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 stable isotope (C, O) record, which documents ocean-climate changes, and provides independent chemostratigraphy.The study section spans nannofossil Zones NP20-NP23 (equivalent to CNE20-CNO4) and lasts 5.6 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 δ 18 O shift -EOIS), which is consistent with previous studies, making Hole 756C one of a few sites globally boasting both the familiar stepped δ 18 O and δ 13 C 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.
The EOT is marked by a two-step shift in the δ18O record of benthic foraminifera. The transition was rapid (500 000 years) and occurred roughly 34 million years ago. It marks an important step in Earth’s climate history where semi-permanent ice-sheets developed over Antarctica and is considered the initiation of the modern glaciated climate that the Earth experience today. Ninetyeast Ridge is a hot spot trail in the Indian Ocean that was formed by the Keguelen/ Ninetyeast hotspot and was one of the drilling targets of ODP leg 121 and among other sites 757 and 756 were drilled. To gain a better understanding of the paleoceanography and better constrain the timing of the EOT in the Indian Ocean, benthic foraminifera stable isotopes in Cibicidoides havanensis was analysed at site 757B and Cibicidoides mundulus at site 756C and was then age calibrated using an age model. The results indicate that the significant two-step shift that characterizes the EOT is present at Site 757B. However the results also indicate that it is not sufficient to use an age model that assumes a constant sedimentation rate since it changed drastically at this site at the E-O boundary. Finally the results indicates that there may have been several water masses operating simultaneously in the Indian Ocean at the EOT, the water masses discussed as most likely in this thesis is AAIW and TISW.
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