1. To augment the biostratigraphic, biogeographic, paleoceanographic, and paleoclimatic history of the Southern Ocean during the Cenozoic, including the evolution and stability of the Antarctic cryosphere; and

2. To construct high- and ultra-high-resolution records during the Quaternary and late Neogene to better understand the role of the Southern Ocean in climate change on orbital and suborbital time scales.

Evolutionary history and stability of the Antarctic cryosphere. The Leg 177 transect will permit reconstruction of latitudinal isotopic gradients and analysis of biogeographic distribution and abundance patterns of microfossil assemblages that should lead to improved understanding of the growth and stability of the Antarctic ice sheets and help address existing discrepancies between land-based, marine isotopic, and sea-level records (Fig. 6).

Thermal isolation of Southern Ocean surface waters by development of the ACC and its associated frontal systems. Thermal isolation of the Antarctic continent was intimately linked to tectonic and paleoceanographic changes that led to the establishment of a zonal circulation system, the ACC. Knowledge of the timing and strength of thermal isolation is important for understanding polar heat transport and its effect on the development and stability of the Antarctic ice sheets. The establishment and expansion of the ACC has also influenced intermediate-, deep-, and bottom-water formation in the Southern Ocean. Accurate reconstruction of frontal boundaries requires a latitudinal transect of sites that encompass the dynamic range of the frontal movements. Leg 177 sites will permit us to reconstruct the changes in the paleolatitudinal position of frontal boundaries, similar to studies carried out on piston cores from the late Quaternary (Prell et al., 1979; Morley, 1989; Howard and Prell, 1992).

History and distribution of sea ice and its seasonal variation to better understand its role in the global climate system. Sea ice is a fast changing environmental parameter, which is presently characterized by strong seasonal variations. Changes in sea-ice distribution have been among the most important controls on the Southern Hemisphere climate during the late Pleistocene and affect gas and heat exchange between ocean and atmosphere, ocean circulation and the formation of water masses by the rejection of salt, atmospheric circulation and wind speeds, surface albedo, and the biological production and distribution of organisms.

History of primary productivity in the Southern Ocean and evolution of the Antarctic biogenic silica belt. Since about 36 Ma, the Southern Ocean has acted as a major sink for biogenic opal, reflecting increased surface-water productivity as a result of polar cooling and upwelling in the circum-Antarctic (Baldauf et al., 1992). Changes in Southern Ocean productivity and the expansion of the biogenic silica belt have significantly influenced the distribution of nutrients in the World Ocean and have probably played a role in atmospheric pCO₂ variation (Keir, 1988). Differences of opinion exist, however, regarding the role that changing primary productivity in the Southern Ocean has had on atmospheric CO₂ and global climate (Kumar et al., 1995, Frank et al., 1996, Pollock, 1997).

Early low-temperature chert diagenesis in sediment from the Antarctic biogenic silica belt. Leg 177 will provide the first continuous Neogene records from the Antarctic biogenic silica belt, which is comprised of nearly pure diatom ooze that accumulates at high sedimentation rates. Very early transformation of silica from opal-A to opal-CT (strongly cemented porcellanites) has been observed at shallow burial depth in a low-temperature environment in cores recovered near TSO 6A and TSO-7C (Bohrmann et al., 1990, 1994). At these sites, it will be possible to study the nature and rates of silica diagenetic reactions in a sediment type that is ubiquitous in the geological record (e.g., Eocene cherts), but rare in the contemporaneous ocean.

Southern high-latitude calcareous and siliceous biozonations. ODP Legs 113, 114, 119, and 120 provided an enormous improvement in southern high-latitude stratigraphy, but further refinement of these biozonations is desirable. Leg 177 will provide the opportunity to improve dating of Neogene biostratigraphic markers by correlation with orbital-tuned paleoenvironmental signals. In addition, Leg 177 sequences will permit study of evolutionary processes (patterns, modes, and timing of speciation and diversification), the development of Southern Hemisphere bioprovinces (e.g., endemism), and the response of the biota to long- and short-term environmental changes.

Changes in the production and the mixing ratios of various deep -and bottom-water masses, and their role in affecting the global climate system. The Southern Ocean is unique in that its deep water (mainly Circumpolar Deep Water) is a mixture of deep-water masses from all ocean basins. As such, monitoring changes in the chemistry of Southern Ocean deep water provides an opportunity to reconstruct changes in the mean composition of the deep ocean. The Southern Ocean is perhaps the only region where fluctuations in the production rate of NADW can be monitored unambiguously (Oppo and Fairbanks, 1987; Charles and Fairbanks, 1992). The South Atlantic sector of the Southern Ocean represents the initial point of entry of NADW into the Circumpolar Current and, therefore, is highly sensitive to changes in the strength of the NADW conveyor. The Leg 177 depth transect will be ideal for reconstructing the long-term evolution of the dominant subsurface water masses in the Southern Ocean (Fig. 2).

Timing and response of Southern Ocean surface and deep waters to orbital forcing, including the phase relationships to climatic changes in the Northern Hemisphere. Relatively little is known about the interhemispheric phase response (lead, lag, or in-phase) between the high-latitude Northern and Southern Hemispheres. Based upon limited data from the Southern Ocean, Imbrie et al. (1989, 1992) suggested an early response of surface and deep waters in the Southern Ocean relative to other regional proxy data. This early response has also been observed by other studies (Charles et al., 1996; Bender et al., 1994; Sowers and Bender, 1995), implying that the Antarctic region plays a key role in the driving mechanism of glacial-to-interglacial climate change during the last climatic cycle. It is not known, however, if this early response of the Southern Ocean was characteristic of the entire 100-k.y. world of the late Pleistocene or whether this phase relationship also extended to the 41-k.y. world of the early Pleistocene and Pliocene. Leg 177 sediments (e.g., from TSO-3C and TSO-5C) will provide the material needed to study the response of the Southern Ocean to orbital forcing and the phase relationships to climatic changes in other regions.

Rapid (suborbital) climate change in the Southern Ocean by correlation and comparison of millennial signals from the Southern Hemisphere with polar ice cores and marine records. Leg 177 will recover cores with highly expanded sections at three sites (SubSAT-1B, TSO-6A, TSO 7C), which will permit the study of climatic variations in the Southern Ocean at suborbital (millennial) time scales. These targets will serve as the Southern Hemisphere analogs to the North Atlantic drift deposits recovered by ODP Legs 162 and 172. These cores will allow us to determine whether abrupt climate changes similar to those documented in Greenland ice cores (Dansgaard et al., 1993) and marine records from the high-latitude North Atlantic (Bond et al., 1993; Bond and Lotti, 1995) have occurred in the southern high latitudes. If high-frequency oscillations can be identified in the Southern Ocean, as recently claimed by A. Hoffmann (per. comm., 1997), then were they synchronous with changes in North Atlantic climate? Expanded sections will also permit study of the structure of glacial and interglacial cycles in the Southern Ocean, including the trajectories of deglacial meltwater from the Antarctic continent (Labeyrie et al., 1986). For example, did pulse-like surges occur in the Antarctic Ice Sheet during the late Pleistocene, similar to Heinrich events in the North Atlantic? What was the nature and structure of terminations in the Southern Hemisphere during the late Pleistocene? Lastly, correlation between Antarctic sediment cores and ice cores from Greenland and Antarctica, which now span the last 400 k.y. at Vostok (Antarctica) (Petit et al., 1997), will reveal the phase relationships between various variables in the atmosphere and ocean systems, and may contribute to identifying the mechanisms responsible for rapid climate change.

Geomagnetic paleointensity. U-channel sampling of cores collected on Leg 177 will be used to construct continuous records of variations in the intensity of Earth's magnetic field. Comparison of these signals from the high-latitude Southern Hemisphere with similar results obtained from the North Atlantic will test whether these observed variations are reflecting changes in the intensity of the Earth's dipole field. If so, then these dipolar paleointensity changes will provide a powerful stratigraphic tool that can be used to correlate cores globally.

Tectonic history of Agulhas Ridge. Drilling a depth transect on the Agulhas Ridge may shed light on the tectonic evolution of this fracture zone ridge. The ridge also has paleoceanographic implications because of its role as an impediment to deep-water flow in the South Atlantic, as well as its topographic influence on the location of oceanic frontal systems.

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