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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.

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Calcareous Nannofossils of Deep Sea Drilling Project Sites 558 and 563, North Atlantic Ocean: Biostratigraphy and the Distribution of Oligocene Braarudosphaerids