by the partial collapse of some tubes. Thin section petrographic analysis suggests that the tube walls could have been remineralized to a partially amorphous material with low birefringence that extinguishes parallel to the tube walls. X-ray diffraction (XRD) analysis of a decalcified sample (see XRD in "Supplementary material") shows the presence of amorphous silica, which may be the mineral that composes the tube walls. The cavities within the mold have been filled by diatom clayey silt. Sparry calcite (possibly siderite; Fig. F12C) precipitates from the tube walls toward the center of the cavities. The tube walls, the sediment in the cavities, and the sparry calcite are completely or partially replaced by a mosaic of very fine (<5 µm) carbonate rhombs (possibly dolomite).

The presence of gas in the sediments caused several types of coring disturbance that affect stratigraphic integrity. Sediment at the top or bottom of the cores below Cores 323-U1345A-7H, 323-U1345C-6H, 323-U1345D-5H, and 323-U1345E-5H was ejected out of the core barrel by gas expansion; 20–150 cm of sediment was extruded onto the rig floor. This sediment was pushed back into a core liner; however, some sediment pieces may be out of order or upside down. Punctures were made in Core 2H and in every core thereafter for all holes, potentially causing a significant loss of sediment from extrusion. Punctures were not always noted in core descriptions because they were not always visible on the cut surface of the core. Numerous cracks and voids occur in all holes between 5 and 20 mbsf and deeper. In addition, wafers of sediment between wide voids were broken and jumbled during core splitting. The resulting gaps affected the measurements of physical properties (gamma ray attenuation [GRA] bulk density, in particular) and color reflectance.

Discussion

Unit I spans the mid-Pleistocene to Holocene. As such, it corresponds to the upper portion of Unit I at other sites, which includes sediments deposited since the early Pleistocene. Site U1345 is distinct among the near-shelf sites because of the abundance and generally coarser texture of the siliciclastic component in the sediments as well as the higher frequency of laminated intervals. Intervals characterized by >25% sand and thin sandy layers occur at all depths in all holes drilled at this site. The laminations and thin-bedded sediments are numerous and well correlated between holes.

Site U1345 is located in the central portion of the modern OMZ. As such, the sediments deposited at this site can provide important information concerning Pleistocene to Holocene variability of bottom water oxygen concentrations. The preservation of laminated and thinly bedded sediments (beds <10 cm in thickness) could be interpreted to result from a reduction in or cessation of the activity of benthic macrofauna as a result of low oxygen concentrations in the bottom waters and surface sediments. Hereafter, references to "laminations" or "laminated intervals" include these thin beds.

Laminations do not have a clear signature in the physical properties or color reflectance data (Figs. F6, F7, F8, F9) like that observed at Site U1342. This is probably because of the dominant presence of the siliciclastic fraction and the fact that the relatively small changes in the relative proportion of biogenic grains have a smaller influence on physical properties and color than they do at sites with more drastic lithologic contrasts between laminated and nonlaminated intervals.

The laminated intervals can be divided into two categories based on the abundance of biogenic grains:

1. Couplets or triplets of diatom oozes, mixed siliciclastic/biogenic sediments, and siliciclastic sediments; or
2. Couplets of siliciclastic sediments of alternating textures that may include minor (<40%) amounts of diatoms (Fig. F13; see "Site U1345 smear slides" in "Core descriptions").

Laminated sediments of the first category are similar to laminated intervals at other sites, which are typically biogenic rich and olive-green, dark olive-gray, or very dark greenish gray. This category of lamination seems to occur mainly during interglacials (Fig. F11). This relationship supports previous observations of a higher flux of diatoms during interglacials than during glacial periods (Okazaki et al., 2005). The variable bottom water oxygen concentrations are probably influenced by the magnitude of the export production and the associated consumption of oxygen by organic matter remineralization, in addition to changes in the ventilation of intermediate watermasses.

The second category of laminated sediments is mainly siliciclastic and is unique to Site U1345. This type of lamination occurs in sediments that are tentatively identified as having been deposited during glacial conditions (Fig. F13). Because these sediments are not biogenic rich, changes in intermediate water ventilation may have been the controlling parameter for bottom water oxygen concentrations during these periods. Detailed reconstructions of productivity and export production as well as ocean circulation proxy records will be required to elucidate the mechanisms responsible for the fluctuations in OMZ intensity and position that occurred in the past.

Intermittent finely disseminated authigenic carbonates are present in sediment below ~30 mbsf in all holes at this site. The sulfate–methane transition zone (SMTZ) is at ~6.5 mbsf, the shallowest observed during Expedition 323 (see "Geochemistry and microbiology"). Calcium and magnesium concentrations in the pore water decrease toward the SMTZ, suggesting active authigenic carbonate precipitation at and below this depth today (see "Geochemistry and microbiology"). Smear slide analysis indicates the presence of authigenic carbonate crystals large enough to be identified at relatively low magnification (400×). Therefore, it is possible that carbonate crystals <2–4 µm were not observed and recorded, even if they were present.

Sediment intervals containing authigenic carbonates are subtly paler or more yellowish in color than the surrounding sediment; however, authigenic carbonates were observed in smear slides even in sediments without an obvious color change. Therefore, it is likely that the occurrence of authigenic carbonates was underestimated, and further study of the mineralogical and isotopic composition of the sediment will be required to comprehensively determine the distribution of authigenic minerals. The remineralized and dolotomized tube worm colony may be evidence of a methane cold seep if it is found to be of a type that generally has chemosynthetic symbionts and/or is found near cold seeps today (e.g., Vanreusel et al., 2009).

In several cases, authigenic carbonates were observed in laminated intervals (Fig. F13). The authigenic carbonates do not appear to preferentially occur in laminations, but they are associated with laminated sediments that are tentatively estimated as having been deposited during marine isotope Stage (MIS) 5 (Fig. F13). A bulk sediment carbonate measurement taken in the MIS 5 interval is the highest CaCO₃ measurement recorded at this site (see "Geochemistry and microbiology").

Few ash layers were observed at Site U1345, but the ash that occurs is light colored, suggesting that its source is explosive rhyolitic volcanism. This site is distant from the nearest likely source of volcaniclastic grains, the Aleutian arc, so the transport mechanism must have been one capable of widespread dissemination.

The lithologies at Site U1345 are sandier than those at any other site from Expedition 323. Lithologies with >25% sand (Figs. F6, F7, F8, F9) and thin sandy layers occur throughout all holes. The presence of this coarse material is probably related to the position of Site U1345 at the crest of an interfluve at the mouth of Navarin Canyon. In contrast, other sites on the continental slope, Sites U1343 and U1344, are more distant from the mouths of major canyons that intersect the shelf. Sand deposition at these latter two sites may be limited to large, singular mass movement events. The proximity of Site U1345 to the center of the canyon may allow for the deposition of sediment being transported downslope.

The siliciclastic grain sizes at Site U1345 contrast even more strongly with those from Site U1339, located on a submarine plateau isolated from the continental shelf. Virtually no sand-sized grains were recorded at Site U1339. This may be due to either less transport of terrigenous material to the site or a high biogenic flux to Site U1339 sediments.

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