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doi:10.2204/iodp.proc.303306.103.2006 LithostratigraphyFive holes were drilled at Site U1302, and two holes were drilled at Site U1303 (Table T1). All cores were recovered using the APC. The sediments at Sites U1302 and U1303 are dominated by varying mixtures of terrigenous components and biogenic debris (primarily quartz, detrital carbonate, and nannofossils) (see “Site U1302 and U1303 smear slides” in “Core descriptions;” Fig. F7), so the most common lithologies are clay, silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay (Fig. F8). Gradational contacts between these lithologies are much more common than well-defined boundaries. Abundances of terrigenous components, as estimated from smear slides, are quartz, 0%–80%; detrital carbonate, 0%–95%; feldspars, 0%–20%; clay minerals (including chlorite), 0%–95%; heavy minerals (especially hornblende), 0%–1%; and volcanic glass, 0%–5%. No discrete ash layers were observed. Dropstones are present throughout these cores and 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, limestone, and dolomite). Abundances of biogenic components, as estimated from smear slides, are nannofossils, 0%–85%; foraminifers, 0%–50%; diatoms, 0%–25%; radiolarians, 0%–10%; and sponge spicules, 0%–25%. Total carbonate contents range from 1 to 48 wt% in these cores (see “Sedimentary geochemistry” in “Geochemistry” and “Site U1302 and U1303 smear slides” in “Core descriptions”). Pyrite (usually associated with burrows) and iron oxide coatings on grains are present locally and were the only authigenic sediment components observed. The sediments at Sites U1302 and U1303 were designated as a single unit composed of Holocene–Pleistocene terrigenous and biogenic sediments due to the gradational interbedding of these lithologies at scales of a few meters or less. This unit was divided into two subunits. Subunit IA is composed of undisturbed sediments, whereas underlying Subunit IB contains abundant intraclasts in a matrix of sand-silt-clay and is interpreted as debris flow deposits. Description of unitsUnit I
Unit I is composed predominantly of clay, silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. Nannofossil ooze, foraminifer silty sand, sandy foraminifer ooze, silty clay with diatoms and sponge spicules, silty nannofossil ooze with diatoms and sponge spicules, and silty clay siliceous ooze with nannofossils are present as minor lithologies. Colors are predominantly gray (5Y 5/1 and 2.5Y 5/1) and dark gray (5Y 4/1 and 2.5YR 4/1), with lesser occurrences of light gray (2.5Y 6/1 and 5Y 6/1), brownish grays (7.5YR 5/2 to 2.5YR 5/2), and reddish brown (10YR 4/1). Contacts between these lithologies are generally gradational; the most common exceptions are the sharp contacts at the base of the foraminifer silty sands and the sandy foraminifer oozes (Fig. F9). Contacts between the other lithologies are sometimes defined by sharp color boundaries. Bioturbation is present through most of this unit; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. In a few cases, discrete burrows or discrete macroscopic pyritized burrows were observed. Dropstones are present throughout Unit I, and their distribution is plotted in Figure F10. In Subunit IA these lithologies occur as horizontally bedded, undisturbed sediments; the only notable exception is a disturbed zone in the lower half of Core 303-U1303A-7H. In Subunit IB, these pelagic and hemipelagic lithologies are present as angular to rounded intraclasts in a deformed matrix of foraminifers, sand, silt, and clay. Extrabasinal clasts originally deposited as dropstones are also present in this interval, which is interpreted to be a debris flow deposit (Fig. F11). The contact between Subunits IA and IB is relatively sharp within a single hole but varies in depth (both mbsf and mcd) between holes because of relief at the uppermost part of the debris flow deposit. Subunit IA
Subunit IA consists of 103–106 m of variable mixtures of terrigenous and biogenous material dominated by quartz, detrital carbonates, and nannofossils. The resulting lithologies are clay, silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. Nannofossil ooze, foraminifer silty sand, sandy foraminifer ooze, silty clay with diatoms and sponge spicules, silty nannofossil ooze with diatoms and sponge spicules, and silty clay siliceous ooze with nannofossils are present as minor lithologies. The uppermost 10–15 cm in Sections 303-U1302D-1H-1, 303-U1302E-1H-1, 303-U1303A-1H-1, and 303-U1303B-1H-1 are reddish brown to brownish gray and are interpreted as surface-oxidized equivalents of the underlying lithologies (Fig. F12). In each of these sections, a zone of concentrated metal oxides is present at the base of the oxidized sediments. Below this surface-oxidized layer, contacts between the major lithologies are generally gradational and defined by progressive color changes. The foraminifer silty sand and the sandy foraminifer ooze form well-defined intervals ranging from 1 to ~50 cm in thickness. These coarser-grained intervals have sharp basal contacts and sharp to gradational upper contacts (Fig. F9). Some intervals also exhibit a fining-upward trend in grain size, as well as millimeter-scale inclined stratification. These beds, especially the thicker ones, may be turbidites. Millimeter-scale lamination, caused by changes in grain size (foraminifers versus silty clay) and/or color, is rare in Subunit IA but is present in several intervals as thick as 20 cm (Fig. F13). These laminated intervals may be contourites or small turbidites, or they may reflect winnowing by downslope currents. Additional analyses will be required to develop sedimentologic interpretations of these intervals. Dropstones are present throughout Subunit IA (Fig. F10). Sediments in Subunit IA are predominantly horizontally bedded and undisturbed, as indicated by the orientations of lithologic contacts and color patterns. The principal exception is in Core 303-U1303A-7H, where intraclasts of the underlying horizontally bedded lithologies lie at various orientations within a deformed sandy matrix of foraminifers, sand, silt, and clay (Fig. F14). This interval is interpreted as a relatively small sandy debris flow unit. Subunit IB
Subunit IB consists of angular to rounded intraclasts as large as 1 dm and composed of clay, silty clay, silty clay with nannofossils, nannofossil silty clay, silty clay nannofossil ooze, and nannofossil ooze with silty clay. These clasts display variegated colors, including shades of gray, brown, light greenish gray, and white and occur in a deformed matrix of foraminifers, sand, silt, and clay (Fig. F11). Rock clasts similar in size and composition to the dropstones recorded in Subunit IA are also present. This interval is interpreted to be a debris flow deposit that was sourced in sediments similar to those forming Subunit IA. However, many of the exotic clasts are much older than Quaternary. DiscussionThe Holocene and Pleistocene sediments at Sites U1302 and U1303 record repeated variations in the input rates of terrigenous and biogenic components. Conditions such as these existed prior to the record cored at Sites U1302 and U1303, as indicated by the intraclasts of similar lithologies contained in the debris flow deposit of underlying Subunit IB. During the deposition of Subunit IA, the relative input of terrigenous and biogenous components 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, and nannofossils (as observed in smear slides) and in the distribution of dropstones. Sources of the terrigenous input can be inferred from the dominance of quartz and detrital carbonate in smear slides and supplemented by the compositions of dropstones observed throughout these cores. Dropstone compositions include acidic intrusive and metamorphic rocks, basic volcanics and metavolcanics, and sedimentary rocks including limestones and dolomites. These carbonate dropstones and the fine-grained detrital carbonate indicate sources immediately upslope or to the northwest, especially in the regions around Hudson Bay. The granitic and gneissic dropstones and the fine-grained quartz could have been derived from nearby sources or from more distant sources; the latter could include regions to the northwest or, potentially, to the north in southern and eastern Greenland. The dropstones with more basic lithologies could have been derived from sources to the northwest or in central eastern Greenland. Input rates of the biogenic components may have varied in response to changes in the paleoceanographic conditions in the overlying and surrounding waters. A preliminary qualitative analysis indicates that the lithologic changes recorded in Subunit IA cannot be linked consistently to glacial–interglacial stages interpreted from magnetic susceptibility and the carbonate records (see “Composite section” and “Whole-core magnetic susceptibility measurements” in “Physical properties”). For example, dropstone distribution maxima occur during apparent glacials, apparent interglacials, and apparent climatic transitions. As a result of these complexities, extracting paleoclimatic records from the range of sedimentary components will require a variety of detailed postcruise studies. Deposition of Subunit IA was dominated by pelagic and hemipelagic processes. Episodically, however, small turbidity currents or contour currents affected the area, depositing the sharp-based coarser-grained beds of foraminifer silty sand and sandy foraminifer ooze and producing the intervals that contain weak to well-developed laminations. One of the major goals of Expedition 303 was to extend the record of Pleistocene millennial-scale climate change beyond that obtained from conventional cores. A reasonable first step toward achieving this objective is to evaluate the ways in which the sediments at Sites U1302 and U1303 record known late Pleistocene and Holocene millennial-scale climatic events, whose commonly recognized characteristics in the North Atlantic have included a basal layer of IRD, increased detrital carbonate content, light color, and increased magnetic susceptibility. Magnetic susceptibility and bulk density data were used to identify the positions of Heinrich events H0–H6, as well as low-detrital carbonate (LDC) and detrital carbonate (DC6) layers, in Hole U1303B (Fig. F26). The overall color reflectance lightness (L*) values, gravel abundances, and visually observed Munsell colors of these intervals were then compared to the characteristics of the overlying and underlying sediments. The L* values show distinct peaks at the levels of H1–H5 and DC6 but do not rise above adjacent background for H0, LDC, and H6. H5 and DC6 are the only intervals that contain significantly more IRD than adjacent background, but the IRD in H5 and DC6 is disseminated throughout each interval, rather than being concentrated at its respective base. Visual color estimates show evidence of increased brown hues for H1 and H2 and lightening for H4 and H5 but no consistent pattern of color change for all of the known millennial-scale events because they are not all rich in detrital carbonate. The detrital stratigraphy at Orphan Knoll represents a proximal analog to the detrital stratigraphy of the classic central Atlantic IRD belt. Identifying and explaining the differences in manifestation of these events and refining the correlation among events will be necessary before the record of older millennial-scale climate change can be examined and interpreted in detail. |
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