The broad scientific themes of Leg 177 were twofold:
The Paleogene section at Site 1090 is well suited for paleoceano-graphic studies of the early Cenozoic because it is shallowly buried, continuously recovered, and well dated. Oxygen isotopic measurements of foraminifers coupled with microfossil distribution and abundance patterns should provide insight into the growth and stability of the Antarctic ice sheet and the ACC during the Paleogene.
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 (Kennett, 1977). Determining 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. Together with sites previously drilled in the South Atlantic sector of the Southern Ocean (e.g., Sites 689, 690, 703, and 704), Sites 1088, 1090, and 1092 will permit us to study the development of the ACC during the Paleogene and early Neogene (Fig. F4). For the late Neogene, Leg 177 sites form a complete north-south transect from 41° to 53°S that is well suited for reconstructing the paleolatitudinal position of the PF, similar to studies conducted on piston cores from the late Quaternary (Prell et al., 1979; Morley, 1989; Howard and Prell, 1992).
Sea ice is presently characterized by rapid and large-scale seasonal variations, and it affects 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. Changes in sea-ice distribution undoubtedly influenced the Southern Hemisphere climate during the late Pleistocene. Analysis of sea-ice diagnostic taxa of siliceous microfossils in sediments from the southern sites (1091-1094) will be used to reconstruct the distribution and seasonality of sea ice in the Southern Ocean during the late Pliocene-Pleistocene.
During glacial periods, export production and accumulation rates of biogenic opal increased substantially within the PFZ and possibly were fueled by iron fertilization of surface water by enhanced eolian dust supply from periglacial Patagonian deserts (Kumar et al., 1995). Different opinions exist, however, regarding whether net productivity increased or remained the same in the Southern Ocean during the last glaciation (Kumar et al., 1995; Frank et al., 1996, in press; Francois et al., 1997). Leg 177 sediments will be important for testing various hypotheses related to glacial-interglacial changes in productivity and nutrient cycling in the Southern Ocean.
Since at least ~36 Ma, the Southern Ocean has acted as a major sink for biogenic opal, reflecting increased surface-water bioproductivity as a result of polar cooling and upwelling in the circum-Antarctic (Baldauf et al., 1992). Expansion of the opal belt may have significantly influenced the distribution of nutrients in the ocean. During Leg 177, two sites (1093 and 1094) were drilled by APC in the circum-Antarctic opal belt, yielding continuous late Pliocene-Pleistocene sequences. Study of these thick sequences of diatom ooze, including laminated diatom mats, will permit estimation of opal accumulation rates, which will be important for assessing the role of these deposits in the dissolved silica budget of the world's oceans.
The Southern Ocean is unique in that its deep water (mainly CDW) is a mixture of deep-water masses from all ocean basins (Fig. F3). As such, monitoring changes in the chemistry of Southern Ocean deep water provides a means to reconstruct changes in the mean water-mass 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 Atlantic sector of the Southern Ocean represents the initial entry point of NADW into the ACC and, therefore, is highly sensitive to changes in the strength of the NADW conveyor. The bathymetric distribution of Leg 177 sites is ideal for reconstructing the long-term evolution of the dominant subsurface water masses in the Southern Ocean, and for assessing their role in global climate change (Fig. F2).
Relatively little is known about the interhemispheric phase response (lead, lag, or in-phase) between the high-latitude Northern and Southern Hemispheres to orbital forcing of climate. Imbrie et al. (1989, 1992) suggested an early response of surface and deep waters in the Southern Ocean relative to climate responses in other regions of the world ocean. This lead has also been confirmed by other studies (Labeyrie et al., 1986; Howard and Prell, 1992; Bender et al., 1994; Sowers and Bender, 1995; Charles et al., 1996), implying that the Antarctic region played a key role in the driving mechanism of glacial-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 mid- to late Pleistocene, which was dominated by 100-k.y. cyclicity, or whether this phase relationship also extends back into the early Pleistocene and Pliocene interval dominated by 41-k.y. cyclicity. Leg 177 sediments (especially Sites 1089, 1091, 1093, and 1094) will provide the material needed to study the response of the Southern Ocean to orbital forcing and its phase relationships with climatic changes in other regions.
Highly expanded sections were recovered at four sites (1089, 1091, 1093, and 1094), which permit the study of climatic variations in the Southern Ocean region at suborbital (millennial) time scales. These sedimentary sequences represent the Southern Hemisphere analogs to the North Atlantic drift deposits recovered during 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. Expanded sections along a north-south transect from 41° to 53°S will also permit study of the structure of glacial-interglacial transitions in the Southern Ocean, including the trajectories of deglacial meltwater from the Antarctic continent (Labeyrie et al., 1986). Lastly, correlation of climate proxies between Leg 177 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.
ODP Legs 113, 114, 119, and 120 provided an enormous improvement in southern high-latitude stratigraphy, but further refinement of these biozonations is desirable. Sediments drilled during Leg 177 provide the opportunity to improve dating of Neogene and Paleogene biostratigraphic markers by correlation with the geomagnetic polarity time scale and orbitally tuned paleoenvironmental signals. In addition, Leg 177 sequences 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.
Although chert is ubiquitous in the geologic record, few examples of recent porcellanites exist except those found in diatom deposits of the Southern Ocean (Bohrmann et al., 1990, 1994). The 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 Site 1094 (Bohrmann et al., 1990, 1994). By sampling interstitial water and solid phases at Site 1094, it will be possible to study the nature and rates of silica diagenetic reactions in these young sediments. In addition, measurements of physical properties and heat flow at Site 1094 will better characterize the conditions under which these young porcellanites were formed.
U-channel sampling of Leg 177 cores 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 reflect changes in the intensity of 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. In addition, the high sedimentation rates of Leg 177 sites offer the opportunity to study transitional field behavior at polarity reversal boundaries and, perhaps, brief excursions and secular variation of the magnetic field in the high-latitude Southern Hemisphere.