Site 1088 (proposed site TSO-2B) is located on the Agulhas Ridge, which extends from the northern tip of the Meteor Rise and terminates abruptly at 40°S, 15°E, where it intersects the northern end of an abandoned spreading-ridge axis in the Agulhas Basin (Figs. F1, F5, both in the "Leg 177 Summary" chapter). In this region, the Agulhas Ridge is a fairly broad topographic feature with ~2250 m of relief (Figs. F1, F5, both in the "Leg 177 Summary" chapter). Piston cores and Parasound echo-sounding profiles across the Agulhas Ridge have demonstrated that this relatively narrow topographic high is draped with sediment consisting of biogenic carbonate oozes, and is not disturbed by turbidites (Figs. F1, F2). A detailed seismic survey was conducted before Leg 177, and Site 1088 was selected in an area of thick (>1000 m) acoustically stratified sediments (Figs. F3, F4). Seismic reflectors at Site 1088 can be traced to the north and south where they crop out at the flanks of the Agulhas Ridge (Figs. F2, F4). Piston cores taken on the ridge top in 2163 (PS2077-1) and 2254 m (PS2076-3) water depth gave ages of late (Chron 7, ~9.3-10.5 Ma) and middle Miocene (Chron 5, ~11-13.3 Ma), respectively (Fig. F1). Extrapolating these depth/age relations to Site 1088, we predicted an age of early Miocene (~12 Ma) at 145 meters below seafloor (mbsf), suggesting that the sedimentary succession was deposited at an average sedimentation rate of 12 m/m.y. Recovery of sediment cores at Site 1088 confirmed that our prediction was basically correct in that the age of the sediment at 200 mbsf in Hole 1088C is ~12 Ma.
Site 1088 represents the northernmost location drilled during Leg 177. The latitude (41°S) places Site 1088 just south of the Subtropical Front (STF), and the water depth (2082 m) places it near the boundary of Circumpolar Deep Water (CDW) and North Atlantic Deep Water (NADW) (Figs. F1, F2, both in the "Leg 177 Summary" chapter). Site 1088 represents the shallow end-member of the Agulhas Ridge depth transect, extending from Site 1088 (2082 m), through Site 1090 (3702 m), to Site 1089 (4620 m). The primary objective at Site 1088 was to recover a Cenozoic carbonate record that could be used to study paleoceanographic changes near the STF and to gain information about thermohaline circulation from an upper mid-water-depth site. Specifically, the objectives were to reconstruct long-term changes in (1) surface-water parameters (e.g., sea-surface temperature and seawater chemistry), and the evolution of the STF and its response to southern high-latitude climate variability; (2) biogenic paleoproductivity north of the Polar Front Zone; (3) the mixing ratio between lower upper CDW and upper NADW, and the evolution of these water masses through time; and (4) the subsidence history of the Agulhas Ridge.
Although problems with drilling operations and coring disturbance prevented us from reaching our depth objective of 700 mbsf, we were able to recover a 230-m-thick section comprising a middle Miocene to Pleistocene sedimentary sequence that was deposited at sedimentation rates averaging 25 m/m.y. during the Miocene and decreasing to 8 m/m.y. during the last 6 m.y. Because sedimentation rates are relatively low at this site, our objective was to study paleoceanographic change at a temporal resolution of 105 to 106 yr. Together with Sites 704 (Ocean Drilling Program [ODP] Leg 114; Wright et al., 1991, 1992) and 1092, located on the Meteor Rise within NADW, the sediments from Site 1088 should provide a history of changes in deep-water circulation, especially during the Miocene. The Neogene sediments at Site 1088 are carbonate rich (ranging from 85 to 95 wt%), have not been deeply buried, and, therefore, should provide reliable stable isotopic stratigraphies that are not compromised by diagenetic alteration.
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