Extinction, Turnover, and the Reorganization of Diatom Communities Across the Eocene/Oligocene Boundary: Equatorial Atlantic Perspective

Abstract. Marine diatoms couple the global carbon and silicon cycles, and their fossil record tracks oceanographic and climatic changes in deep-time. The Eocene/Oligocene Transition (EOT) marks the onset of Antarctic glaciation and major ocean reorganization and is a key interval in diatom evolutionary history. Although high-latitude plankton responses to polar cooling are extensively studied, it remains challenging to determine how cooling-driven changes in circulation, stratification and nutrient supply propagated and shaped low-latitude assemblages. Here we reconstruct species-level diatom diversity from exhaustive full-assemblage counts and integrate these data with diatom and radiolarian productivity from Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, equatorial Atlantic) spanning 38–32 Ma. Diatom diversity at DSDP 366 varies in step with Southern Ocean diversity records across the same interval. Extinction rates and community-structure metrics indicate a major reorganization of tropical diatom communities that is consistent with changes in upper-ocean stratification. We identify a sharp shift in community structure at ~33.5 Ma, pointing to a rapid ecological response in the earliest Oligocene.