Deep Sea Drilling Project Sites 558 and 563, located on the west flank of the Mid-Atlantic Ridge between 33 and 37°N latitude, yielded lower Oligocene through Pleistocene nannofossil-bearing sediments. A detailed study of the abundance, preservation, and biostratigraphic distribution of calcareous nannofossils at Sites 558 and 563 shows that preservation ranges from moderate to good and nannofossil assemblages are reasonably diverse. These long, continuously cored sections add substantially to the record of Tertiary nannofossiliferous sediments recovered in the North Atlantic. A hiatus of early Pliocene age is noted in Hole 558A, where the nannofossil Subzones CNlOb, CNlOc, and CNlla are missing. An increase in the relative abundance of triradiate forms of Discoaster brouweri proved useful in determining the LAD (last appearance datum) of D. brouweri in the presence of considerable reworking.A significant occurrence of abundant braarudosphaerids, along with holococcoliths, was noted in the lower Oligocene section at Hole 563, but not at Hole 558, 400 km to the north, an indication that a provincial nannofossil boundary separates the two sites. This first report of abundant Oligocene braarudosphaerids from the central North Atlantic Ocean suggests that they should be pervasive in Northern Hemisphere mid-latitude and subtropical lower Oligocene sediments deposited above 2700 m. Together with Southern Hemisphere occurrences, this North Atlantic occurrence of braarudosphaerids may indicate a bipolar distribution in the maximum abundance of Oligocene Braarudosphaera.

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.

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.

During Leg 110 of the Ocean Drilling Program, sediment was recovered from six sites in the vicinity of the Lesser Antilles Forearc. Hole 67IB, drilled near the toe of the Barbados deformation front, was the first-ever penetration of the decollement between the underthrusting Atlantic Plate and the off scraped Barbados accretionary prism. Stratigraphic repetitions in sequence associated with tectonic movement along the decollement zone, first observed on DSDP Leg 78A, were further documented at four ODP Leg 110 sites. A significant biostratigraphic inversion is present at Site 671 at 128 mbsf in which upper Miocene sediments rest atop lower Pleistocene strata. Smaller repetitions in sequence are recorded at Sites 671, 673, 674, and 676. Leg 110 sediments range from middle Eocene to early Pleistocene in age. Pliocene/Pleistocene assemblages are generally well preserved; however, Miocene assemblages have undergone extensive dissolution at all Leg 110 sites. Paleogene sediments are sometimes recrystallized and the nannofossils contained within exhibit a range in preservation from poor to good.