Integrated calcareous nannofossil and magnetostratigraphic record of ODP Site 709: Middle Eocene to late Oligocene paleoclimate and paleoceanography of the Equatorial Indian Ocean

We investigated the calcareous nannofossil biostratigraphy and magnetostratigraphy of middle Eocene – lower Oligocene sediments from ODP Hole 709 C, equatorial Indian Ocean. The new bio-magnetostratigraphic analyses have resulted in an accurate biochronology of the interval spanning Chrons C20r (middle Eocene) to C12r, in which 29 bioevents were investigated, in a 12 myr interval. The magnetostratigraphic signal is less clear across the Eocene-Oligocene transition (EOT) but becomes more reliable at the top of Chron C13n to Chron C12r (early Oligocene). Quantitative analyses of calcareous nannofossil assemblages allowed recognition of the Middle Eocene Climatic Optimum (MECO) and the long cooling trend leading to the glacial state starting in the early Oligocene. We identify two hiatuses, in the lower middle Eocene and across the Eocene-Oligocene Transition (EOT). Across the latter unconformity, a major transition from oligotrophic to eutrophic favoring nannofossil taxa highlights the enhanced sea surface nutrient availability during the transition to the early Oligocene glacial system. Finally, a late Oligocene warming event is recorded at this site by the increase in calcareous nannofossil taxa that preferred warm water.