At Site 1101, 217.7 m of predominantly hemipelagic clayey silts were drilled at a distal site within Drift 4 on the Antarctic Peninsula continental rise (Rebesco et al., 1996, 1997). The sediments of Site 1101 contain a nearly continuous distal glacial record of the past 3.1 m.y. and can be divided into three lithostratigraphic units (Fig. F4). Unit I (0-53.3 mbsf) and Unit II (53.3-142.7 mbsf) are composed of alternating biogenic-bearing massive clayey silts and laminated clayey silts that are interpreted to have been deposited during interglacial and glacial periods, respectively. Within Unit I, diatom-bearing layers are correlated to warm oxygen isotope stages. Unit II has at least 19 discrete foraminifer-bearing layers that alternate cyclically with barren laminated or massive intervals. These biogenic intervals are interpreted to represent interglacials, during which varying controls on marine productivity and oceanographic conditions resulted in calcareous instead of siliceous biogenic sedimentation in Unit II. Unit III (142.7-217.7 mbsf) lacks the regular alternation of biogenic and terrigenous intervals observed in the overlying units. Above 198 mbsf in Unit III, the biogenic component is low, and massive clayey silt and diamict occur. The diamict facies was deposited directly under glacial influence by iceberg rafting, and the associated massive barren facies could have been rapidly deposited by turbid plumes or sediment gravity flows from glaciers near the edge of the continental shelf. The lower part of Unit III may represent a warmer interval with deposition of diatom-bearing massive and laminated facies.
Unit I consists of 53.3 m of fine-grained sediments, primarily clayey silt (83% of Unit I) and silty clay (17% of Unit I; Figs. F4, F5). Silt laminae, 1-2 mm thick, occur throughout Unit I but form only a minor part of the total thickness. Three normally graded silt beds (as much as 3 cm thick) occur within Cores 178-1101A-2H and 4H. A 3-cm-thick graded bed of volcanic ash with a sharp basal contact occurs at 9.82 mbsf (interval 178-1101A-2H-1, 112-115 cm; Fig. F6). Two facies alternate with gradational contacts within Unit I: a massive facies with a variable biogenic component, and a laminated facies. These are described in more detail below.
Facies M (massive) ranges in color from dark grayish brown (2.5Y 4/2) to olive gray (5Y 4/2) and is characterized by bioturbation indicated by color mottling and burrows. For example, Planolites burrows occur in the interval 178-1101A-7H-2, 90-100 cm. Scattered pebbles, granules, and sand are abundant within the massive facies. The biogenic component is variable and includes both diatoms and foraminifers (Fig. F5). Muddy diatom ooze occurs in the upper 20 cm of Core 178-1101A-1H (Fig. F4). An 80-cm-thick bed of diatom-bearing foraminiferal clayey silt occurs in the massive facies at 8.7 mbsf (interval 178-1101A-2H-1, 0-80 cm). Two other thin, diatom-bearing massive beds occur at 19.7 mbsf (interval 178-1101A-3H-1, 0-50 cm) and 23 mbsf (interval 1101A-3H-4, 30-100 cm). A 1.4-m thick, diatom-bearing, massive bed occurs at 28.2 mbsf. Five intervals of massive facies occur in Unit I and range in thickness from 1.5 m to 11 m (Fig. F4).
Laminated facies, commonly dark gray (5Y 4/1), occur in six intervals, ranging in thickness from 1.6 to 7 m in Unit I (Fig. F4). The facies descriptions used at Sites 1095 and 1096 can also be applied here (see Fig. F9 in the "Explanatory Notes" chapter). Most commonly, the laminated facies is formed by faint discontinuous silt lamination, without any repetitive vertical organization, which appears as color banding with variable bioturbation (Facies C, as described at Site 1096) and 1- to 3-mm-thick graded silts with sharp bases (Facies L2 and L3). The graded silt laminae occur in groups of 3-7 (Fig. F7). Also within Unit I are three normally graded, thin beds (as much as 5 cm thick) of silt to fine sand (Facies L1). These beds have sharp basal contacts and grade into the overlying mud.
Unit I sediments are dominantly hemipelagic, deposited under a regime of weak bottom currents. Clayey silts with faint discontinuous lamination are interpreted as contourites (similar to Site 1096). The thin groups of silt laminae with sharp bases (L2) are interpreted as distal muddy turbidites. Low-energy turbidity flows probably deposited the three thicker and more coarsely graded beds. The massive facies record periods of ice-free open water (allowing higher diatom productivity), intense bioturbation, and probably lower sedimentation rates. As at Site 1096, the massive facies with a higher biogenic component is thought to occur during oxygen isotope warm Stages 1 (seafloor sediment), 5e, 7 (occurrence of Hemidiscus karstenii; see "Biostratigraphy"), and 9. Stages 11-17 cannot presently be identified, but Stage 19 coincides with the Brunhes/Matuyama boundary, which is placed at 55.1 mbsf (see "Paleomagnetism"). A vitric ash layer occurs at the base of Stage 5 and appears very similar to an alkaline ash in an equivalent stratigraphic position in piston cores from near Site 1095 (Fig. F6; Pudsey and Camerlenghi, 1998). Pebbles and dispersed granules and sand were transported to Site 1101 by iceberg rafting (Fig. F5).
Unit II is 89.4 m thick and begins at the first occurrence downcore of foraminifer-bearing silty clay (Fig. F4). Massive and laminated facies alternate throughout the unit as discussed below. Within Unit II, 19 discrete layers of foraminifer-bearing, massive clayey silt or silty clay occur, ranging in thickness from 45 to 171 cm. Each of these layers corresponds to a trough in the magnetic susceptibility curve (Fig. F8), which is expected because the addition of carbonate to this primarily terrigenous sediment should produce lower magnetic susceptibility (see "Physical Properties"). The foraminifer-bearing intervals are also lighter in color, and each corresponds to a peak in the chromaticity parameter b* and in percent carbonate (Fig. F8, see "Organic Geochemistry", Table T2). The magnetic susceptibility curve shows two more troughs, which do not correspond to observed foraminifer-bearing intervals. These occur near core boundaries, which are disturbed and where sediment may have been lost. Therefore, it is likely that 21 foraminifer-bearing intervals are present within Unit II at Site 1101. However, we cannot verify that we have recovered the entire sediment record because only a single APC hole was drilled at this site.
Massive facies can be subdivided into foraminifer-bearing (Mf) and barren (Mb). A massive foraminifer-bearing facies (Mf) with as much as 3% nannofossils is repeated at fairly regular intervals downcore in Unit II (Figs. F4, F9). This facies ranges from greenish gray (5Y 5/1) to dark greenish gray (5GY 4/1) and is composed of clayey silt with 10%-90% biogenic component (Fig. F5). The coarser grain size of the terrigenous silt fraction and the addition of larger foraminifer tests or fragments produce a rough appearance on the core surface that is visible in core images (Fig. F9). In some intervals, foraminifers are abundant enough for the sediment to be classified as an ooze, and the tests can be clearly seen on the core surface using a hand lens. Mf is from structureless to burrow mottled with scattered sand and granules. Zoophycos and Planolites burrows occur within Mf. Massive barren facies (Mb) has the same characteristics as Mf but lacks the foraminifer biogenic component.
The laminated facies within Unit II is dark greenish gray (5GY 4/1) to dark gray (N 4/0) and includes three different subfacies. The most common is Facies C (see "Lithostratigraphy" in the "Site 1096" chapter), which contains faint, bioturbated, silty lamination that sometimes appears as faint color banding (Fig. F10). The second, less common, facies is sharp-based, parallel-laminated, 1- to 3-mm graded silts (Facies L2 above; Fig. F10). In some laminated intervals, such as in Core 178-1101A-10H, Facies C occurs between regularly spaced L2 facies (Fig. F10). Thicker beds of coarse silt or fine sand grading into mud (Facies L1) occur uncommonly in Unit II. These beds range in thickness from 0.5 to 4 cm and have irregular sharp (erosive?) basal contacts and fairly sharp upper contacts (Fig. F11). The thicker beds occur at 66 mbsf (Section 178-1101A-8H-5), 87.6 mbsf (Section 178-1101A-11H-2), 116.7 mbsf (Section 178-1101A-14H-2), and 136.8 mbsf (Section 178-1101A-16H-3). The 4-cm silt bed at 136.8 mbsf is cemented with carbonate and is the only lithified interval in Unit II.
Laminated intervals correspond to high-frequency peaks in the magnetic susceptibility curve (see "Physical Properties"). The highest peaks corresponding to the thicker, coarser grained beds are especially noticeable (Fig. F8). The carbonate silt bed at the base of Unit II (Core 178-1101A-16H) produces a trough because of its higher carbonate content compared to the rest of the laminated intervals.
Massive foraminifer-bearing intervals from Unit II represent a significantly different paleoenvironment from that of the laminated facies (see "Biostratigraphy"). Both Mf and Mb are deposited from hemipelagic settling of mud from low-density turbid flows, together with biogenic components. Lower sedimentation rates than in laminated intervals are inferred because of the presence of bottom-dwelling infauna that were able to bioturbate the seafloor sediments completely. Large amounts of ice-rafted debris appear within massive facies because of the increased flux of coarse-grained sediment delivered by iceberg rafting, relative to fine sediment.
Laminated facies in Unit II include Facies C, which suggests a low-energy setting dominated by contour currents alternating with Facies L2, indicating distal low-density turbidity currents. Intervals with laminated facies are most common toward the middle of Unit II (Cores 178-1101A-9H through 11H; Fig. F4). In the magnetic susceptibility data, this corresponds to a 20-m-thick interval from 65 to 85 m of regular short-wavelength L2 peaks separated by troughs corresponding to Mf facies (Fig. F8). The same alternating facies occur above and below this interval, although the repetition is not as regular (Figs. F4, F8). The thicker silt and sand beds are probably distal turbidites deposited at this site by occasional more vigorous turbidity flows.
Unit III is 75 m thick, ranging from dark greenish gray (5GY 4/1) to brown (5GY 5/1), and composed primarily of massive facies with a low biogenic content (Mb; Fig. F5). Unit III has relatively low magnetic susceptibility and comprises three alternating lithofacies: massive and laminated clayey silt, and diamict (Fig. F4). Foraminifers are not present within this unit, and diatom-bearing silty clay occurs in both massive and laminated lithofacies below 198.5 mbsf (from Section 178-1101A-23X-1 to the bottom of the hole [Fig. F4]). The highest sedimentation rates occur within Unit III (see "Paleomagnetism") from 166 mbsf to the bottom of the hole (Fig. F5). Diamict is restricted to an interval between 193 and 202 mbsf (Cores 178-1101A-22X and 23X) interbedded with laminated facies. Pyrite is present in silt laminae and in burrow fills within Unit III. A 6-cm bed of aragonite-cemented silt occurs at 162.7 mbsf. The lithofacies in Unit III are described in detail below.
The upper part of Unit III is structureless clayey silt (Mb) with minimal bioturbation. Ice-rafted debris increases downcore within the massive interval along with the biogenic component (Fig. F5). Massive diatom-bearing silty clay (Md) occurs near the bottom of the hole (Fig. F4).
Laminated Facies C and L2, similar to those in Units I and II, occur within Unit III. Thin, faint lamination, without any repetitive vertical organization (Facies C), is most common (Fig. F12). This facies is burrowed and shows evidence of minor to moderate bioturbation. Below 198 mbsf, thin "green"-colored (5GY 4/1) parallel silt laminae (as much as 3 mm thick) that contain 15% glauconite occur in diatom-bearing silty clay (an example of Facies L2). A 5-cm-thick fine sand bed with sharp upper and lower contacts occurs at 206.5 mbsf (Fig. F13). This bed is thicker and coarser grained than all the other beds, and it produces the highest magnetic susceptibility value for the entire hole (Fig. F4).
Clast-poor diamict (see "Lithostratigraphy" in the "Explanatory Notes" chapter for definition) occurs in beds 15-60 cm thick within Cores 178-1101A-22X and 23X (Fig. F14). These dark greenish gray (5GY 4/1) to dark gray (5Y 4/1) clast-poor beds have gradational contacts with the surrounding laminated or massive facies. There is indication of a faint stratification produced by segregation of clasts into bands within the diamict. Each diamict bed appears to have a similar range of clast size, shape, and origin (mostly volcanic and plutonic). In Core 178-1101A-23X, diamict is interbedded with massive diatom-bearing silty clay containing scattered pebbles, granules, and sand.
The facies in Unit III were deposited by processes that are similar to those within the other two units, but the sediment flux through the water column was much greater at the bottom of Unit III than higher in the hole. Based on the magnetostratigraphy for Site 1101 (see "Paleomagnetism"), the sedimentation rate below 166 mbsf is roughly one-third greater than in the top of Unit III (Fig. F6).
As much as 57 m of sediment in Unit III shows strong glacial influences on deposition. The gradational contacts of the diamict and its low clast content indicate that it was deposited by rainout through the water column of sediment transported by icebergs. Because the magnetostratigraphy at Site 1101 indicates that deposition was the most rapid at this time, the presence of the coarse-grained diamict intervals within otherwise fine-grained sediment must represent an intense period of iceberg rafting, not merely a reduction in the flux of fine-grained sediment. The massive barren facies overlying the diamict beds was also deposited rapidly. It is described as structureless, with thin intervals that are thinly laminated (Facies L2), with generally low magnetic susceptibility values, little evidence of bioturbation, and low biogenic content. For these reasons, Mb facies may have been deposited by different processes from those operating during deposition of the massive facies within Units I and II. One possibility is that Mb originates from deposition from turbid meltwater plumes originating at the grounded glacier terminus on the nearby continental shelf. Similar massive deposits containing ice-rafted debris have been described as plumites and occur on the Labrador Sea continental slope near the outflow of the Laurentide Ice Sheet (Hesse et al., 1997). At the base of the hole, in Core 178-1101A-23X, laminated facies (L1, L2, and C) occur, and diatoms are present. Bottom processes and hemipelagic settling were more important than rainout at this time.
Ice-rafted debris occurs throughout Units I, II, and III at Site 1101 (Fig. F6). It is present as scattered sand grains, granules, isolated pebbles (lonestones), and beds of weakly stratified diamict. Pebble abundance is lowest in Unit I, averaging 0-1 pebble per core (Fig. F6). In Unit II, pebbles increase in abundance downcore (to a maximum of 8 per core) until ~90 mbsf, then decrease in number to the base of Unit II. Although in Unit III the total number of pebbles between 0.5 and 6 cm in diameter is relatively low, the presence of diamict indicates an episode of very high coarse-grained sediment flux to Site 1101 most probably delivered from icebergs during this period. Igneous pebbles with local sources on the Antarctic Peninsula occur most commonly, including volcanic (basalt), volcaniclastic, and intrusive (granite or granodiorite) igneous rocks. One basalt pebble recovered from Section 178-1101A-6H-5 shows striations and a faceted surface, characteristics of subglacial transport (Fig. F15).
Site 1101 is located on the distal portion of Drift 4 described by Rebesco et al. (1996, 1997). This site was influenced by weak bottom currents and appears to have been an area of near-continuous deposition over the period sampled (see "Seismic Stratigraphy"). Site 1101 contains a record of glacial-interglacial climate history on the Antarctic Peninsula over the past 3.1 m.y. Available evidence suggests that the ice sheet on the Antarctic Peninsula is highly responsive to climatic variations (Larter and Barker, 1991b; Bart and Anderson, 1995). Within Units I and II, massive, biogenic-enriched facies and laminated facies appear to alternate in a regular pattern similar to the glacial-interglacial facies described in the Australian-Antarctic Basin during Leg 119 (Ehrmann and Grobe, 1991). By analogy with those sediments, the laminated facies at Site 1101 represents increased downslope sediment transport from the shelf (turbidites), a higher terrigenous sediment flux, and low biogenic productivity that accompany glacial periods. Massive facies, with a high biogenic component (both diatoms and foraminifers), bioturbation, and high amounts of ice-rafted debris represent deposition during interglacial periods when less sediment was transported from the continental shelf and sea ice extent was less, so that marine productivity was high. The extension of this facies interpretation to the Antarctic Peninsula is further supported by identification of warm oxygen isotope Stages 1 and 5 within massive biogenic-rich facies in piston cores from Drift 7 (Pudsey and Camerlenghi, 1998).
Another significant climate signal is the presence of foraminifers within the massive facies of Unit II. Today, calcareous biogenic sediment is deposited north of the Polar Front and siliceous tests dominate sediments south of the Polar Front; foraminifers are common in deep-water sediments only in a few areas such as the southeastern Weddell Sea (Mackensen et al., 1989). Although the factors that influence biogenic accumulation are complex, the record appears to indicate regular glacial-interglacial cycles during the deposition of Unit II that were of a different form from those recorded by Units I and III, above and below, which do not contain regular foraminifer-bearing layers (see "Biostratigraphy").
Unit III does not have biogenic-enriched facies that alternate in a regular pattern downcore. The upper part of Unit III (142.7 to 198.5 mbsf) is mostly massive clayey silt with a low biogenic content, together with thin beds of diamict. This interval coincides with the highest sedimentation rates in the core (see "Paleomagnetism") and appears to be strongly glacially influenced, with intense rainout from icebergs depositing the diamict facies. Multichannel seismic profiles show the thickness of Unit III increasing landward, which supports a continental shelf or slope source for these sediments (see "Seismic Stratigraphy"). The massive clayey silt in Unit III may have been rapidly deposited from turbid meltwater plumes (plumites). At the base of Unit III, both massive and laminated facies are diatom-bearing, indicating a less direct glacial influence. Within the laminated facies at 206.5 mbsf, an 8-cm-thick sand bed occurs that is interpreted as a turbidite.