LITHOSTRATIGRAPHY

Overview

Site 1094 was drilled to a TD of 171 mbsf. Sediments from this site are of Pleistocene age and predominantly consist of olive gray to gray mud-bearing diatom ooze, with minor and varying amounts of foraminifers and nannofossils (Figs. F5, F6). Smear-slide analysis (see "Site 1094 Smear Slides") shows that diatom abundance varies from 60% to 100%, total biogenic carbonate components (foraminifers + nanno-fossils) range from 0% to 40%, and mud ranges from 0% to 20% (Fig. F7). Total carbonate contents estimated by smear-slide analysis are somewhat higher, by ~0% to 30%, than those determined by coulometry (Fig. F7). X-ray diffraction (XRD) results (Table T2, also in ASCII format in the TABLES directory) indicate that opal ranges from 50 to 95 wt% and siliciclastics range from 5 to 60 wt% (Fig. F7).

Biogenic calcareous components (foraminifers + nannofossils) show an increase below 95 mcd, diatom abundance decreases below this point, and mud abundance remains fairly constant throughout. Several carbonate-bearing intervals are present within the sedimentary column (Fig. F5). Those with the highest carbonate content are thin, pink- or salmon-colored intervals that contain both foraminifers and nannofossils with a combined abundance of 20% to 30% and a maximum abundance of 45%. Spacing between the carbonate-bearing intervals decreases and the thickness of the intervals increases below 95 mbsf. This increase in carbonate below 95 mbsf marks the boundary between two lithostratigraphic subunits.

Dispersed sand- to gravel-sized volcanic material is a minor but ubiquitous component throughout the section, particularly in dark-colored, mud-rich intervals. Although diatoms are the dominant component at this site, pure laminated diatom oozes, which were abundant at Sites 1093 and 1091 situated further north in the circum-Antarctic opal belt, are less common at this site (Fig. F5). Several intervals of fragmented diatom-rich laminae are observed, possibly remnants of a laminated diatom ooze subjected to bioturbation. Fragments of porcellanite were recovered, indicating porcellanite layers at 68, 104, and 164 mbsf (Table T3). At 142 mbsf in Hole 1094A, a porcellanite concretion was found that displays concentric "growth" rings (Fig. F8). XRD analyses reveal an opal-CT composition of the porcellanite concretions (Fig. F9).

Description of Lithostratigraphic Unit

The lithostratigraphic characteristics of the sediments are defined primarily on the basis of visual core descriptions and sediment smear-slide analyses. Additional information is obtained from diffuse spectral reflectance measurements, XRD analyses for opal abundance, and calcium carbonate contents measured by coulometry. One lithostratigraphic unit that comprises two subunits was defined.

Unit I

Subunit IA

Intervals: 177-1094A-1H through 11H (0-98.0 mbsf; 0-100.83 meters composite depth [mcd]); 177-1094B-1H through 4H (0-33.2 mbsf; 0.52-36.05 mcd); 177-1094C-1H through 8H (0-67.2 mbsf; 0.28-69.52 mcd); 177-1094D-2H through 8H (28.6-93.2 mbsf; 32.25-96.55 mcd). Note: in Hole 1094D, the interval 0-28.6 mbsf was drilled ("washed") without coring.

Age: Holocene to Pleistocene

The upper 95 mbsf of sediment in Hole 1094A consists primarily of olive gray diatom ooze containing significant amounts of mud and minor amounts of carbonate. In the upper ~6 mbsf, the diatom ooze is quite soupy as a result of very high water content (see "Physical Properties"). Thin, pink- or salmon-colored intervals of carbonate-bearing to calcareous diatom ooze are present throughout Subunit IA. These intervals contain both nannofossils and foraminifers, with nannofossils showing higher relative abundances.

A 3-cm-thick vitric volcanic ash layer at 66.9 mbsf in Hole 1094A exhibits no size grading and is not scoured at its base (Fig. F10). Rare occurrences of concentrated volcanic glass were also observed as burrow fills. Dispersed sand to gravel-sized volcanic material is present throughout the subunit. Its coarse grain size suggests ice rafting by icebergs and/or sea ice as a likely mode of transport to the site. The volcanic origin and the apparent lack of coarse-grained crystalline rock fragments suggest a nearby volcanic island (Bouvet) or the Scotia Arc as the primary source area rather than the Antarctic continent.

A porcellanite layer at 68 mbsf in Hole 1094A (Fig. F11) indicates low-temperature silica diagenesis (Bohrmann et al., 1994). As a result of the fragmented nature of the recovered porcellanite and disturbance during core splitting, the position and thickness of this layer is difficult to ascertain. Features unique to Subunit IA are large Echiurid-type burrows similar to those reported from ODP Site 846 in the eastern equatorial Pacific. Several of the large burrows (some >40 cm in length) are found in all four holes, but most extensively in Hole 1094A (8-45 mbsf) (Fig. F12; Table T4). Though not abundant or extensive, laminated diatom ooze composed predominantly of Thalassiothrix sp. is present throughout the upper portion of the subunit and, except for isolated fragments of laminated diatom ooze, is absent in the lower portion of the subunit below ~52 mbsf.

Subunit IB

Intervals: 177-1094A-11H through 18H (98.0-157.6 mbsf; 100.83-160.11 mcd); 177-1094D-9H through 16H (93.2-171.1 mbsf; 96.55-169.75 mcd)

Age: Pleistocene

Below 95 mbsf in Hole 1094A, the sediments are predominantly gray carbonate-bearing diatom ooze. Both nannofossils and foraminifers show higher abundances in Subunit IB relative to Subunit IA. The increase in total biogenic carbonate content, however, is primarily a result of an increase in the abundance of foraminifers (from typically 10% in Subunit IA to typically 10%-25% in Subunit IB), whereas nannofossil abundance shows only a minor increase. In addition to the general downhole increase in foraminifer abundance throughout this subunit, the thin, pink- or salmon-colored carbonate-rich intervals seen in Subunit IA are present throughout Subunit IB with greater frequency and increasing thickness. In Hole 1094A, a second and third porcellanite interval are present at 104 and 142 mbsf, respectively. Intact, laminated diatom ooze is present only rarely within Subunit IB, and even fragmented laminated diatom ooze is sparse. As in Subunit IA, small volcanic dropstones are observed throughout.

Interpretation

The alternation of diatom ooze and carbonate-rich intervals may reflect lithologic changes corresponding to glacial-interglacial cycles. In general, darker diatom oozes include cold-water diatom assemblages (see "Biostratigraphy") and contain more abundant sand- to gravel-sized dropstones, thus supporting the hypothesis that these sediments accumulated during glacial intervals. The lighter, carbonate-rich layers lack or contain only a few coarse-grained dropstones, suggesting deposition during interglacial intervals. This variable pattern of dropstone occurrence is seen in both Subunit IA and the lower, relatively carbonate-rich Subunit IB.