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Low‐Latitude Calcareous Nannofossil Response in the Indo‐Pacific Warm Pool Across the Eocene‐Oligocene Transition of Java, Indonesia

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 CO2 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.