Initial Reports of the Deep Sea Drilling Project - Volume 73

This volume covers Leg 73 of the cruises of the Drilling Vessel Glomar Challenger. Leg 73 was the middle of five legs on South Atlantic paleoenvironments planned by the JOIDES Ocean Paleoenvironment (OP) Panel during the years from 1975 to 1980. The objectives and tactical planning for a transect across the east flank of the Mid-Atlantic Ridge just north of 30°S were outlined by the OP Panel. Drilling was proposed at three sites clustered near the ridge to obtain information on the middle Miocene CCD crisis; a second group of four sites, farther to the east, would be drilled to obtain information on the Eocene-Oligocene paleoceanographic revolution triggered by the onset of Antarctic glaciation. In addition, crustal sampling was planned at all sites to obtain datable material for a more precise determination of the seafloor spreading history in this region. Two broad aims of the cruise were spelled out, namely (1) to investigate the Cenozoic oceanography of the South Atlantic, particularly temperature gradients, circulation patterns, water chemistry, and biotic communities and their environmental adaptation and evolution, and (2) to develop the correlation between the reversal history of the Earth's magnetic field and biostratigraphic zonations. Precisely dated samples would be used to determine the timing and duration of specific paleoceanographic events during the last 70 m.y. In addition, paleomagnetic samples were to provide field direction and intensity data for a study of Cenozoic geomagnetic field phenomena. The Glomar Challenger left Santos, Brazil on April 13, and arrived at Cape Town, South Africa, on June 1,1980. During the 49 days at sea, we drilled 13 holes at six sites and recovered more than 1000 m of core.