Unraveling Southern Ocean diatom diversity across the Eocene–Oligocene transition

Abstract. The Eocene–Oligocene transition (EOT) was a critical interval of global cooling and circulation change that reshaped marine ecosystems. However, current knowledge of diatom diversity and community dynamics during this interval relies mainly on biostratigraphic compilations, which largely document common species and thus likely underestimate true diversity. This study provides a more complete picture of Southern Ocean diatom communities across the EOT by using comprehensive quantitative surveys from four sites spanning the Atlantic and Indian Ocean sectors. Results reveal a consistent pattern across sites, with two major shifts in community composition around 37–36 and 34 Ma, linked to major environmental change. Following a prolonged interval of diversification without substantial extinction in the late Eocene, we identify a pronounced extinction event approximately 1 Myr after the Eocene–Oligocene boundary (E–O, ∼33.9 Ma). This suggests that significant changes in surface ocean conditions lagged the boundary and varied across the Southern Ocean. These findings highlight the interplay between global climate changes and local environmental factors in shaping diatom communities. By capturing detailed patterns of diatom diversity and extinction, our study improves our understanding of past ecological responses.