Site 1093 (proposed site TSO-6A) is located north of Shona Ridge near the present-day position of the Polar Front (PF) but north of the average winter sea-ice edge (Fig. F1 in the "Leg Summary" chapter). This area also represents the southernmost extent of North Atlantic Deep Water as it mixes with Circumpolar Deep Water (CDW) and inserts itself into the Antarctic Circumpolar Current (Fig. F2 in the "Leg Summary" chapter). The water depth of 3626 m places the site within lower CDW. The region is marked by thick, moderately laminated pelagic drift deposits (Tucholke and Embley, 1984) that accumulated at very high sedimentation rates within the circum-Antarctic opal belt (Fig. F9 in the "Leg Summary" chapter). The high flux of biosiliceous material to the seafloor in this region offers an unprecedented opportunity for the study of climate change at high temporal resolution. Moreover, Site 1093 is located near a sediment trap station (Mooring PF) that has monitored vertical particle fluxes to the seafloor since 1987 (Abelmann and Gersonde, 1991). This region is also known for laminated diatom mats that accumulated rapidly at very high sedimentation rates and may be important for studying past changes in the budget of dissolved silica in the global ocean. At Site 1093 cores were recovered for the first time from the circum-Antarctic opal belt using modern techniques (i.e., multiple advanced hydraulic piston corer [APC] holes and construction of a composite section).
A multichannel seismic line (AWI94090) shot across Site 1093 during Polarstern Cruise ANT-XI/3 shows a thick sequence of acoustically laminated sediments (Figs. F1, F2). Acoustic basement is estimated at 1100 meters below seafloor (mbsf) on the basis of interval velocities (Table T1). Basement is very smooth in the region of Site 1093 and may have formed by lava flows from the Shona Hotspot, which is presently thought to reside between 50º and 52.5ºS near an anomalously shallow segment of the Mid-Atlantic Ridge (Hartnady and le Roex, 1985; Douglass et al., 1995). Sediment cores PS1654-2 and TTN057-17-PC1, recovered during two site-survey cruises with a piston corer near Site 1093 (Fig. F1), consist of upper Quaternary diatom ooze and diatom mud that were deposited at very high sedimentation rates (800 m/m.y.) during the Holocene. These rates decreased greatly, however, during glacial periods, when diatoms indicate that this region was covered by sea ice. Foraminifers are present throughout the cores and will be used to establish stable isotopic stratigraphies.
The drilling strategy at Site 1093 was to recover the entire Pleistocene section to the depth of APC refusal and to drill a single deep hole to obtain the older, upper Cenozoic sedimentary record. Specific objectives include the study of (1) the evolution of surface-water masses and the PF, (2) sea-ice distribution in the Southern Ocean, (3) paleoproductivity changes (e.g., biogenic silica, organic carbon export rates) and the history of the circum-Antarctic opal belt, (4) deep-water circulation including changes in the physical and chemical properties of CDW, and (5) silica diagenesis (see also "Site 1094" chapter).
Site 1093 forms part of a north-south transect of sections at 41ºS (Site 1089), 47ºS (Site 1091), 50ºS (Site 1093), and 53ºS (Site 1094) that accumulated at sedimentation rates exceeding 100 m/m.y. during the late Pliocene-Pleistocene (Fig. F15 in the "Leg Summary" chapter). These sites will be used to reconstruct past movements of the PF and Antarctic sea-ice field and will permit correlation of millennial scale climate oscillations recorded in the marine sediments of the Southern Ocean with climate signals in ice cores from Greenland and Antarctica.
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