Middle Eocene-Lower Oligocene calcareous nannofossil biostratigraphy and paleoceanographic implications from Site 711 (equatorial Indian Ocean)

• Revision of the nannofossil biostratigraphy of the middle Eocene-lower Oligocene at Site 711
• Discussion on the reliability of bioevents for the late Paleogene at low latitudes
• Magnetobiochronology of Site 711
• Evaluation of the nannofossil response during the MECO and the EOT

Nannofossil data from ODP Site 711 (equatorial Indian Ocean) yield a set of consistent, reliable biohorizons that form the basis of a revised calcareous nannofossil biostratigraphy for the low-latitude Eocene-Oligocene. We discuss 31 biohorizons occurring over an 11 myr time interval which we correlate to previous magnetostratigraphic data. Calcareous nannofossils from the middle Eocene through the lower Oligocene of the studied section are characterized by moderately well preserved assemblages consisting largely of low latitude and cosmopolitan species.A significant nannofossil dissolution interval is evidenced at the middle Eocene Climatic Optimum (MECO). We document a significant increase in late Eocene nannoplankton exhibiting a eutrophic preference. Analysis of the assemblage suggests important changes in the equatorial oceanic regime just before the onset of the Eocene-Oligocene transition (EOT), that foreshadow the more dramatic climatic shift of the early Oligocene.