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doi:10.2204/iodp.proc.303306.105.2006

Lithostratigraphy

Three holes were drilled at Site U1305 (Table T1). Core recovery at Site U1305 was 104%. All cores were recovered using the APC. The sediments at Site U1305 are dominated by varying mixtures of terrigenous components and biogenic material (primarily quartz, detrital carbonate, and nannofossils) (see “Site U1305 smear slides” in “Core descriptions;” Fig. F7), so the most common lithologies are silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay (Fig. F8). Gradational and burrowed contacts between these lithologies are much more common than well-defined or sharp boundaries. In addition to these lithologies, we observed many intervals of silty clay with sandy silt laminae, which are designated as a distinct lithologic type, separate from silty clay. Abundances of terrigenous components, as estimated from smear slides, are quartz, 0%–85%; detrital carbonate, 0%–85%; feldspars, 0%–30%; clay minerals (including chlorite), 0%–85%; heavy minerals (especially hornblende), 0%–1%; and volcanic glass, 0%–5%. No discrete ash layers were observed. Dropstones are present in low numbers (~1 entity per 3 m of core) throughout these cores and display a wide range of compositions, including acidic intrusive and metamorphic (granites, gneisses, and granitoids), basic igneous and/or metamorphic (basalts and metabasalts), and sedimentary and metasedimentary (sandstone and limestone). Abundances of biogenic components, as estimated from smear slides, are nannofossils, 0%–88%; foraminifers, 0%–20%; diatoms, 0%–20%; radiolarians, 0%–10%; and sponge spicules, 0%–10%. Total carbonate contents range from 1 to 49 wt% in these cores (see “Geochemistry;” Table T21). Pyrite (usually associated with burrows) and iron oxide coatings on grains are present occasionally, constituting the only authigenic sediment components observed.

The sediments at Site U1305 are designated as a single unit composed of Holocene–uppermost Pliocene (see “Biostratigraphy” and “Paleomagnetism”) terrigenous and biogenous sediments, which are gradationally interbedded at scales of a few meters or less.

Description of units

Unit I

• Intervals: Sections 303-U1305A-1H-1, 0 cm, to 30-CC, 29 cm; 303-U1305B-1H-1, 0 cm, to 28H-CC, 16 cm; and 303-U1305C-1H-1, 0 cm, to 31H-CC, 25 cm

• Depths: Hole U1305A: 0–280.0 mbsf, Hole U1302B: 0–265.3 mbsf, and Hole U1305C: 0–287.9 mbsf (0–314 meters composite depth [mcd])

• Age: Holocene–Late Pliocene

Unit I is composed predominantly of silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. Throughout the section, numerous centimeter- to decimeter-scale intervals of sandy silt laminae are interbedded with silt clay. Clay, foraminifer nannofossil ooze with silty clay, silty clay with diatoms, silty clay with sponge spicules, and silty clay nannofossil ooze are present as minor lithologies. All lithologies occur as horizontally bedded, undisturbed sediments. The uppermost 10–15 cm in Sections 303-U1305B-1H-1 and 303-U1305C-1H-1 are reddish brown to brownish gray and are interpreted from smear slide data as surface-oxidized equivalents of the underlying lithologies (Fig. F9). In both of these cores, a zone of concentrated iron oxides is present at the base of the oxidized sediments. Dropstones are present throughout Unit I, and their distribution is plotted in Figure F10.

Bioturbation is present throughout most of this unit; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. In some cases, discrete burrows or macroscopic pyritized burrows were observed. At greater depths in all three holes, the remnants of burrows are preserved as sandy silt blebs. The bioturbation index for the silty clays ranges from absent to moderate, but most silty clays display moderate bioturbation.

Color variation within the silty clays is minor, varying between dark gray (5Y 4/1 and N 4/1) to very dark gray (5Y 3/1 and N 3/1). The nannofossil lithologies, in general, are a lighter shade of gray (5Y 5/1) and tend to have moderate to common bioturbation. The sandy silt laminae occur most frequently in the silty clay intervals. Hence, the laminated intervals are predominantly dark (5Y 4/1) to very dark (5Y 3/1) gray, with lesser occurrences of gray (5Y 5/1) and olive-gray (5Y 4/2). Contacts between these lithologies generally are gradational or burrowed; the most common exceptions are the sharp contacts at the bases of the sandy silt laminae and CaCO₃-rich silty clay intervals (Fig. F11). Basal contacts between these two lithologies and the underlying silty clays are sharp and commonly scoured. In contrast, the upper boundaries of these intervals (the sandy silt laminae and CaCO₃-rich silty clays) are often heavily burrowed and commonly gradational (Fig. F12). Some intervals also exhibit a fining-upward trend in grain size with the basal laminae having more sand-sized particles than the silt laminae above. Millimeter-scale cross lamination is observed in a majority of the sandy silt laminae (Fig. F11).

Discussion

Sediments in Unit I at Site U1305 reflect the input of biogenic and terrigenous components. These sediments represent pelagic and hemipelagic deposition modified by the interaction of deep ocean currents and bottom topography. The resulting sediment composition reflects environmental changes at the ocean's surface that caused variations in phytoplankton and zooplankton assemblages, aerosol inputs, and the discharge and source of icebergs. Because it is located in the lee of the Eirik Drift crest, Site U1305 accumulates sediments at high rates when bottom currents (NADW) are strong. As proposed in the Expeditions 303 and 306 Scientific Prospectus (Channel, Sato, Kanamatsu, Stein, Malone, et al., 2004), another source that might contribute sediment to Site U1305 is the transport of sediments down the NAMOC. Sediments moving through this channel may overflow the levees from time to time and spread laterally, possibly reaching Site U1305. Thus, this site is potentially well positioned to examine the relationships between the high-latitude surface ocean, cryosphere, and bottom circulation. Furthermore, the combination of records from this deep location (3459 m) and the shallower Site U1306 (2271 m) provides a unique opportunity to understand deep ocean circulation and climate variations throughout much of the Pleistocene epoch.

Large-scale patterns in the distribution of important lithologies at Site U1305 are presented in Figure F8. Of the 867 m of sediment recovered at this site, 757 m (87%) is silty clay and related derivatives (e.g., silty clay with nannofossils and silty clay with diatoms). Nannofossil ooze and its variations (e.g., silty clay nannofossil ooze or nannofossil ooze with silty clay) comprise 77 m (9%) of the sediments, whereas sandy silt laminae are the dominant lithology in 33 m (4%) of the sediments. The most notable downhole lithologic change is the increased abundance of nannofossil ooze found between ~60 and 100 mcd, corresponding roughly to Cores 6H through 10H (or 60–100 mcd) in each hole (Fig. F8). Preliminary age estimates suggest that this interval corresponds to the ages of ~300–500 ka (see “Biostratigraphy”). Thus, the higher occurrence of nannofossil ooze may correspond to the interglacial conditions during MIS 9, 11, and 13.

Sandy silt laminae are found throughout the recovered section in all holes and may be related to either mass-sediment transport events or intensification of bottom currents. Cross lamination is evident in many of these intervals (Fig. F11). These sandy silt laminae produce the very high peaks observed in the magnetic susceptibility records (see “Physical properties”). Similar peaks can be seen in piston and gravity cores collected from this region (Hillaire-Marcel et al., 1994), indicating that the sandy silt layers represent regional events. As such, understanding their significance should be an important focus in postcruise studies.

In addition to the sandy silt laminae, centimeter- to decimeter-scale intervals of silty clay composed largely of detrital carbonate are present in the Site U1305 sediments. These carbonate intervals are recognized easily by their olive-gray color and fine clayey texture and are observed in all holes (Table T2). Sandy silt laminae are sometimes present within these intervals, but not always. Basal contacts of the detrital carbonate layers are sharp and sometimes irregular in appearance (Fig. F12). The top boundaries are commonly bioturbated and grade into the above sediments. The nature of the contacts indicates that the detrital carbonate layers were probably deposited rapidly. Although these layers appear throughout Unit I, a distinct change in the thickness of individual layers occurs at ~160 mcd. The average thickness of detrital carbonate layers above 156 mcd is 7 cm, whereas the average thickness below 156 mcd is 25 cm. Interestingly, this change appears to occur at a level where the sediments have normal polarity assigned to the Jaramillo Subchron (~1 Ma) (see “Paleomagnetism”) and corresponds roughly to the time when the climatic signal transitioned from a 40 k.y. rhythm to a 100 k.y. beat.

Gravel counts, defined as the number of clasts that are >2 mm in a 10 cm interval, were made on each section in all three holes, yielding a total of 295 clasts or ~1 clast for every 3 m of core (Fig. F10). Therefore, the cryospheric imprint at this site will need to be evaluated in postcruise studies using microscopic and/or other techniques.

One of the goals of Expedition 303 is to examine millennial-scale climate variability beyond the scope obtained by conventional piston coring efforts. Reconstructing climate variability on this scale requires sedimentation rates >10 cm/k.y. The sediment record recovered at Site U1305 appears to be free of hiatuses and to have sedimentation rates sufficiently high to allow studies of millennial-scale change (see “Composite section”). One encouraging observation is that several horizons at Site U1305 have distinct textural and compositional characteristics and probably correspond to the Heinrich (H) events. For example, layers with distinct color and rich in detrital carbonate suggest that the H2 and H4 layers are present in intervals 303-U1305C-2H-3, 115–120 cm, and 2H-4, 57–63 cm, respectively (Fig. F13). Knowing the position of these layers in the core allows us to identify H0 and H1 layers in intervals 303-U1305C-2H-3, 32–40 cm, and 2H-3, 67–72 cm, respectively. The H3 layer is more difficult to identify, but may be tentatively placed in interval 303-U1305C-2H-3, 38–140 cm. These are preliminary assignments and are subject to revision with postcruise studies. However, the preliminary interpretation does demonstrate that there are clear lithologic differences that apparently correspond to recognized millennial-scale climate events.

As noted previously, the magnetic susceptibility record from Site U1305 correlates well with magnetic susceptibility records for other cores from this region (e.g., Hillaire-Marcel et al., 1994), affording an opportunity to further evaluate the potential for this site to yield high-resolution climate records (Fig. F14). This correlation indicates that sediment accumulation is enhanced in the region of Site U1305 during MIS 1, 5e, 7, 9, and 11. In particular, the inferred base of the Holocene is at ~9 mbsf (9.9 mcd) in Hole U1305C. The Core MD99-2227 record also shows that interglacials are associated with high magnetic susceptibility values (Turon and Hillaire-Marcel, 1999). Lightness (L*) measurements from Site U1305 mirror the magnetic susceptibility values, such that high L* (light) values generally correlate with low magnetic susceptibility values and vice versa (Fig. F15). This is not surprising given that sedimentary CaCO₃ is usually light in color and reduces the magnetic susceptibility value by dilution of the other terrigenous components. Based on the correlation with the magnetic susceptibility record from Core MD99-2227, glacial terminations and the early interglacial conditions have low L* (dark) values and high magnetic susceptibility values. The pattern observed at Site U1305 suggests that sediment accumulation from increased bottom current activity swamps the biogenic contribution during the termination and early interglacial intervals. Higher L* values and lower magnetic susceptibility values characterize the transition into a glacial. This may arise from decreased contribution of the detrital component as the bottom current weakened, allowing more CaCO₃ to accumulate at Site U1305. Shipboard CaCO₃ measurements are too widely spaced to test this hypothesis. High-resolution measurements of CaCO₃ and other sedimentary proxies (e.g., grain size measurements) will need to be performed as part of postcruise studies to decipher the relative importance of sediment contributions to Site U1305 from the surface ocean, cryosphere, and bottom currents.

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