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Site 1091 (proposed site TSO-5C) is located on the western flank of the Meteor Rise at a water depth of 4363 m near magnetic Anomalies 24-25 (early Eocene-late Paleocene), which represents the oldest oceanic crust between the Meteor Rise and the Mid-Atlantic Ridge (MAR). Oceanic crust began to form between the MAR and Meteor Rise as a result of a jump in the spreading center from the fossil ridge axis in the Agulhas Basin to the Meteor-Islas Orcadas Rises at ~62 Ma (Fig. F5 in the "Leg 177 Summary" chapter). Site-survey Thompson Cruise TTN057 collected single-channel seismic profiles in the region surrounding Site 1091 (Fig. F1). Seismic profiles indicate a thick (~1200 m) package of sediment overlying normal oceanic crust (Fig. F2). We surveyed two areas that appeared promising on the basis of Parasound profiles collected by the Polarstern. The region to the southwest was deemed superior by virtue of its higher sedimentation rates and relatively uniform upper stratigraphic succession. A 13-m piston core (TTN057-10-PC3) recovered in the vicinity of Site 1091 contains latest Pleistocene sediments deposited at an average sedimentation rate of 80 m/m.y. The lithology consists of alternating diatom and calcareous ooze representing glacial and interglacial periods, respectively.
Site 1091 is located in the central Polar Front Zone (PFZ), ~2º north of the present-day position of the Polar Front (PF), ~2º south of the Subantarctic Front, and ~7º-8º south of the Subtropical Front (Fig. F1 in the "Leg 177 Summary" chapter). Because of the close proximity to the PF, the sedimentary environment at Site 1091 is highly sensitive to changes of the frontal position. During glacial periods, opal accumulation rates increased markedly whereas carbonate export production and preservation declined. During the last glaciation, maximum biological productivity occurred within the PFZ and probably was fueled by iron fertilization of surface water, when iron was possibly delivered by enhanced eolian dust input from periglacial Patagonian deserts (Kumar et al., 1995). However, a part of the glacial increase in accumulation rates can be attributed to lateral advection and sediment focusing (Frank et al., in press; Diekmann et al., in press). Site 1091 will be an important site for reconstructing the history of PFZ movement and for testing hypotheses related to glacial-interglacial changes in biological productivity and nutrient cycling in the Southern Ocean.
The high sedimentation rates at Site 1091 complement the record at Site 1089 (41ºS) in the northern Subantarctic Zone and Sites 1093 and 1094 in the Antarctic Zone to the south. The north-south transect of high-resolution sites drilled during Leg 177 will be used to reconstruct past movement of the PFZ and Antarctic sea-ice field. Site 1091 will provide a high-resolution record in the northern PFZ and will permit the study of climate variability at orbital and suborbital (millennial) time scales. The record at Site 1091 can also be compared with lower sedimentation-rate signals obtained at Site 1092 and Ocean Drilling Program (ODP) Site 704 (Hodell and Venz, 1992; Hodell, 1993), located only ~60 km to the east of Site 1091 on the crest of the Meteor Rise. The water depth of Site 1091 (4363 m) places it within lower Circumpolar Deep Water (CDW) (Fig. F2 in the "Leg 177 Summary" chapter). Together with Site 704 (2532 m) and Site 1092 (1988 m), Site 1091 forms a depth transect that can be used to study the history of middle to upper North Atlantic Deep Water (NADW) and lower CDW, respectively.
The primary objective of Site 1091 was to recover a high-resolution sequence within the PFZ that could be used to study
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