PREVIOUS DRILLING IN THE SOUTHERN OCEAN
Previous deep-sea drilling in the Southern Ocean (Fig. 4), especially cores recovered with the Advanced Hydraulic Piston Corer (APC) and Extended Core Barrel (XCB) systems (Deep Sea Drilling Project [DSDP] Leg 71, Ocean Drilling Program [ODP] Legs 113, 114, 119, and 120), have provided a basic understanding of the paleoceanographic and paleoclimatic evolution of the southern high latitudes during the Cenozoic (see Kennett and Barron, 1992), a period of paleogeographic changes that permitted the development of the Antarctic Circumpolar Current (Kennett, 1977). Sections recovered by previous Antarctic drilling are often incomplete, however, because the APC and XCB systems miss intervals at core breaks even under ideal conditions and apparent 100% recovery in single holes (Ruddiman et al., 1986). In addition, cores are easily disturbed when recovered in the high seas often encountered in the Southern Ocean. Problems with incomplete core recovery, core disturbance, the presence of hiatuses, and diminished carbonate preservation at the high latitudes of the Southern Hemisphere have hampered efforts to obtain continuous paleoclimatic records, especially those of Neogene age, in the Southern Ocean. One of the primary goals of Leg 177 was to recover complete sections by drilling multiple holes that are used to construct continuous composite sections in real time.
A major deficiency in the distribution of ocean-drilled cores is the lack of late Neogene sequences from the southern high latitudes that would permit the generation of continuous stratigraphic and paleoenvironmental signals. Compared to the superb records now available from the high-latitude North Atlantic Ocean (Legs 94, 154, 162, and 172), the Southern Ocean had relatively few sites suitable for high-resolution paleoclimatic studies. Expeditions by ODP to the South Atlantic and Indian sectors of the Southern Ocean during 1987-1988 (Legs 113, 114, 119, and 120) acquired good Paleogene sequences, but relatively few late Neogene records were recovered that would be suitable for studies of Neogene paleoclimatology. Of the 32 sites drilled during these four legs, only one site (ODP Leg 114, Site 704), had sufficient stratigraphic continuity and carbonate content during the Pliocene-Pleistocene to be suitable for high resolution paleoclimatic studies. Thus, the recovery of long, continuous sequences from the Southern Ocean was one of the major goals behind Leg 177.
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