Calcareous nannofossils across the Eocene-Oligocene transition at Site 756 (Ninetyeast Ridge, Indian Ocean): implications for biostratigraphy and paleoceanographic clues

The timing and modalities of calcareous phytoplankton community and evolutionary responses to the Eocene-Oligocene transition (EOT, ~34 Ma) are still under-investigated. In order to better constrain the dynamics of these pelagic primary producers during the climate transition, we conducted high resolution assemblage analysis on calcareous nannofossils across a ~19 m-thick interval of nannofossil ooze at Ocean Drilling Program (ODP) Site 756 (Ninetyeast Ridge, Indian Ocean; Peirce et al. 1989) (paleolatitude ~43° S; Zachos et al. 1992). We explored the diversity patterns against a new integrated planktonic foraminifera and calcareous nannofossil biostratigraphy produced for the site, as well as new benthic foraminifera and bulk sediment stable isotope (C, O) records, which documents ocean-climate changes, and provides independent chemostratigraphy. The study section spans nannofossil Zones NP20-NP23 (equivalent to CNE20-CNO4) and lasts 5.5 Myr. The results show that the hankeninid extinction falls within the ~4.5 m-thick EOT isotopic interval (0.67 m below the base of the second positive δ18O shift – EOIS), which is consistent with previous studies, making Hole 756C one of a few sites globally boasting both the familiar stepped δ18O and δ13C structure of the EOT and the primary biostratigraphic marker defining the base of the Oligocene. A series of potentially useful new calcareous nannofossil bioevents were identified that could help improve dating and correlation of this crucial interval. In this context, changes in calcareous nannofossil assemblages observed across EOT are interpreted in terms of modifications of paleoecological parameters that typically control the abundance and distribution of different taxa. Variations in sea surface temperature and nutrient availability are considered to be the most likely triggers for the calcareous phytoplankton changes observed across EOT. Specifically, our data suggest that increased nutrients in the mixed layer played a key role in shaping the late Eocene – early Oligocene calcareous nannofossil assemblages

Studio biostratigrafico e paleoecologico delle associazioni a nannofossili calcarei alla transizione Eocene-Oligocene al Site ODP 756 (Ninetyeast Ridge, Oceano Indiano)

Calcareous nannofossil biostratigraphic and paleoecological study at the Eocene Oligocene Transition at ODP Site 756 (Ninetyeast Ridge, Indian Ocean

Calcareous nannofossils, bulk and benthic foraminifera stable isotope from ODP Hole 121-756C

The Eocene-Oligocene transition (EOT, ~34 Ma) is marked by changes in calcareous nannofossil assemblages and coincides with variations in sea-surface temperature and nutrient availability. During the EOT, calcareous nannofossil assemblages underwent an abrupt turnover with a permanent decline of warm-oligotrophic taxa, a decrease in species diversity and a remarkable increase in eutrophic taxa. Here, we combined calcareous nannofossil and planktonic foraminifera biostratigraphy from ODP Hole 756C (Ninetyeast Ridge, Indian Ocean), as well as new benthic foraminifera and bulk sediment stable isotope (C, O) records, which documents ocean-climate changes and provides independent chemostratigraphy. Our results provide further support to previous records documenting an invigorated Southern Ocean circulation associated with the transport of cooler and nutrient-enriched waters toward low-middle latitudes