The general goals of Leg 177 were twofold: (1) to document the biostratigraphic, 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 records from other ocean basins and with ice cores from Greenland and Antarctica.
More than 4000 m of sediments were 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 bulk density, and spectral color reflectance). Leg 177 cores, for which spliced composite sections were constructed, represent the most complete sections 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 isotopic measurements of diagenetically unaltered foraminiferal calcite. Site 1090 will likely become a deep-sea type section for biomagnetostratigraphic correlations, potential development of an astronomically tuned time scale, and paleoceanographic studies in the Southern Ocean for the middle Eocene through early Miocene. This time period included the buildup of ice on the Antarctic continent, as well as major paleogeographic changes in the Southern Ocean, and marked a shift in Earth's climate from a warm- ("hothouse") to a cold-climate ("icehouse") mode. The study of Site 1090 will help to decipher processes linked to early thermal isolation of Antarctica from warm subtropical gyres, which led to ice-sheet development and attendant changes in sea level. Similar studies can be achieved using late Miocene sequences that recovered at two locations (Sites 1088 and 1092) with lower sedimentation rates.
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 (i.e., 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 to orbital forcing and the phase relationships to climate change in the North Atlantic. The location of the cores on a north-south transect between subtropical waters and the Antarctic Zone is optimal for monitoring some key aspects of the climate system including the Antarctic sea-ice field, frontal boundary movements within the ACC, changes in paleoproductivity and opal export rates, and changes in the input of North Atlantic Deep Water to the Southern Ocean.
Shipboard measurements of physical properties (diffuse spectral reflectance, gamma-ray attenuation, 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 millennial-scale climate oscillations detected in marine sediments of the Southern Ocean and the ice-core signals on Greenland and Antarctica offer 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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