Holes 1096A, 1096B, and 1096C penetrated to a maximum depth of 607.7 mbsf, recovering predominantly fine-grained and terrigenous sediments. They can be divided into three lithostratigraphic units (I, II, and III; Table T2; Fig. F5), although further subdivision of Unit III may ultimately be justified.
Unit I (0-32.8 mbsf) consists of laminated and massive, commonly intensely bioturbated, diatom-bearing silty clays. Sedimentation rates averaged 7 cm/k.y. The top of Unit II is placed at the first appearance downcore of common parallel-laminated silt and mud turbidites. Unit II is partly a turbidite succession ~140 m thick (32.8-173.0 mbsf), with generally low biogenic content. Sedimentation rates averaged 9 cm/k.y. Although the turbidite silts are thin and subordinate to muds, two coarsening-upward sequences may be discerned in the upper part of Unit II. A massive, well-sorted sand turbidite occurs at 112 mbsf. Unit III extends from 173 mbsf to the bottom of the hole at 607.7 mbsf. It is composed of (1) very thinly laminated and generally nonbioturbated clays deposited from muddy turbidity currents, and (2) intensely bioturbated homogenous silty clays. Overall, Unit III has a sedimentation rate of 18 cm/k.y., although variations in deposition rates may be recorded by intervals of intense bioturbation with abundant ice-rafted debris.
In Units II and III, the alternation of laminated turbidite facies and bioturbated hemipelagic facies records cyclic fluctuations in sediment supply and transport processes. Some of these fluctuations may be related to glacial-interglacial (Milankovich) cycles along the Antarctic Peninsula margin similar to those identified within Unit I, but longer period cycles also exist.
Interval: Cores 178-1096A-1H through 4H; Core 178-1096B-1H through Section 5H-1, 50 cm
Age: Holocene to late Pleistocene (0.0-0.48 Ma)
Depth: 0-32.8 mbsf
Unit I consists of laminated and massive, commonly intensely bioturbated, clay and diatom-bearing silty clay. Weak bottom (contour) current activity influenced depositional patterns, although much fine-grained sediment was probably delivered from the margin by turbidity currents. Sedimentation rates averaged 7 cm/k.y. (see "Sedimentation Rates"). Unit I shows a well-defined alternation of biogenic-rich and terrigenous horizons that are the stratigraphic expression of late Quaternary glacial cycles.
Three facies are recognized in Unit I: two massive facies (diatom rich and diatom poor) and one laminated facies consisting mainly of clay with very rare silts.
Facies Md (massive, diatomaceous), comprising diatomaceous silty clay and diatom-bearing silty clay, occurs at the core top and in three units downcore (Fig. F6). It is dark grayish brown to brown and olive brown (10YR 4/2, 4/3, 5/3, 6/4; 2.5Y 4/2) and is intensely mottled by burrows, with original depositional structures having been obliterated (Fig. F7). Diatoms are well preserved, and foraminifers, radiolarians, and calcareous nannofossils are also present. Scattered sand grains and granules occur. This facies corresponds to high values of the chromaticity parameters a* and b* and to low magnetic susceptibility (Fig. F6). There is a thin (1 cm) vitric ash at 8.83 mbsf at the base of one interval of Facies Md (178-1096B-2H-4, 53 cm).
Facies Mb (massive, barren) consists of silty clay containing few, poorly preserved diatoms. Bioturbation is common to rare, and colors are generally lighter than Facies Md (grayish brown, pale brown, and olive gray: 2.5Y 5/2, 10 YR 6/3, 5Y 5/2), locally darkening to dark gray (5Y 4/1). Very faint darker and lighter bands are the only primary structures seen, where bioturbation is less intense. Sand grains and granules occur in centimeter-thick burrowed layers, and also dispersed in layers up to 3 m thick.
Facies L (laminated) is subordinate to the massive facies (Md and Mb), occurring as one thick interval from 0.6 to 6.0 mbsf and three thinner intervals downcore (Fig. F6). These consist of clay (20% silt, estimated from smear slides), with only a trace of biogenic material in the form of fragmented diatoms and foraminifers. Facies L is faintly color banded in shades of gray and dark gray (5Y 5/1, 5Y 4/1, N 4), becoming greener downcore to greenish gray (5GY 5/1) in Core 178-1096B-4H. Darker gray bands several centimeters thick are internally finely laminated with a few small bedding-parallel burrows (Fig. F8). Facies L has generally low values of chromaticity parameters a* and b* and high but variable magnetic susceptibility.
In the lowest 10 m of Unit I, the laminated facies includes four thin (5-10 cm) packets of interlaminated silt and clay. Although silt forms only 20%-30% of these packets, each is clearly visible as a trough in magnetic susceptibility (Fig. F6). The laminae in the packets at 23.7 and 24.6 mbsf are disrupted by minor bioturbation; in the lower two packets, the laminae have sharp bases and tops and form small thinning-upward sequences.
Unit I sediments are fine grained and record deposition mainly from hemipelagic settling in a regime of weak bottom currents. The rare thin packets of beds with sharp-based, parallel-laminated silts are interpreted as distal turbidites (compare Facies L2, below). The more common clays showing weak, diffuse parallel lamination are interpreted as the deposits of bottom currents (contourites; similar to Facies C below). The intermixing of contourites and turbidites indicates a setting influenced mainly by weak bottom-current transport of fine sediment. Occasionally, dilute turbidity currents reached the site and deposited the silts. Dispersed sand grains and granules were transported to the area by ice rafting. The massive diatomaceous facies records periods of seasonally open water allowing diatom productivity; these periods may represent warm oxygen isotope Stages 1 (seafloor sediment = Holocene), 5e, 7 (occurrence of Hemidiscus karstenii; see "Biostratigraphy"), and 9. Stage 11 and additional warm stages down to the Brunhes/Matuyama boundary at 55 mbsf (see "Paleomagnetism") are not represented by diatom-bearing layers.
Interval: Cores 178-1096A-5H through 13H; Section 178-1096B-5H-1, 50 cm, through Core 22X; Core 178-1096C-1H through Section 2H-4
Age: late Pleistocene to late Pliocene (0.48-2.1 Ma)
Depth: 32.8-173.0 mbsf
Unit II is a partly turbiditic succession 140 m thick. The top of Unit II is placed at 32.8 mbsf at the first appearance downcore of common parallel-laminated silt and mud couplets (Core 178-1096B-5H-1, 50 cm). This corresponds to a downcore increase in high-frequency variability of magnetic susceptibility (Fig. F6). Although the turbidite silt beds are thin and subordinate to muds, two coarsening-upward trends (first-order cycles) may be discerned in the upper part of Unit II. A massive, well-sorted sand turbidite bed occurs at 112 mbsf.
The sediments can be divided into four facies that record regimes where mud was deposited by different processes. Two related laminated facies are characterized by a well-developed repetitive internal organization of sedimentary structures. The first, L3 (we retain the numbering scheme of Site 1095 where the same facies were seen), consists of thinly laminated silty clays. The second and less common, L2, shows sharp-based, parallel-laminated silts and laminated silty clays.
The other two facies show no consistent internal organization of structures and occur in beds that are poorly defined and commonly moderately to intensely bioturbated. Facies C (not seen at Site 1095) comprises faintly laminated bioturbated silty clays with starved ripples; Facies M consists of massive silty clay, commonly with foraminifers or diatoms. Ice-rafted debris is present throughout Unit II but is most abundant in Facies M.
Facies L2 is distinguished by the presence of sharp-based, commonly graded silt laminae. Silts are 1-12 mm thick, and gray, dark gray, or very dark gray (5Y 5/1, 4/1, 3/1). The base of silt laminae are generally sharp, although thicker laminae may have slightly erosional bases. Internally, the thicker silts (>3 mm) show parallel lamination or slightly wavy lamination. Bed tops are sharp or gradational into overlying laminated or massive silty clay (Figs. F9, F10). The silty clay is gray to dark gray (5Y 5/1, 4/1; N 4) and faintly parallel laminated. Bioturbation is not extensive and is limited to small Chondrites and Planolites burrows, belonging to the deep-water bathyal Nereites ichnofacies assemblage. Facies L2 accounts for ~9% of the thickness of Unit II. It occurs either as isolated silt laminae grading up into laminated mud or as packets 10-60 cm thick of interlaminated to thinly interbedded silt and silty clay (Fig. F10). Some of these packets are thinning-upward sequences (e.g., in Core 178-1096A-5H), with silts typically 10 mm thick at the base and 1 mm thick at the top; silt forms 10%-20% of each packet. From 70 to 101 mbsf, there are packets of silt laminae partly cemented by aragonite (Fig. F10). Ice-rafted debris is rare in Facies L2, although isolated pebbles occur.
This facies is distinguished by the presence of well-defined lamination and only minor bioturbation. It consists of thinly laminated (millimeter scale) and laminated (<1 cm) silty clay, with thin bioturbated layers at bed tops (Fig. F9). The lamination is visible as subtle color changes or as partings within drilling biscuits. Small-scale color banding is common, and upward transitions from dark to light hues in the silty clays suggest normal grading in grain size. Greenish gray to dark greenish gray (5GY 5/1, 4/1) colors are characteristic. Bioturbation is not extensive in this facies and is limited to small Chondrites and Planolites burrows, belonging to the deep-water bathyal Nereites ichnofacies assemblage. This facies accounts for ~40% of Unit II. Ice-rafted debris is rare in Facies L3, although isolated pebbles and lenses of sand grains and granules occur.
Facies C is distinguished by faint lamination (generally visible as diffuse color banding) and minor to moderate bioturbation, together with very thin discontinuous silt laminae that occur in isolation instead of in packets. Facies C accounts for <5% of the total stratigraphic thickness of Unit II and occurs as intervals up to 40 cm thick, which are transitional from underlying hemipelagic intervals (TE3-H) of Facies L2 and L3, rather than as discrete, well-defined beds (Fig. F11). Facies C shows a weak lamination, commonly wavy, with isolated (starved) silty ripples, irregular and discontinuous silt laminae (Fig. F12), and variable bioturbation by Planolites, Chondrites, and Zoophycos ichnofauna. A distinctive color mottling caused by bioturbation and mixing of dark greenish gray, locally glauconitic clay (5GY 4/1) and dark gray (5Y 4/1) silt laminae with prominent gray burrows is also observed. Ice-rafted debris occurs locally, in the form of dispersed sand grains and granules, as well as isolated pebbles.
Facies M is distinguished by moderate to intense bioturbation that has obliterated most primary sedimentary structures. It occurs in intervals up to 6 m thick (more commonly 0.5-2 m thick) and accounts for ~45% of the thickness of Unit II. The base of Facies M is usually poorly defined as a result of mixing by bioturbation and typically shows large (1 cm) Planolites burrows; interval tops are commonly the sharp bases of overlying Facies L2 silts. Colors include gray, dark gray, and dark greenish gray (N 5/0, N 4/0, 5Y 4/1, 5GY 4/1). Facies M includes silty clay and diatom-bearing silty clay; diatom preservation is poor to moderate. Most intervals are homogeneous, but the greener layers show very faint color banding on a 5- to 15-cm scale. Color changes are gradational and burrow mottles (Planolites and Zoophycos) are easiest to observe where there are color contrasts (Fig. F13). Ten thin (0.2-1.0 m) intervals of foraminifer-bearing to foraminiferal silty clay occur between 48 and 123 mbsf (shown in blue in Fig. F5, illustrated in Fig. F13; see "Organic Geochemistry"). Ice-rafted debris is common in Facies M and includes scattered sand grains and angular granules, a few thin (1-2 cm) sandy layers, and isolated pebbles as much as 2-3 cm in diameter.
In addition to these four facies, Unit II contains a single bed of massive sand, 68 cm thick, at 112 mbsf (178-1096B-13H-2, 147 cm, to 13H-3, 62 cm; Fig. F14). The sand is well sorted and medium grained, with subangular to subrounded grains. The base and top are sharp, and the bed shows no internal sedimentary structures. It is a quartz-lithic sand, and most of the lithic grains are low-grade metasediments.
In the upper part of Unit II (33-110 mbsf), the occurrence of silt-based turbidites (Facies L2) delineates two coarsening-upward sequences, shown as thin arrows in Figure F5. In the lower sequence, Facies L2 forms 4% of the sediment in Core 178-1096B-13H, 9% in Core 178-1096B-12H, and 16% in Core 178-1096B-11H. Core 178-1096D-10H, the base of the upper sequence, contains no silt beds; Facies L2 forms 11% of the sediment in Cores 178-1096B-9H, 8H, and 7H, 15% in Core 178-1096B-6H, and 31% in Core 178-1096B-5H.
Laminated silts and silty clays similar to those of Unit II (Facies L2, L3) are described in the literature as "parallel silt-laminated muds," "mud turbidites," and "thin-bedded turbidites" (see reviews by Stow and Piper, 1984; Stow and Wetzel, 1990). These facies are characteristic of deep-marine depositional environments dominated by muddy sediment gravity flows (e.g., Pickering et al., 1988; Alonso and Maldonado, 1990) and are interpreted as being those recognized at Site 1095 (see "Lithostratigraphy" in the "Explanatory Notes" chapter).
Facies C and M clearly record very different depositional regimes from the turbidite Facies L2 and L3. The lack of clear lamination and of well-defined bedding sequences, the occurrence of dispersed ice-rafted debris (sand grains and granules), and more intense bioturbation are distinguishing characteristics. Facies C probably records deposition of hemipelagic mud from weak bottom currents and is transitional upward from the TE3-H hemipelagic division of Facies L2 and L3. Rare "starved" silt ripples and discontinuous silt laminae indicate limited current reworking. Current velocities were insufficient to generate significant size sorting, continuous silt laminae, or ripple cross-lamination. This is consistent with a regime dominated by hemipelagic settling of mud in an area affected by weak bottom currents. Facies C is therefore interpreted as contourites. Commonly, a waning low-density turbidity current supplies suspended fine sediment to the lower water column (benthic nepheloid layer), where it is redistributed by bottom currents. Such processes are well described by Stow and Wetzel (1990). Contourite facies typically show pervasive bioturbation (e.g., Gonthier et al., 1984). However, some deposits attributed to muddy contourites at Site 1096 are not uniformly intensely bioturbated. This can be attributed to a high input of hemipelagic mud.
Facies M is closely related to Facies C and results from hemipelagic settling of mud from the benthic nepheloid layer. The nepheloid layer can be supplied with suspended sediment by low-density turbidity currents and by biogenic components settling from the sea surface. Supply of fine sediment from sinking glacial meltwater plumes is another possibility because the continental shelf edge (the nearest possible position of the grounded ice margin) is ~100 km from the site. Facies M differs from C in being almost structureless as a result of intense bioturbation, which suggests deposition rates low enough to allow the benthic fauna to rework seafloor sediments. A high proportion of coarse ice-rafted debris in this facies is also consistent with a reduction in the supply of fine-grained sediment (although it may also record a greater influx of ice-rafted debris).
In the massive sand bed, the absence of sedimentary structures such as cross-stratification may rule out deposition from a traction current (i.e., a strong bottom current). Beds of massive sand in deep-sea deposits may also be interpreted as sediment gravity flows, and a wide variety of terms has been employed (fluxoturbidite, grain flow, fluidized flow, liquefied flow, and hyperconcentrated flow).
Interval: Cores 178-1096B-23X through 32X; Section 178-1096C-2H-5 through Core 42X
Age: late Pliocene to early Pliocene (2.1-4.7 Ma)
Depth: 173.0-607.7 mbsf
Unit III extends from 173 mbsf to the bottom of the hole and is dominantly fine grained. It is composed of thinly laminated and generally nonbioturbated silty clays (Facies L3) and intensely bioturbated homogenous silty clays (Facies M). Diatom content increases downward. The sedimentation rate of 18 cm/k.y. is twice as high as in Unit II. Intervals of intense bioturbation with abundant ice-rafted debris may record fluctuations in deposition rates. Cyclicity in sedimentation is present on various scales.
Unit III is mainly composed of two of the facies previously described, L3 (laminated silty clay) and M (massive, bioturbated silty clay). Silty turbidite facies (L2) and fine-grained contourite facies (C) are extremely rare. The contact between Units II and III occurs at 173 mbsf in Section 178-1096C-2H-4, 150 cm, at the top of a very thick interval of Facies M (Fig. F5). This stratigraphic level also coincides with a downward increase in the siliceous biogenic component (Fig. F5) and marked changes in physical properties of the sediments (see "Physical Properties"), which confirms a change in depositional style between Units II and III.
Facies L3 lamination is seen very clearly within drilling biscuits (Fig. F15). Biogenic content increases downcore. Facies M in Unit III shows some differences from Unit II and thus is described separately. It is distinguished by moderate to intense bioturbation that has obliterated most primary sedimentary structures. It occurs in intervals 0.5-30 m thick (generally <8 m thick) and constitutes 52% of the recovered thickness of Unit III. The biogenic content increases downcore from silty clay through diatom-bearing silty clay to diatom silty clay, with some muddy diatom ooze intervals below 450 mbsf (Fig. F5). Foraminifers are absent. In the upper part of Unit III (above 225 mbsf), Facies M shows diffuse color banding (gray to dark gray and greenish gray; 5Y 5/1, 4/1, 5GY 5/1). Elsewhere, Facies M is dark greenish gray (5G 4/1) and appears conspicuously green compared with the dark greenish gray (5GY 4/1) of surrounding Facies L3. Ice-rafted debris in the form of scattered sand grains and angular granules is very common in Facies M (Fig. F16). It occurs throughout some intervals but in others is concentrated in decimeter- to meter-thick layers.
Massive and laminated facies alternate downcore on a 0.3- to 20-m scale (Fig. F5). Because of incomplete core recovery, it is not known whether the 30 m of Facies M at the top of Unit III and the 55 m of Facies L3 from 395 to 450 mbsf are continuous. Facies M occurs from 173 to 204 mbsf at the top of Unit III and is then very subordinate to Facies L3 until 450 mbsf. From 450 to 524 mbsf, Facies M forms nearly half the cored thickness; there is a clear correspondence between sediment facies, spectral reflectance, and magnetic susceptibility in this interval (Fig. F5). Texturally, Unit III is quite uniform downcore as shown by sand:silt:clay ratios estimated from smear slides (Fig. F17). Below 524 mbsf, the alternations of Facies M and L3 are generally thin (0.3-5 m).
The massive facies generally has lower magnetic susceptibility than the laminated facies (Fig. F5), perhaps because of dilution of ferromagnetic minerals by biogenic silica. The magnetic susceptibility curve also exhibits a low-frequency cyclicity of ~1 cycle per 40 m (~200 ka; see "Physical Properties"). The high-frequency variability in magnetic susceptibility cannot be related to any visible features in the cores, in contrast to Unit I.
As described above for Unit II, the laminated Facies L3 can be attributed to distal turbidite flows. The massive Facies M records a setting dominated by hemipelagic deposition and ice rafting. Although more intense bioturbation in Facies M would suggest a lower deposition rate of mud allowing complete sediment reworking by the benthos, there appears to be no significant difference in high sedimentation rate (18 cm/k.y.; see "Sedimentation Rates") between the parts of Unit III dominated by Facies L3 and the part below 450 mbsf, where Facies L3 and M alternate.
Ice-rafted debris occurs throughout Units I, II, and III at Site 1096 (Fig. F18). It is present as scattered sand grains and granules, as isolated pebbles (lonestones), and as lenses of granules and sand. An average of 1.9 pebbles (0.5-6 cm in diameter) per core was observed on split-core surfaces from Holes 1096A, 1096B, and 1096C. Pebble abundance is lowest in Unit I, averaging 0-2 pebbles per core (Fig. F18). In Unit II, ice-rafted pebbles reach a peak in abundance at 112-114 mbsf. Unit III appears to have a cyclic trend in pebble abundance that peaks several times downhole. To understand the flux in ice-rafted debris at this site through time, it is necessary to quantify the contribution of the sand and granule ice-rafted debris fraction and to compare ice-rafted debris concentrations with sedimentation rates and facies. Ice-rafted debris lithologies are primarily volcanic (basalt and rhyolite), volcaniclastic, and intrusive igneous (granite and granodiorite), with smaller contributions of metamorphic (granitic gneiss) and sedimentary rocks (siltstone). Most or all rock types have local sources on the Antarctic Peninsula.
The concentration of ice-rafted debris at Site 1096 appears to be more than twice as high as at Site 1095. The total accumulation rate is three times higher at Site 1096 than 1095, even further increasing the difference in ice-rafted debris between Sites 1095 and 1096. A detailed study of ice-rafted debris at both Sites 1095 and 1096 should yield important information about the iceberg flux from the continental shelf over time.
The elevated position of Site 1096 (water depth of 3152.5 m) near the crest of Drift 7, ~600 m above the channel to the southwest and 380 m above the smaller channel to the southeast, indicates that turbidity currents derived from the continental margin may bypass the site. This possibly has been the case throughout the drift-maintenance stage, as a considerable difference in height between paleodrift crest and paleochannel floor has been maintained (Rebesco et al., 1996). Probably the finest fractions of suspended flows were available for redistribution over the drift by gentle bottom currents. In the long term, the occurrence of the laminated (turbidites) and massive (hemipelagic) facies may be related to the history of channel development on either side of the drift (see "Seismic Stratigraphy"). The southwestern channel is a very long-lived feature and has existed throughout the 4.7 m.y. represented by Site 1096 sediments. Although there is a deeply buried channel northeast of the drift, the northeastern channel in its present position appears to be younger. Reflectors at 400 ms TWT below the seafloor (320 mbsf and deeper) can be traced beneath it, which suggests that it did not exist for much of Unit III time. The young channel margins truncate reflectors at 200 ms TWT below the seafloor (i.e., 155 mbsf), which is in the lower part of Unit II. The occurrence of distal turbidites in Unit II may be related to reactivation of the northeastern channel, which in turn may be a reflection of greater sediment supply from the margin related to climatic changes. The possibility of a local sedimentological, not a regional, climatic explanation for changes in sedimentation, as here, was part of the rationale for sampling a different sediment drift (see "Lithostratigraphy" in the "Site 1101" chapter).
The upward decrease and disappearance of silt-based turbidites from Unit II to Unit I could have resulted from a reduction in sediment supply in the late Pleistocene. The alternation of hemipelagic facies (C and M) with turbidite facies (L2 and L3) at Site 1096 could be the stratigraphic "overprint" of glacial-interglacial climate cycles along the Antarctic Peninsula. In Unit III, the sedimentation rate appears to be uniformly high (see "Sedimentation Rates"), and the reasons for the apparent changes in cycle length (L3/M alternations varying from meters to tens of meters; Fig. F5) are not yet completely understood. Very thick (tens of meters) cycles probably do not bear a simple relationship to glacial-interglacial cycles; however, the interval from 525 to 560 mbsf has eight cycles (i.e., 1/44 k.y.), close to the obliquity period of 41 k.y.
Cyclic, climatically driven changes of sedimentary facies are well known along the Antarctic continental margin. Ehrmann and Grobe (1991) and Ehrmann et al. (1991) described significant changes in ice-rafted debris content, grain size, and biogenic components from glacials to interglacials at Sites 745 and 746 in the Australian-Antarctic Basin drilled during Ocean Drilling Program (ODP) Leg 119 (Barron, Larsen, et al., 1991). They recognized two facies. Facies A (interglacial) had a high content of siliceous microfossils and a relatively low content of fine terrigenous sediment and ice-rafted debris. Facies B (glacial) was distinguished by a smaller biogenic silica content and enhanced terrigenous sedimentation. Here, probably because of proximity to the continental shelf edge, the direct terrigenous sediment supply (Facies L) is dominant during glacials, leading to a greater ice-rafted debris component within the much more slowly deposited interglacial sediments (Facies C and M).