Quantitative Calcareous Microfossil Diversity Fluctuations across the Eocene/Oligocene Boundary in Tanzania: A Comparison Between the Phytoplankton and Zooplankton Response

The Eocene/Oligocene (E/O) boundary interval is characterised by extinction and turnover within marine and terrestrial organisms, a significant drop in the calcite compensation depth and a >1 per mil increase in oxygen isotopes reflecting cooling and the establishment of a major ice cap on Antarctica. While this interval has been recovered in marine drill cores from multiple locations, the preservation of calcareous microfossils has hindered documentation of the biotic response of nannoplankton and planktonic foraminifera across the greenhouse-icehouse transition. Drill holes through the E/O boundary in southern Tanzania contain exceptionally well preserved and diverse assemblages of calcareous microfossils. Here we document a high-resolution quantitative record of assemblage and diversity fluctuations in planktonic foraminifera and calcareous nannoplankton through this critical interval, which is often dissolved and/or recrystallised in carbonate-rich facies. We compare diversity fluctuations between the zooplankton (planktonic foraminifera) and phytoplankton (calcareous nannofossils).

The E/O boundary is marked by the abrupt extinction of all remaining species of the family Hantkeninidae and a distinct size reduction in the genus Pseudohastigerina. Quantitative analysis of assemblages reveals significant changes in the abundance of certain species and the composition of the assemblages. A major shift in evenness within the planktonic foraminifera occurs at ~33.8 Ma coupled with the extinction of the Turborotalia cerroazulensis group. Within the calcareous nannoplankton two key transitions occur. The first transition, at ~34.0 Ma, coincides with a distinct interval of very low nannofossil abundance. The second, at ~33.63 Ma, is immediately after the E/O boundary and is associated with a major drop in nannofossil diversity. Both nannoplankton transitions involve significant reductions in the abundance of holococcoliths and other oligotrophic taxa. These changes in calcareous plankton assemblages indicate a widespread acute ecological disturbance and significant perturbations to the low-latitude surface ocean.