Site 1094 (proposed site TSO-7C) was the highest latitude site drilled during Leg 177 and represents the southernmost anchor of sites drilled along a north-south transect across the Antarctic Circumpolar Current (ACC) (Fig. F1 in the "Leg Summary" chapter). The site is located in the center of the ice-free Antarctic Zone bounded to the north by the Polar Front (PF) and to the south by the Weddell Gyre/ACC boundary, which coincides approximately with the present average winter sea-ice edge. The site is presently north of the winter sea-ice edge, but the presence of sea-ice diagnostic diatoms indicates that Site 1094 was covered by sea ice during the last Ice Age. The water depth of 2807 m places the site within the core of Circumpolar Deep Water (CDW) (Fig. F2 in the "Leg Summary" chapter).
Site 1094 is located in a small sedimentary basin north/northeast of Bouvet Island. A multichannel seismic line (AWI 94080) shows a thick package of acoustically stratified sediment with basement estimated at ~1100 meters below seafloor (mbsf) on the basis of interval velocities (Figs. F1, F2; Table T1). The site is close to the Bouvet Fracture Zone, and the basement age cannot be determined precisely from magnetic anomalies in the area (Fig. F6 in the "Leg Summary" chapter). If the results of Cande and Kent (1992) are extrapolated to the area of Site 1094, basement age would be late Miocene.
Parasound echosounder profiles near Site 1094 indicate the presence of high-amplitude reflectors at ~66 and 105 mbsf (Fig. F3), which are associated with porcellanite in nearby piston cores (PS2089-1, 2) collected to the east of Site 1094 (Fig. F4). The porcellanite is found in sediments belonging to marine isotope Stage (MIS) 11, suggesting a very young formation age (~400 ka; Bohrmann et al., 1994). On the basis of the porcellanite reflector at 66 mbsf (Fig. F3), we deduced an average sedimentation rate of 170 m/m.y. The presence of porcellanites at Site 1094 will permit geochemical studies of low-temperature silica diagenesis by studying sediments and interstitial waters. The porcellanite beds in the sediment cores are only a few centimeters thick and, thus, were not expected to impede penetration of the advanced hydraulic piston corer (APC).
Sediment cores (PS2089-1, 2, PS2090-1, and TTN057-13-PC4) collected in the vicinity of Site 1094 with a piston corer are composed of diatom ooze and diatom mud, but foraminifers are sufficiently abundant to permit the development of stable isotopic stratigraphies. Variations in magnetic susceptibility and natural gamma radiation (NGR) show strong contrasts between high values in glacial sediments and low values in interglacial sediments, which is useful for identifying glacial-interglacial cycles downhole.
The purpose of Site 1094 was to obtain a high-resolution record of biosiliceous sediments south of the present-day position of the PF. Site 1094 (53šS) represents the southernmost anchor of high-resolution sites across the ACC, including Sites 1089 (41šS), 1091 (47šS), and 1093 (50šS) (Fig. F15 in the "Leg Summary" chapter). The paleoclimatic record from this site will be used to study rapid climate change on suborbital time scales and will be correlated to similar high-resolution climate records from other marine cores, as well as to climatic signals from Antarctic and Greenland ice cores. Specific objectives include the reconstruction of (1) surface-water parameters (sea-surface temperatures, water chemistry) south of the present Polar Front Zone; (2) sea-ice distribution in the Southern Ocean during Pleistocene glacial-interglacial cycles; (3) paleoproductivity changes (e.g., biogenic opal, organic carbon export rates) in relation to surface-water mass changes and sea-ice distribution; (4) deep-water circulation, including changes in the physical and chemical properties of CDW; and (5) early low-temperature silica diagenesis.
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