Benthic foraminifers from Holes 522, 523, and 524 of DSDP Leg 73 (eastern South Atlantic) were quantitatively analyzed by using three methods: analysis of local originations and extinctions, factor analysis of species relative abundances, and population structure analyses of species diversity and equitability. The results of all three forms of analysis are compatible and show a sharp faunal change in the late Eocene. This change involves an almost complete cessation of local originations, a small increase in local extinctions, a dramatic shift in species relative abundances, and a sharp increase in species equitability. Sporadic small-scale variations in the same parameters during the middle Eocene precede the major change. These results indicate initiation of the psychrosphere in the latest Eocene, with precursor events in the middle Eocene. The faunal evidence is consistent with a northeast Atlantic source for the cold bottom water.

We established a composite oxygen-and carbon-isotope stratigraphy for the Pliocene in the central South Atlantic. Monospecific samples of benthic and planktonic foraminifers from pelagic sediments from DSDP Sites 519, 521, 522, and 523 were analyzed isotopically. The resulting benthic oxygen-isotope stratigraphy allowed three paleoclimatic periods in the Pliocene to be distinguished. During the early Pliocene (5.2-3.3 Ma), low-amplitude climatic changes prevailed in a world that was less glaciated than during the Pleistocene. A net increase in global ice volume is documented in a 0.5% 0 positive shift in the average 18 O composition of the benthic foraminifers at 3.2 Ma. The middle Pliocene (3.3-2.5 Ma) is not only characterized by a more widespread glaciation of the Southern and Northern hemispheres but also by more drastic isotopic differences between glacial and interglacial times. A minor shift in the average 18 O composition of the benthic foraminifers marks the beginning of the late Pliocene-early Pleistocene climatic period (2.5-1.1 Ma). Alternating cold and warm climate is documented in both the oxygen-isotope record and in the pelagic sediments. During cold periods, sediments with a lower CaCO 3 content indicate more corrosive bottom-water conditions. More negative 13 C signals in the benthic foraminifers from these sediments suggest that the Antarctic Bottom Water current was intensified in glacial times. The oxygen-isotope composition of the measured planktonic foraminifers suggests that the surface water in this part of the South Atlantic remained relatively warm during the growth of the Pliocene glaciers.

A stable-isotope stratigraphy was established for planktonic and benthic foraminifers from upper Miocene-lower Pliocene pelagic sediments from the Mid-Atlantic Ridge. A correlation of stable-isotope and biostratigraphic data with magnetostratigraphic age revealed the following: (1) the late Miocene carbon-isotope shift in the South Atlantic bottom waters was minute compared with the shift reported for other deep-sea locations (Haq et al., 1980), (2) a significant cooling or continental ice-volume increase occurred between 5.7 and 5.2 Ma, and (3) a period of warming or ice-volume decrease followed, with the rate of warming increasing beginning at 4.5 Ma and reaching a climax at 4.3 Ma. The timing of these paleoceanographic events is correlated with the onset and termination of the Messinian salinity crisis in the Mediterranean Sea.

The recovery of calcareous-rich sediments of Paleogene age from three sites in the eastern South Atlantic Ocean provided the opportunity to document the rich abyssal benthic foraminiferal fauna contained within them. The faunal dominance patterns observed at these sites are generally similar to those observed elsewhere in the abyssal Atlantic. There is no unusual faunal turnover at the Oligocene/Eocene boundary, although faunal changes are marked between the middle and upper Eocene.

Several suites of undisturbed cores obtained by continuous hydraulic piston coring provided material for the investigation of Cenozoic paleoceanography. The faunal record is complete, except for the early Eocene. The isotope records have gaps in the early Eocene and in the early and middle Miocene. Hundreds of samples were studied and thousands of analyses were carried out. Detailed results and conclusions are presented in several separate chapters of this volume. This report is a synthesis.Studies of calcite dissolution in pelagic sediments indicate that calcite compensation depth (CCD) and lysocline underwent two types of changes during the Cenozoic. The first-order changes, which have a periodicity of some I0 7 yr., have amplitudes (depth difference from the medium) of more than 1,000 m. The Eocene and Miocene were epochs of high CCD. The deepening of the CCD in the late Eocene and early Oligocene was stepwise, but it was more abrupt in the early Pliocene. We attribute the higher CCD in the Eocene and Miocene to a relatively low production rate of calcareous plankton in the open oceans; during those epochs many of the nutrients had been consumed by phosphate deposition and by siliceous plankton. The second-order changes, which have a periodicity of I0 4 or I0 5 yr., represent CCD variations in response to changes from interglacial to glacial paleoenvironments during the Pliocene and Quaternary; the amplitude was a few hundred meters only. Evidence suggests that increased dissolution due to the more active bottom waters during the glacial stages in the Atlantic far overshadowed the effect of possible increases in fertility.The oxygen-isotope record shows an overall increase of δ 18 θ values since the middle Eocene, with a moderate reversal of the general trend during the late Oligocene and early Miocene. Whereas the early Paleogene trend of the Atlantic middle-latitude sites closely simulated that of the southern oceans, vertical and latitudinal gradients began to develop in the Oligocene and continued to steepen with time, so that the present thermal structure of the ocean waters above our sites is similar to that of the equatorial Pacific. The cause of the increased δ 18 θ values should have been related to increased ice volume on Earth and to temperature declines. However, the relative importance of the two different factors at any given time cannot be ascertained. We favor the hypothesis that the Antarctic Ice Cap started to form in the late Eocene and suggest that the early Oligocene ice volume was larger than it is today. The middle Miocene oxygen shift has also been registered at one of our sites, but its magnitude is smaller, and the corresponding environmental changes should have been less impressive, than those in the late Eocene and early Oligocene.The carbon-isotope record shows a parallel trend between the planktonic and benthic foraminifers since the beginning of the Oligocene and a divergent trend as well as a steeper gradient during the middle Eocene. Peak values of δ 13 C are found in ...