Late middle Eocene to early Oligocene radiolarian biostratigraphy in the Southern Ocean (Agulhas Ridge, ODP Leg 117, Site 1090)

The analyses of abundant and well-preserved late middle Eocene to early Oligocene radiolarian assemblages were carried out on ODP Site 1090 (Leg 177) sediments in the Atlantic Sector of the Southern Ocean. A total of 83 radiolarian events are identified and correlated with the geochronologic time scale and paleomagnetic polarity data. The high latitude Eucyrtidium spinosum and Eucyrtidium antiquum Interval Zones are recognized in the studied interval at Hole 1090B, and three new subzones are proposed herein for the Eucyrtidium spinosum Zone: Eucyrtidium nishimurae Interval Subzone (Subchmns C17n.2r to C16r), Dendrospyris stabilis Interval Subzone (Subchrons C16n.2n to C13r), and Dicolocapsa microcephala Interval Subzone (Subchmn C13r). The bases of these subzones are defined by the first occurrences of Eucyrtidium spinosum, Dendrospyris stabilis and Dicolocapsa microcephala, respectively. The proposal of these subzones could significantly improve the biostratigraphic resolution of the middle to late Eocene interval from high latitude regions. The FOs of Axoprunum irregularis and Eucyrtidium antiquum are nearly synchronous in the Southern Ocean, and could be used to estimate the Eocene/Oligocene boundary. Other radiolarian events such as the FOs of Dendrospyris stabilis, Dicolocapsa microcephala, and Zygocircus biitschli, and the LO of Eucyrtidium nishimurae are synchronous or approximately synchronous in the Southern Ocean. In contrast, the LOs of Axoprunum pierinae, Periphaena decora, and Siphocampe quadrata, and the FO of Artostrobus annulatus are commonly diachronous among different sites in the Southern Ocean.

Automated summary

The study analyzed late middle Eocene to early Oligocene radiolarian assemblages in sediments from ODP Site 1090 in the Atlantic Sector of the Southern Ocean. Three new subzones were proposed, improving biostratigraphic resolution in the region. The synchronous first occurrences of Axoprunum irregularis and Eucyrtidium antiquum in the Southern Ocean can be used to estimate the Eocene/Oligocene boundary. Other radiolarian events show diachronous patterns among different sites in the region.