1.Document 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.Construct records during the Quaternary and late Neogene with millennial or high temporal resolution to better understand the role of the Southern Ocean in climate change on orbital and suborbital time scales.
Paleoceanographic and Biogeochemical Objectives
Evolutionary History and Stability of the Antarctic Cryosphere
The Paleogene section at Site 1090 is shallowly buried, and thus oxygen isotopic measurements of foraminifers are not likely to have been compromised by diagenetic alteration. The study of oxygen isotopic variation coupled with microfossil distribution and abundance patterns may provide insight into the growth and stability of the Antarctic Ice Sheet and the ACC during the Paleogene.
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 (Kennett, 1977). 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 Sheet. 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. 4). For the late Neogene, Leg 177 sites form a complete latitudinal transect from 41° to 53°S that permits reconstruction of the paleolatitudinal position of the Polar Front, similar to studies carried out on piston cores from the late Quaternary (Prell et al., 1979; Morley, 1989; Howard and Prell, 1992).
History, Distribution, and Seasonal Variation of Sea Ice
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 may have been among the most important controls on Southern Hemisphere climate during the late Pleistocene. Analysis of siliceous microfossils indicative of sea ice in southern sites (Sites 1091 through 1094) will be used to reconstruct the distribution and seasonality of sea ice in the Southern Ocean during the late Pliocene-Pleistocene.
History of Southern Ocean Primary Productivity and its Effect on Atmospheric pCO2
During glacial periods, opal accumulation rates and export production may have increased substantially within the PFZ and may have been fueled by iron fertilization of surface water delivered by aeolian input from glacial Patagonian deserts (Kumar et al., 1995). Differences 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; Francois et al., 1998). Leg 177 sediments will be important for testing various hypotheses related to glacial-to interglacial changes in productivity and nutrient cycling in the Southern Ocean.
Evolution of the Antarctic Biogenic Silica Belt and its Effect on the Global Marine Silica Budget
Since ~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 circumantarctic (Baldauf et al., 1992). Expansion of the biogenic silica belt may have significantly influenced the distribution of nutrients in the ocean. Leg 177 drilled two sites (Sites 1093 and 1094) in the Antarctic biogenic silica belt that represent the first verifiably complete late Pliocene-Pleistocene sequences from this region. Study of these thick sequences of diatom ooze, including laminated diatom mats, will permit estimation of silica accumulation rates, which will be important for assessing the role of these deposits in the dissolved silica budget of the world's oceans.
Changes in the Production and Mixing Ratios of Various Deep- and Bottom-Water Masses
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 (Fig. 3). 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 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 assessing their role in global climate change (Fig. 2).
Timing and Response of Southern Ocean Surface and Deep Waters to Orbital Forcing
Relatively little is known about the interhemispheric phase response (lead, lag, or in phase) between the high-latitude Northern and Southern Hemispheres. Imbrie et al. (1989, 1992) suggested an early response of surface and deep waters in the Southern Ocean relative to other regional climate responses. This lead has also been observed by other studies (Howard and Prell, 1992; Labeyrie et al., 1986; Charles et al., 1996; Bender et al., 1994; Sowers and Bender, 1995), implying that the Antarctic region played 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 middle to late Pleistocene the interval 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.
Suborbital Climate Change by Comparison with Ice Cores and Other Marine Sediment Records
Highly expanded sections were recovered at four sites (Sites 1089, 1091, 1093, and 1094), which permit the study of climatic variations in the Southern Ocean 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 latitudinal transect from 41° to 53°S 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). Lastly, correlation 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.
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. Sediments drilled during Leg 177 provide the opportunity to improve dating of Neogene and Paleogene biostratigraphic markers by correlation with 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.
Early Low-Temperature Chert Diagenesis in Sediment from the Antarctic Biogenic Silica Belt
Although chert is ubiquitous in the geological past (e.g., Eocene cherts), few examples of recent porcellanites exist in the geologic record except those found in diatomaceous deposits of the Southern Ocean (Bohrmann et al., 1990, 1994). 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 pore fluids 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 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.
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