PROPOSED PRIMARY SITES

Site TSO-2B
Proposed Site TSO-2B is located on the northern Agulhas Ridge in a water depth of 2104 m near the boundary between the upper CPDW and the NADW (Fig. 1 and Fig. 5). The Agulhas Ridge in this region is a narrow feature that trends approximately northeast/southwest, and is covered by a thick (>1000 m) package of sediments. Quaternary and Neogene sediments in the region consist of biogenic calcareous oozes that have sedimentation rates of 1-3 cm/k.y. and are not disturbed by turbidites. We plan to drill an 700-m-thick Cenozoic calcareous sequence at Site TSO-2B to gain information from an upper mid-water depth (2104 m) site that is presently located near the northern boundary of the ACC. Because of the low sedimentation rates at Site TSO-2B, it is doubtful that a complete Pliocene-Pleistocene sequence can be recovered. Consequently, only a single APC hole is planned at this site. This is not a major compromise, however, because surface water signals at this latitude will be captured at Site SubSAT-1B.

The primary objective at Site TSO-2B is to recover a Cenozoic carbonate record that can be used to reconstruct long-term changes in

• surface-water parameters and the evolution of the Subtropical Front (STF) and its response to southern-high latitude climate variability;
• paleoproductivity north of the PFZ;
• the mixing ratio between lower upper CPDW and upper NADW properties and the evolution of these water masses during the Cenozoic; and
• the paleodepth history of the Agulhas Ridge.

Site SubSAT-1B
Proposed Site SubSAT-1B is located on the northern flank of the Agulhas Ridge (Fig. 1) where a thick package of sediment overlies topographically irregular basement consisting of normal oceanic crust. This site was targeted because of the remarkably high-resolution isotopic record of the last climatic cycle obtained from nearby Core RC11-83 (Fig. 7, Charles et al., 1996), which has sedimentation rates of about 20 cm/k.y. Although the site is deep (4622 m), the high rain rate and quick burial of sediment promote the preservation of calcium carbonate, which averages 35% in Core RC11-83. Site SubSAT-1B will be cored by triple APC/XCB to a depth of 300 m to recover a complete Pleistocene section at sedimentation rates of >20 cm/k.y. The primary objective is to recover an expanded Pleistocene section north of the present-day PFZ that can be used to study

• the response of the Southern Ocean to orbital forcing and the phase relationships to climatic events occurring in low and northern high latitudes;
• rapid climate change on suborbital time scales in the Southern Ocean and its relation to climate signals from ice cores recovered from Greenland, Antarctica, and tropical glaciers;
• glacial-to-interglacial variations in the physical and chemical properties of bottom-water masses in the South Atlantic Ocean and their relation to high-latitude climate change; and
• glacial-to-interglacial variations in Southern Ocean productivity, nutrient cycling, and pCO2 and their role in global biogeochemical cycles.
Site TSO-3C
Proposed Site TSO-3C is located in the central part of the Subantarctic Zone on the southern flank of the Agulhas Ridge (Fig. 1). The topography of the ridge is more intricate here compared with its northern component. The water depth at Site TSO-3C is ~3700 m, which is close to the boundary between NADW and underlying lower CPDW (Fig. 2). Site TSO-3C will recover carbonate sediments at sedimentation rates averaging 3 cm/k.y. from the Subantarctic Zone of the ACC. Oxygen isotope stratigraphy of a 13-m piston core (TN057-6) taken at the location of TSO-3C revealed a complete record back to marine isotope Stage 12 (Fig. 8, Hodell et al., in prep.). Together with Sites TSO-2B (2104 m) on the Agulhas Ridge and SubSAT-1C (4620 m) north of the ridge, the sites form a depth transect that intersects most of the major water masses of the South Atlantic Ocean (Fig. 2). Site TSO-3C will be cored by APC to a depth of 200 m, which should penetrate to the late Miocene (~7 Ma), assuming a constant sedimentation rate. Specific objectives at Site TSO 3C include the recovery of a continuous Pliocene-Pleistocene composite section at moderate sedimentation rates to study

• past migrations in the position of the PFZ;
• changes in the mixing ratios of lower NADW and CPDW in the Southern Ocean and its relation to high-latitude climate change;
• response of the Southern Ocean to orbital forcing and the phase relationships (leads and lags) to climatic changes in the low and high northern latitudes; and
• stability of the Antarctic cryosphere during "warmer-than-present" climate during the Pliocene prior to the initiation of Northern Hemisphere glaciation.
Site TSO-5C
Proposed Site TSO-5C is located on the western flank of the Meteor Rise in the Polar Frontal Zone of the ACC (Fig. 1). It is located at about the same latitude as ODP Site 704, drilled on the Meteor Rise, but is significantly deeper (4418 m) than Site 704 (2532 m). Late Pleistocene sediments at Site TSO-5C consist of alternating diatom ooze with calcareous intercalations and an average sedimentation rate of 8 cm/k.y. It will be cored by triple APC to a depth of 200 m to obtain a continuous late Pliocene-Pleistocene record within the PFZ. Comparison of the deeper water record of Site TSO-5C with shallower results from ODP Site 704 on the Meteor Rise will be useful to study the history of lower CPDW/AABW and mid- to upper NADW, respectively (Fig. 2). Specific objectives at Site TSO-5B include the reconstruction of

• surface-water parameters and the evolution of the PFZ;
• paleoproductivity changes in the present PFZ region; and
• lower CPDW and AABW properties and their response to high-latitude climate and the changes in flux of NADW during glacial and interglacial cycles.
Site TSO-6A
Proposed Site TSO-6A is located to the north of Shona Ridge (water depth 3680 m) close to the present-day Polar Front (Fig. 1). This area also represents the southernmost extension of the southward spreading tongue of the NADW, which mixes with cold Antarctic waters to form the CPDW (Fig. 2). This site is located in the northern part of the biogenic silica belt, which is characterized by high biosiliceous accumulation rates. Cores recovered at and near the location of Site TSO-6A are marked by high biosiliceous accumulation with sedimentation rates as high as 80 cm/k.y. during the Holocene. These rates decrease greatly, however, during glacial periods. Today, Site TSO-6A is located about 5° north of the average winter sea-ice edge, but diatoms from glacial aged sediments indicate that Site TSO-6A was covered by sea ice during late Quaternary glaciations (Gersonde, unpublished). Sediments consist of late Quaternary diatom ooze and diatomaceous mud, but foraminifers are present throughout the core for the establishment of stable isotopic stratigraphies. Site TSO-6A will be triple APC cored to 200 m, followed by single XCB coring to approximately 400 m, and RCB coring to a depth of 700 m to recover late Cenozoic biosiliceous and calcareous high-resolution sediments in mid-water depths on the Shona Ridge. Specific objectives include the study of

• surface-water parameters and the evolution of the Polar Front;
• sea-ice distribution in the Southern Ocean;
• paleoproductivity changes (e.g., silica, carbon export rates) and the history of the circum-Antarctic biogenic silica belt in relation to surface-water mass changes, deep-water circulation, and sea-ice distribution;
• deep-water circulation, including changes in the physical and chemical properties of CPDW; and
• silica diagenesis (see also Site TSO-7C).
Site TSO-7C
Proposed Site TSO-7C is located in a small sedimentary basin north of Bouvet Island at a water depth of 2850 m. The site is located in the center of the ice-free Antarctic Zone and is bounded to the north by the Polar Front and to the south by the Weddell Gyre/ACC Boundary (Fig. 1). During glacial times, however, the site was covered by winter sea ice as indicated by the occurrence of sea ice diagnostic diatoms. TSO-7C is one of two sites located in the biogenic silica belt south of the present-day Polar Front. Sedimentation rates in the late Quaternary range between 15 and 60 cm/k.y. An interesting feature of Site TSO-7C is the occurrence of a very early porcellanite in Marine Isotopic Stage 11 (Bohrmann et al., 1994). The occurrence of early porcellanites at TSO-7C will permit geochemical studies of low-temperature silica diagenesis in sediments and pore waters. The porcellanite beds in the sediment cores are only a few centimeters thick and thus should not impede APC penetration. Site TSO-7C will be cored by triple APC to a depth of 200 m to obtain an ultra-high-resolution record of biosiliceous sediments south of the present-day position of the Polar Front. Any time remaining at the end of Leg 177 will be used to deepen TSO-7C to a maximum depth of 730 m. The paleoclimatic record from this site will be used to study rapid climate change on suborbital time scales in Antarctic surface waters and will be correlated to similar ultra-high resolution records from other marine cores, as well as to Antarctic and Greenland ice core records. Specific objectives include the reconstruction of

• surface-water parameters south of the present PFZ;
• sea-ice distribution in the Southern Ocean;
• paleoproductivity changes (e.g., silica, carbon export rates) in relation to surface-water mass changes and sea-ice distribution;
• deep-water circulation, including changes in the physical and chemical properties of CPDW; and
• early low-temperature silica diagenesis.

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