The role of the Eocene-Oligocene transition in Cenozoic radiolarian evolution is still insufficiently known, especially when compared to that for other microplankton groups such as planktonic foraminifers and calcareous nannoplankton. Some faunal change has been observed in low latitudes (Riedel and Sanfilippo, 1986; Sanfilippo et al., 1985), but it appears minor in comparison to the severe taxonomic turnover reported in planktonic foraminifers (Keller et al., 1992) and calcareous nannoplankton (Aubry, 1992). The climatic deteriorations around the Eocene/Oligocene boundary usually play a major role in the scenarios explaining plankton changes in the Southern Ocean. Most commonly, changes are attributed to an increase in productivity (Baldauf, 1992; Baldauf and Barron, 1990; Diester-Haass, 1995), eventually favoring opal preservation and promoting carbonate dissolution.
As part of a larger project concerning radiolarian faunal change, the original aim of this study was to document the radiolarian faunal change during the late Eocene to the early Oligocene in the Southern Ocean in relation to the outstanding climate change during this time period. Previous studies were devoted mostly to developing a stratigraphic zonation (Caulet, 1991; Takemura, 1992; Takemura and Ling, 1997) but did not characterize overall faunal change in relation to Southern Ocean cooling (Zachos et al., 1999). So far, this has only been done by Lazarus and Caulet (1993), who reported a considerable faunal turnover.
Unfortunately, no radiolarians of Eocene age are preserved in any of the studied sites. Thus, our study is limited to the Oligocene suite and can only document faunal changes within this interval. We document qualitative and quantitative radiolarian faunal characteristics through a large part of the Oligocene and relate this to published paleoclimatic and productivity data. Taxonomy, stratigraphy, diversity, abundance, and preservation of radiolarians are described. A significant accomplishment of this study is the documentation, albeit preliminary, of the complete recovered Oligocene radiolarian assemblage, including (in open nomenclature) several previously undescribed species.