We reconstruct the calcareous nannofossil response to the Eocene-Oligocene Transition (EOT)-the most significant climate transition of the Cenozoic-in the Indo-Pacific Warm Pool. Data from south central Java consist of species relative abundance counts of well-preserved nannofossil assemblages. From the late middle Eocene to early Oligocene species biodiversity declined, with the most rapid species loss occurring across the latest Eocene rosette-shaped discoaster extinction event (DEE;~34.44-34.77 Ma). A decline in abundance of oligotrophic indicator taxa across the DEE indicates increased nutrient supply to the tropical surface ocean in the early stages of the EOT. The mean lith size of reticulofenestrids also increases across the DEE driven by a marked reduction in the abundance of small Reticulofenestra morphotypes (<3.5 μm). There is no preferential loss of larger Reticulofenestra cell sizes (coccoliths > 8 μm) across the EOT, indicating that coccolith size was apparently not limited by atmospheric CO 2 concentrations at this time. Overall, the main phase of tropical phytoplankton ecosystem change preceded the interval of rapid Antarctic ice sheet growth and is closely associated with the biotic perturbations that define the start of the EOT. This suggests that enhancement of Southern Ocean controls on tropical ocean biogeochemistry and nutrient pathways may have played a role in triggering the transition to an icehouse climate state.

Abstract. The fossil record of marine microplankton provides insights into the evolutionary drivers which led to the origin of modern deep-water plankton, one of the largest components of ocean biomass. We use global abundance and biogeographic data, combined with depth habitat reconstructions, to determine the environmental mechanisms behind speciation in two groups of pelagic microfossils over the past 15 Myr. We compare our microfossil datasets with water column profiles simulated in an Earth system model. We show that deep-living planktonic foraminiferal (zooplankton) and calcareous nannofossil (mixotroph phytoplankton) species were virtually absent globally during the peak of the middle Miocene warmth. The evolution of deep-dwelling planktonic foraminifera started from subpolar–mid-latitude species, during late Miocene cooling, via allopatry. Deep-dwelling species subsequently spread towards lower latitudes and further diversified via depth sympatry, establishing modern communities stratified hundreds of metres down the water column. Similarly, sub-euphotic zone specialist calcareous nannofossils become a major component of tropical and sub-tropical assemblages during the latest Miocene to early Pliocene. Our model simulations suggest that increased organic matter and oxygen availability for planktonic foraminifera, and increased nutrients and light penetration for nannoplankton, favoured the evolution of new deep-water niches. These conditions resulted from global cooling and the associated increase in the efficiency of the biological pump over the last 15 Myr.

The Nanggulan section in south central Java comprises open marine sediments and volcanic deposits of Eocene-Oligocene age that accumulated in a marginal basin within the young Sunda Arc complex. A new borehole captures the stratigraphy and showcases the exceptional preservation of calcareous microfossils across an apparently complete Eocene-Oligocene Transition (EOT), a time interval significant for the initiation of continental scale glaciation on Antarctica. Low-resolution benthic and planktonic foraminifera oxygen and carbon stable isotopes (δ18O and δ13C) record increasing δ18O and δ13C in the basal Oligocene, allowing correlation to global records. Isotopic values imply warm temperatures and relatively high nutrients along the SE Java margin. The Nanggulan EOT is a valuable archive for reconstructing ocean-climate behavior and plankton evolution and extinction in the Indo-Pacific warm pool. The borehole also adds to understanding of the early stages of Sunda Arc volcanism.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5429453

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Abstract. The fossil record of marine microplankton provides insights into the evolutionary drivers which led to the origin of modern deep-water plankton, one of the largest component of ocean biomass. We use global abundance and biogeographic data combined with depth habitat reconstructions to determine the environmental mechanisms behind speciation in two groups of pelagic microfossils over the past 15 million years. We compare our microfossil datasets with water column profiles simulated in an Earth System model. We show that deep-living planktonic foraminiferal (zooplankton) and calcareous nannofossil (mixotroph phytoplankton) species were virtually absent globally during the peak of the middle Miocene warmth. Evolution of deep-dwelling planktonic foraminifera started from subpolar-midlatitude species during late Miocene cooling, via allopatry. Deep-dwelling species subsequently spread towards lower latitudes and further diversified via depth sympatry, establishing modern communities stratified hundreds of meters down the water column. Similarly, sub-euphotic zone specialist calcareous nannofossils become a major component of tropical and sub-tropical assemblages through the latest Miocene to early Pliocene. Our model simulations suggest that increased organic matter and oxygen availability for planktonic foraminifera, and increased nutrients and light penetration for nannoplankton, favored the evolution of new deep water niches. These conditions resulted from global cooling and the associated increase in the efficiency of the biological pump over the last 15 million years.