Sediments in the southeast Atlantic sector of the Southern Ocean were cored during Ocean Drilling Program Leg 177 to study the paleoceanographic history of the Antarctic region on short (millennial) to long (Cenozoic) time scales. Seven sites were drilled along a north-south transect across the Antarctic Circumpolar Current (ACC) from 41° to 53°S: three sites at ~41°S near the Agulhas Ridge (Sites 1088, 1089, and 1090), two sites at ~47°S near and at the Meteor Rise (Sites 1091 and 1092), and two sites at 50° and 53°S within the circum-Antarctic opal belt (Sites 1093 and 1094). The water depths of the sites range from 1974 to 4620 m, intersecting all of the major deep- and bottom-water masses in the Southern Ocean.
The general goals of Leg 177 were twofold: (1) to document the bio-stratigraphic, biogeographic, and paleoceanographic history of the Paleogene and early Neogene, a period marked by the establishment of the Antarctic cryosphere and the ACC; and (2) to target expanded sections of late Neogene sediments, which can be used to resolve the timing of Southern Hemisphere climatic events on orbital and suborbital time scales, and can be compared with similar sedimentary and climatic records from other ocean basins and with climatic signals documented in ice cores from Greenland and Antarctica.
More than 4000 m of sediment was recovered at an average recovery rate of 81%, ranging in age from the middle Eocene to the Holocene. Composite records were constructed at each site from cores in multiple holes by aligning features in the signals of core logging data (magnetic susceptibility, gamma-ray attenuation [GRA] bulk density, spectral color reflectance, and lithologic markers). Leg 177 cores, for which spliced composite sections were constructed, represent the most complete sections yet obtained from the Southern Ocean.
A continuous 330-m sequence of middle Eocene to lower Miocene sediments, recovered at Site 1090, includes cyclic variations in lithologic parameters and a superb geomagnetic polarity reversal record. The shallow burial depth of this Paleogene section will enable oxygen iso-topic measurements of diagenetically unaltered foraminifers. Site 1090 will likely become a deep-sea type section for biomagnetostratigraphic correlations, and the potential exists for development of an astronomically tuned time scale. Site 1090 will be used to reconstruct the paleoceanographic history of the Southern Ocean for the middle Eocene through early Miocene. This time period marked a shift in Earth's climate from a warm- ("hothouse") to a cold-climate ("icehouse") mode, and included the buildup of ice on the Antarctic continent and major paleogeographic changes in the Southern Ocean. The study of Site 1090 will help to decipher processes linked to the thermal isolation of Antarctica from warm subtropical gyres, as well as Antarctic ice-sheet development and attendant changes in sea level. Similar studies can be conducted on late Miocene sequences that were recovered at two other locations (Sites 1088 and 1092).
During Leg 177, we succeeded in recovering complete and expanded sequences at 41° (Site 1089), 47° (Site 1091), 50° (Site 1093), and 53°S (Site 1094) that accumulated at average rates ranging from 130 to 250 m/m.y. These sequences are well-suited for paleoceanographic studies of the late Pliocene-Pleistocene (particularly the past 1.5 m.y.) at a temporal resolution of less than 1 k.y. These sites represent the Southern Hemisphere analogs to North Atlantic drift deposits drilled during Legs 162 and 172, and they will be useful for studying the response of the Southern Ocean environment to orbital forcing and the phase relationships to climate change in the North Atlantic. The location of the sediment cores on a north-south transect between subtropical waters and the Antarctic Zone is optimal for monitoring some key aspects of the climatic system, including the extent of the Antarctic sea-ice field, frontal boundary movements within the ACC, changes in biological paleo-productivity and opal export rates, and changes in the input of North Atlantic Deep Water to the Southern Ocean.
Shipboard examination of lithologic components and measurements of physical properties (diffuse spectral reflectance, GRA bulk density, magnetic susceptibility, and natural gamma radiation) show distinct evidence of cyclicity at Milankovitch time scales, but millennial and perhaps centennial scale changes should be resolvable at some sites. Correlation of climate oscillations on millennial time scales detected in marine sediments of the Southern Ocean and in ice cores from Greenland and Antarctica offers the opportunity to study the linkages between atmosphere (temperature and CO2) and ocean dynamics (sea-surface temperature, productivity, and deep-water circulation) over the past four climatic cycles of the late Pleistocene.
The high-quality sedimentary sequences recovered during Leg 177 fill a critical gap in the distribution of drilled ocean sites and will anchor the southern end of the global array of sites needed to decipher the role of the Southern Ocean in the history of Earth's climatic system.
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2Shipboard Scientific Party addresses.