Previous works from Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) sites in the North Atlantic and Indian Ocean have shown the relationship of calcareous nannofossil assemblages with sea surface productivity. By using quantitative methods, Olafsson (1989) and Fornaciari et al. (1990) underlined some sharp rise in abundance at the beginning and at the end of the nannofossil ranges. Similar sharp rises and falls in abundance have been observed within the ranges of several species. In addition to these studies on productivity, Gartner (1992) introduced several new biohorizons that are recognized in this study and complemented by the addition of new events.

The objective of this study was to investigate the possible correlations between the calcareous nannofossil events and global sea level variations described by Haq et al. (1987). The first phase of the work consisted of precisely locating the first and last occurrences, noting the distribution patterns, and characterizing the turnovers within the calcareous nannofossil assemblages at each site. The second phase consisted of using precise taxonomic concepts to define a new succession of biohorizons for the Oligocene-Miocene interval for the four sites. Bioevents are reported in summary tables. Most of the taxonomic work was done on Hole 900A. Many taxonomically ambiguous species were observed in Hole 900A. During the study of the three other holes, a better understanding of these ambiguous forms emerged and some of these taxa are considered very useful for the biostratigraphy of the Oligocene-Miocene interval.

The variability of and the different trends in the nannofossil assemblages have been studied in order to identify paleoceanographic influences. We believe that calcareous nannofossil evolution is directly related to paleoceanographic changes. These include changes in the available amount of nutrients, temperature, salinity, position of the calcite compensation depth (CCD), and in dissolved carbonate content. The occurrence and abundance of new species, genera, or families of calcareous nannofossils have been compared to the sedimentary record to determine possible correlations with the sequence stratigraphy and sea level curve defined by Haq et al. (1987).

Hole 900A was chosen as reference site for this study because from it was recovered the most complete and extended Oligocene-Miocene interval (about 391 m).

The Oligocene-Miocene sedimentary succession recovered in Hole 900A consists of a lower contourite-turbidite-pelagic sequence (Unit II, see Comas et al., this volume) and an upper turbidite-pelagic sequence (Unit I). To eliminate the uncertainties caused by the presence of reworked nannofossils, only pelagic intervals represented by nannofossil ooze or nannofossil chalk were collected. The nannofossil assemblages are, therefore, interpreted as indicative of open ocean conditions. From the early Oligocene to the late Miocene, most of the regressive/transgressive periods can be recognized by variations in the calcareous nannofossil assemblages.