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doi:10.2204/iodp.proc.303306.104.2006

Lithostratigraphy

Four holes were drilled at Site U1304 to a total depth of 244 mbsf (264 meters composite depth [mcd]) (T1). All cores were recovered using the APC. Recovery was excellent in Holes U1304A and U1304B but decreased in Holes U1304C and U1304D because of worsening weather conditions (see “Operations”).

The sediments at Site U1304 are predominantly interbedded nannofossil oozes and diatom oozes, with less common intervals of clay and silty clay, which also contain abundant nannofossils and/or diatoms (Figs. F6, F7, F8). The color of the sediments throughout the section is various shades of gray. Most contacts between nannofossil ooze and clay intervals are gradational, although sharp contacts are also observed. The contacts between diatom ooze beds and the other lithologies are generally sharp. Redeposited beds of silt- and sand-sized particles are rare throughout the hole, and disturbed units related to mass-transport processes (e.g., slumps and debris flows) are also rare. Thus, the section cored at Site U1304 apparently represents a relatively continuous pelagic section. The sediments at Site U1304 have been designated as a single unit, composed of Holocene–Upper Pliocene sediments. The various lithologies throughout the hole are generally interbedded on a scale of centimeters to decimeters (see “Site U1304 visual core descriptions” in “Core descriptions”). These frequent changes of lithology at this scale do not allow for effective subdivision into multiple units or subunits.

Description of units

Unit I

• Intervals: Sections 303-U1304A-1H-1, 0 cm, to 26H-CC, 25 cm; 303-U1304B-1H-1, 0 cm, to 26H-CC, 10 cm; 303-U1304C-1H-1, 0 cm, to 8H-CC, 13 cm; and 303-U1304D-1H-1, 0 cm, to 21H-CC, 05 cm

• Depths: Hole U1304A: 0–239.46 mbsf, Hole U1304B: 0–242.45 mbsf, Hole U1304C: 0–69.38 mbsf, and Hole U1304D: 52.0–243.74 mbsf (0–263.8 mcd)

• Age: Holocene–Late Pliocene

Unit I is composed predominantly of interbedded nannofossil ooze (Fig. F6) and diatom ooze (Figs. F7, F8). Additional lithologies recognized are nannofossil ooze with clay, nannofossil ooze with silty clay, nannofossil ooze with diatoms, nannofossil ooze with foraminifers, diatom ooze with clay, diatom ooze with nannofossils, clay, silty clay, silty clay nannofossil ooze, silty sand with diatoms, and foraminifer sand. The colors of the nannofossil ooze are generally gray (5Y 6/1 and 5Y 5/1), dark gray (5Y 4/1), greenish gray (5GY 5/1), and olive-gray (5Y 4/2 and 5Y 6/2) (Fig. F6). The diatom ooze is highly laminated and displays an even wider range of colors including gray (5Y 6/1 and 5Y 5/1), greenish gray (5GY 5/1), dark greenish gray (5GY 4/1), dark olive-gray (5Y 4/2), light olive-gray (5Y 6/2 and 7Y 6/2), and pale yellow (5Y 7/3) (Figs. F7, F8). Colors of the other lithologies are predominantly gray (5Y 6/1 and 5Y 5/1), dark gray (5Y 4/1), and very dark gray (e.g., Fig. F6). In contrast to these gray colors, the upper 13 cm of sediments in Hole U1304B (interval 303-U1304B-1H-1, 0–13 cm), which occurs just below the sediment/water interface, is brown nannofossil ooze and has a zone of iron oxide laminae from 9 to 11 cm. Intervals of similar brown sediment with this iron-rich zone were observed in the uppermost sediments of Sites U1302 and U1303 and, in fact, have been previously observed in the uppermost meter of seafloor sediments in numerous cores taken throughout the world ocean (e.g., McGeary and Damuth, 1973; Damuth, 1977, and references therein).

Contacts between the various lithologies are generally gradational, except for contacts between diatom ooze and other lithologies, which are sharp (Figs. F6, F8). However, sharp contacts between the other lithologies are rare. Bioturbation is ubiquitous throughout this unit in all lithologies except the diatom oozes. Most sediments are moderate to heavily burrowed and are recognized by subtle centimeter-scale color mottling or by millimeter-scale pyritic burrow fills. In a few cases, discrete burrows or discrete macroscopic pyritized burrows were observed.

Intervals of laminated diatom ooze comprise much of the section in Unit I. The intervals in which the diatom ooze dominates the lithology are 30–60, 85–115, and 180–265 mcd (Fig. F9). The beds of diatom ooze generally show millimeter-scale laminations, which are more apparent in sections split by saw blades. In contrast, sections split by pulling a wire through the core tends to destroy the millimeter- scale structure because of the resistance of the diatom mats to the wire (Figs. F7, F8). Nearly all individual beds of diatom ooze are centimeter or decimeter scale, and individual beds are commonly separated by laminae and thin beds of nannofossil ooze. Only in interval 303-U1304A-19H-1, 0 cm, to 19H-CC, 21 cm (171.0–175.0 mbsf), are beds thicker than 1 m, and here the entire core is composed of diatom ooze.

Beds of discrete silt- and sand-sized particles are rare throughout Unit I. Where present, these beds are only centimeters thick and are predominantly composed of foraminifer sand. These beds appear to be formed by winnowing by bottom currents. Redeposited sediments formed by mass-transport processes are also rare throughout Unit I. Interval 303-U1304B-19H-3, 107–138 cm, contains a deposit of deformed, folded nannofossil ooze and diatom ooze, which is interpreted as a 31 cm thick intraformational slump deposit (Fig. F10). Interval 303-U1304B-24H-6, 136 cm, to 24H-CC, 21 cm, appears to be a disturbed interval with numerous rock clasts of granule to gravel size and is probably a small debris flow deposit. Interval 303-U1304B-26H-1, 0 cm, to 26H-2, 145 cm, and 26H-4, 65 cm, to 26H-5, 55 cm, also appear to be small slumps or debris flows. Dropstones are absent to rare throughout much of the section, and where present, only a few per 10 cm interval were observed (Fig. F11). Total carbonate contents range from 4 to 71 wt% in these cores (see “Geochemistry” and “Site U1304 smear slides” in “Core descriptions”). Pyrite (usually associated with burrow fills; e.g., Fig. F6) and iron oxide coatings on grains are common and are the only authigenic sediment components observed.

Abundances of terrigenous components, as estimated from smear slides, are quartz, 0%–90%; detrital carbonate, 0%–50%; feldspars, 0%–15%; clay minerals (including chlorite), 0%–70%; heavy minerals (especially hornblende), 0%–1%; and volcanic glass, 0%–3% (note: one slide had 70%). Abundances of biogenic components, as estimated from smear slides, are nannofossils, 0%–95%; foraminifers, 0%–65%; diatoms, 0%–90%; radiolarians, 0%–1%; and sponge spicules, 0%–1% (see “Site U1304 smear slides” in “Core descriptions”). Figure F12 shows downhole plots for diatoms, quartz, and nannofossils for all smear slides. In general, quartz and diatom contents appear to be relatively low between 50 and 95 mcd. Figure F13 shows downhole plots for coccoliths, diatoms, quartz, detrital carbonate, and clay using only smear slide data from nannofossil ooze layers. Although no strong trends are observed, diatom content in general tends to increase upsection, whereas the quartz content apparently decreases.

Discussion

The sediments at Site U1304 represent mainly pelagic deposition. This is clearly demonstrated by comparison of digital imaging system core images from Holes U1304A and U1304B. When the same intervals from each hole are compared side by side, individual beds, especially the laminated diatom oozes, correlate extremely well. This correlation indicates the absence of significant reworking of sediments by bottom currents, as well as input of sediment from downslope processes such as turbidity currents and related gravity-controlled mass flows. Thus, we accomplished the objective at Site U1304 to obtain a high-resolution Pliocene–Quaternary pelagic record from within the central Atlantic IRD belt at a water depth sufficient to sample NADW (Fig. F9).

The most striking sediments recovered at Site U1304 are the thick deposits of laminated diatom ooze (Figs. F7, F8, F9). The presence of these thick diatom ooze layers in the Quaternary sediments of the North Atlantic was only recently discovered by Bodén and Backman (1996), who recovered a 3 m section of laminated diatom ooze (termed “LDO” by them) in Ewing Core EW9303-17. The LDO section occurs only in Stage 5e and consists of Thalassiothrix longissima. However, this core recovered only MIS 3–6. The occurrence of LDOs is apparently localized to a narrow subarctic convergence zone between the cold, less saline surface water associated with the Labrador Sea Current and the warmer North Atlantic Current. Site U1304 sediments show that thick LDOs have not been accumulating at this site during the latest Quaternary (Fig. F9); however, Site U1304 extends the record of LDO accumulation back to the Late Pliocene and shows that thick LDO successions accumulated periodically at this site since at least that time (Fig. F9). Postcruise studies should reveal whether the LDO deposition can be tied to the subarctic convergence zone and, if so, Site U1304 will provide a 1.8 m.y. record of movements of the convergence zone during the Quaternary.

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