Low-latitude biostratigraphy and diversity of planktonic foraminifera from the middle Eocene to early Oligocene

The middle Eocene through early Oligocene was an important interval for Cenozoic climate evolution, having a substantial impact on global palaeoceanography and the biosphere. At the Eocene–Oligocene Transition (EOT), planktonic foraminifera experienced their highest extinction rates since the Cretaceous–Paleogene mass extinction, but the exact extinction mechanisms are poorly constrained. Low-latitude sites that span the EOT are particularly rare in part because of poor preservation of carbonate in many ocean basins in the Eocene. Here we present new planktonic foraminiferal assemblage and biostratigraphic data from the Foz do Amazonas Basin located in the western equatorial Atlantic Ocean, shedding light on the biotic response of tropical planktonic foraminifera to long-term planetary cooling and the establishment of Antarctic glaciation. The samples yielded a rich planktonic foraminiferal assemblage totalling 116 species, enabling the recognition of three Cenozoic tropical planktonic foraminiferal zones (E9, E10, E14) across the middle Eocene–early Oligocene (∼ 44–34 Ma), with several intervals undifferentiated. Assemblages indicate increased upwelling and eutrophication of surface waters possibly associated with fluctuations within water column structure across the EOT. These alterations are likely associated with regional and global perturbations within oceanic circulation and palaeoceanographic variations attributable to the Antarctic glaciations of the earliest Oligocene. The effects of Cenozoic cooling are seen within the planktonic foraminiferal assemblages, wherein a reduction in symbiotic mixed-layer taxa is accommodated by an increase within sub-thermocline dwellers consistent with substantial restructuring of oceanic stratification through the EOT and cold-water expansion.