CALCAREOUS NANNOPLANKTON RESPONSE AND POPULATION RESTRUCTURING ACROSS THE EOCENE-OLIGOCENE TRANSITION

The Eocene-Oligocene transition (EOT) was a profound climatic interval marking the culmination of the long term Paleogene shift from a ‘greenhouse’ to an ‘icehouse’ world. The EOT saw disruption to physical, chemical and biological parameters in the marine realm. Significant global change in this interval is highlighted by prominent positive shifts in oxygen isotope records, evidence for initiation and expansion of significant Antarctic ice sheets and subsequent global sea level fall, >1 km deepening of the calcite compensation depth and high levels of marine biotic disturbance, with elevated rates of plankton extinction and turnover.

Here we will document the response of the calcareous nannoplankton to the EOT using the recently drilled section at IODP Site U1411, which has high sedimentation rates and exceptionally well preserved calcareous nannofossils. Semiquantitative data and relative abundance counts highlight diversity change, species bioevents and major shifts in abundance patterns. Determining the precise timing of these events allows us to examine the relationship between plankton evolution and the strongly shifting paleoceanographic conditions in the North Atlantic.

To place these new observations into a wider context we have compiled calcareous nannofossil data from a range of global sites across the EOT that vary in latitude, ocean basin and oceanographic setting. Our results identify significant and coeval population restructuring within this phytoplankton group prior to the early Oligocene climatic shift. These data reveal that elevated rates of extinction and striking reorganization within the dominant reticulofenestrid group was a particular feature of the interval, reflecting the major climatic/oceanographic changes. The evidence suggests that elevated rates of extinction are associated with a reduction of optimal habitat space for many coccolithophores and that global population shifts reflected increases in taxa that favored elevated levels of nutrients or pulsed delivery of nutrients or with more efficient carbon utilisation strategies under declining atmospheric pCO₂.