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doi:10.2204/iodp.proc.303306.107.2006

Lithostratigraphy

Two holes were drilled at Site U1307 (Table T1). All cores were recovered using the APC, and recovery was good (102.0%). The sedimentary record at Site U1307 reflects the different degrees of mixing between terrigenous and biogenic material (primarily quartz, detrital carbonate, nannofossils, and foraminifers) (see “Site U1307 smear slides” in “Core descriptions;” Fig. F3), so the most common lithologies are silty clay, silty clay with foraminifers, foraminifer silty clay, foraminifer sandy clay, foraminifer ooze, nannofossil silty clay, silty clay nannofossil ooze, and sandy clay (Fig. F4). The top of the succession is dominated by olive-brown and olive-gray colors, whereas different shades of greenish gray are more abundant toward the bottom. Variations in color between light gray and very dark gray are common and represent high to low nannofossil contents, respectively. Bioturbation is generally moderate to abundant, as indicated by the presence of numerous distinct pyritized silty clay-filled and sandy silt-filled burrows. Gradational and burrowed contacts between the various sediment types are much more common than well-defined, sharp boundaries. However, irregular, sharp contacts typically occur at the bases of foraminifer ooze layers.

Smear slide analyses recorded the abundances of terrigenous components, the most abundant of these being quartz, 2%–95%; detrital carbonate, 0%–35%; clay minerals, 0%–70%; and heavy minerals (especially hornblende), 0%–10% (Fig. F3). Among the biogenic components, nannofossils are widely distributed in the upper and lower intervals of the succession, whereas foraminifers are found predominantly in the upper part of the succession. The relative abundances of biogenic components are foraminifers, 0%–56%; nannofossils, 0%–70%; diatoms, 0%–12%; and sponge spicules, 0%–5%. Total carbonate content ranges from <1 to 58 wt% (see “Geochemistry”). Pyrite (usually associated with burrows) and iron oxide coatings on grains are present locally and were the only authigenic components observed.

Dropstones are present in low numbers throughout the entire cored interval. The dropstones display a wide range of compositions, including acidic intrusive and metamorphic (granites and granitoids), basic igneous and/or metamorphic (basalts and metabasalts), and sedimentary and metasedimentary (sandstone, quartzite, and mudstone) rocks.

Large-scale patterns in the distribution of major lithologies at Site U1307 are represented in Figure F4. Of the 317 m recovered at this site, 279 m (88%) is silty clay, 16 m (5%) is nannofossil-dominated sediments (silty clay nannofossil ooze and nannofossil ooze), 16 m (5%) is foraminifer-dominated sediments (silty clay foraminifer ooze and foraminifer ooze), and 5.6 m (1.7%) is sandy clay. The sediments at Sites U1307 have been divided into three units: Unit I comprises Holocene–lower Pleistocene terrigenous and biogenic sediments that contain both nannofossil and foraminifers; Unit II is mainly composed of terrigenous materials of early Pleistocene– Late Pliocene age that lack biogenic carbonate; and Unit III comprises the nannofossil-bearing, but foraminifer-barren, lower Upper Pliocene succession.

Description of units

Unit I

• Intervals: Sections 303-U1307A-1H-1, 0 cm, to 5H-7, 33 cm; and 303-U1307B-1H-1, 0 cm, to 6H-2, 102 cm

• Depths: Hole U1307A: 0–46.6 mbsf and Hole U1307B: 0–45.1 mbsf (0 to 49.37–49.55 meters composite depth [mcd])

• Age: Holocene–early Pleistocene

Unit I is composed predominantly of silty clay with foraminifers, foraminifer silty clay, foraminifer sandy clay, silty clay foraminifer ooze, foraminifer ooze, nannofossil ooze, silty clay nannofossil ooze with foraminifers, and nannofossil silty clay. Foraminifer silty sand and sandy foraminifer ooze beds are present as minor lithofacies. Colors are predominantly olive-brown (2.5Y 4/3 and 2.5Y 4/4), olive-gray (5Y 4/2), light olive-gray (5Y 6/2), gray (5Y 5/1), dark gray (5Y 4/1 and N/4), and very dark gray (5Y 3/1 and N/3), with lesser occurrences of light gray (5Y 6/1), which corresponds to foraminifer ooze. Contacts between these color changes are typically gradational or bioturbated (Fig. F5). The most common exceptions are the sharp contacts at the bases of the foraminifer silty sands and the sandy foraminifer oozes (Fig. F6). These coarser-grained intervals have irregular basal contacts and gradational or bioturbated upper contacts (Figs. F7, F8). A 1.1 m thick foraminifer ooze layer in Cores 303-U1307A-5H and 303-U1307B-6H marks the bottom of Unit I (Fig. F9) and its irregular base is the boundary between Units I and II. The boundary may represent a hiatus in the sedimentary record, although this has not been indicated by micropaleontologic analyses (see “Biostratigraphy”). Paleomagnetic data indicate that the foraminifer ooze layer is coeval with the base of the Jaramillo Subchron (see “Paleomagnetism”) and may correlate with ~20 cm thick foraminifer ooze layer of the same age observed at Site U1306. Bioturbation is common to abundant through most of this unit; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale sandy silt or pyritized burrow fills. In a few cases, discrete burrows or discrete macroscopic pyritized burrows were observed. Dropstones are present throughout Unit I in low numbers, and their distribution is plotted in Figure F6.

Unit II

• Intervals: Sections 303-U1307A-5H-7, 33 cm, to 15H-CC, 20 cm; and 303-U1307B-6H-2, 102 cm, to 14H-4, 150 cm

• Depths: Hole U1307A: 46.6–125.7 mbsf (49.37–49.55 mcd) and Hole U1307B: 45.1–124.8 mbsf (132.98–133.86 mcd)

• Age: early Pleistocene–Late Pliocene

Unit II is composed almost exclusively of silty clay. Single layers of sandy silty clay and foraminifer ooze are the only other lithologies within this 80 m thick interval. The dominant colors are dark greenish gray (5GY 4/1), gray (5Y 5/1), dark gray (5Y 4/1 and N/4), and very dark gray (5Y 3/1 and N/3). Boundaries between these color variations are generally gradational or bioturbated. Common to abundant bioturbation occurs through most of this unit, as seen by the diffuse, centimeter-scale mottling; millimeter-scale sandy silt or pyritic burrow fills; and discrete macroscopic pyritized burrows. Dropstones are slightly less numerous than in the overlying unit (Fig. F10). Micropaleontological analyses point to a hiatus with a duration of ~0.24 m.y., or a condensed interval, between Samples 303-U1307A-7H-1, 10–11 cm, and 7H-5, 10–11 cm. There is, however, no visual evidence for an erosional truncation that could indicate the presence of a hiatus in this interval.

Unit III

• Intervals: Sections 303-U1307A-16H-1, 0 cm, to 19H-CC, 27 cm; and 303-U1307B-14H-5, 0 cm, to 17H-5, 125 cm

• Depths: Hole U1307A: 125.7–162.6 mbsf (0–131 mcd) and Hole U1307B: 124.8–154.6 mbsf (163.82–173.6 mcd)

• Age: Late Pliocene

Unit III consists of at least 30–38 m of variable mixtures of terrigenous and biogenic material, dominated by quartz and nannofossils. The resulting lithologies are silty clay, silty clay with nannofossils, silty clay nannofossil ooze, silty clay nannofossil ooze with diatoms, and nannofossil silty clay. Note that lithologies containing <25% nannofossils are not classified as nannofossil ooze and are represented with the symbol of the major lithology in Figure F4. Therefore, although Unit III in Hole U1307B contains nannofossils, these do not appear in the graphic representation. Contacts between the major lithologies are generally gradational and defined by progressive color changes. The main colors are greenish gray (5GY 5/1), dark greenish gray (5GY 4/1), gray (5Y 5/1), dark gray (5Y 4/1), and very dark gray (5Y 3/1). As in the younger units, bioturbation is common to abundant through most of this unit, represented by diffuse centimeter-scale mottling, millimeter-scale sandy silt or pyritized burrow fills, and distinct macroscopic pyritized burrows. The abundance of dropstones is rarely more than 1 per 10 cm (Fig. F10).

Discussion

The Holocene–Upper Pliocene sedimentary succession at Site U1307 records variations in the input rates of terrigenous and biogenic components. Pelagic and hemipelagic depositional patterns at this site are principally governed by the interplay between thermohaline contour currents and the regional seafloor morphology, leading to the formation of a field of accreting sedimentary waves in an area of focused sedimentation. Winnowing and/or erosion may have occurred during periods of intensified current velocities. Evidence for unconformities in the sedimentary record at this site were not observed, but further examination of micropaleontological and paleomagnetic data will provide more precise sedimentation rate estimates.

The sediment composition reflects changes in paleoceanographic conditions of the overlying and surrounding water masses, with these being translated into changes in the phytoplankton and zooplankton assemblages and the discharge and source of icebergs.

The relative input of terrigenous and biogenic components during the deposition of Units I and III appears to have varied on timescales of tens to hundreds of thousands of years, as indicated by repeated broad changes in the abundances of quartz, detrital carbonate, foraminifers, and nannofossils (as observed in smear slides) and in the distribution of dropstones. Unit II, however, represents a time when terrigenous supply clearly dominated over the biogenic component with little variation.

The top of the succession, corresponding to Unit I, is characterized by olive-brown, olive-gray, and varying shades of gray. The lighter colors generally indicate higher nannofossil and/or foraminifer content. In Units II and III, dark greenish gray and dark to very dark gray colors dominate. The origin of the diffuse dark greenish gray banding that influences the coloration of the lower part of the succession has not been discerned but may be diagenetic in origin.

Based on smear slide observations, nannofossil abundances are higher within Units I and III than within Unit II (Fig. F3). The quartz profile appears to show an inverse pattern to biogenic carbonate, and relatively high values are estimated throughout the two holes. Detrital carbonate content is low and values do not exceed 30%.

Gravel counts, defined as the number of clasts >2 mm in every 10 cm interval of each section, yielded a total of 305 clasts or ~2 clasts per 2 m of core. Thus, dropstones are more abundant at Site U1307 than at Sites U1305 and U1306 (~1.7 and ~1 clast for every 3 m of core, respectively). Dropstone distribution maxima occur at the same level as the foraminifer-rich layers of Unit I (15–22 mcd) and Unit II (85–95 mcd).

Sources of terrigenous input may be differentiated by the dominance of quartz and detrital carbonate in smear slides, supplemented by the composition of dropstones observed throughout the succession. Dropstone compositions include acidic intrusive and metamorphic rocks, basic volcanics and metavolcanics, and sedimentary rocks (mainly mudstones). The source of granitic and gneissic dropstones is uncertain, whereas dropstones with more basic lithologies could have been derived from sources to the northwest or in central eastern Greenland. The dominance of quartz in the sediment composition may also point toward eastern Greenland as a likely source of the terrigenous fraction. On the other hand, the low detrital carbonate content indicates that the input from Hudson Bay is either limited at Site U1307 or diluted by very high input from other source areas.

Lightness (L*) values at Site U1307 are similar to those at the two previous sites, ranging from 34% to 50%. The sediment lightness record was compared to CaCO₃ values to evaluate the effect of carbonate content on sediment color. In general, the two records show similar patterns, although the CaCO₃ record has much lower resolution than L*. As at Sites U1305 and U1306, carbonate content is low, with most CaCO₃ values lower than 12 wt% (see “Geochemistry;” Fig. F11). At ~50 mcd, a 1.1 m thick foraminifer ooze layer was observed in both holes that yielded a CaCO₃ content of 57.6 wt%, which produced a distinct peak in the L* curve (~49%).

Unit III reflects pulses of varying inputs of terrigenous and biogenic (mostly nannofossil dominated) components. The shift to terrigenous-dominated deposition in Unit II might reflect increased terrestrial dilution of the biogenic fraction or decreased surface water productivity. Micropaleontological analyses show that the foraminifer ooze layers in Unit I are well sorted, probably due to reworking, indicating frequent changes in bottom-current regime. Deposition of Unit I was dominated by pelagic and hemipelagic processes. Episodically, however, more pervasive bottom currents seem to have affected the area, scouring the seabed, leaving sharp and irregular contacts and depositing coarser-grained beds of foraminifer silty sand and sandy foraminifer ooze. Eight of these beds are observed and only two of them are thicker than 80 cm (Table T2; Fig. F9). These intervals yield high L* and carbonate values and their locations are indicated in Figure F11.

One of the main aims of Expedition 303 was to extend the record of Pleistocene millennial-scale change beyond that obtained from conventional piston cores. Site U1307 appeared to be a good candidate for coring the upper to middle part of the Pliocene succession using the APC to recover older strata than at the previous sites. A qualitative analysis indicates that the lithologic changes recorded in Units I, II, and III cannot be linked consistently to glacial–interglacial stages as interpreted from magnetic susceptibility and carbonate records (see “Composite section” and “Physical properties”). Extracting paleoclimatic records from the range of sedimentary characteristics will require a variety of detailed postcruise studies.

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