Calcareous Nannofossils and Paleoclimatic Evolution Across the Eocene‐Oligocene Transition at IODP Site U1509, Tasman Sea, Southwest Pacific Ocean

The Eocene-Oligocene transition (EOT; similar to 34 Ma) was one of the most prominent global cooling events of the Cenozoic, coincident with the emergence of continental-scale ice-sheets on Antarctica. Calcareous nannoplankton experienced significant assemblage turnover at a time of long-term surface ocean cooling and trophic conditions, suggesting cause-effect relationships between Antarctic glaciation, broader climate changes, and the response of phytoplankton communities. To better evaluate the timing and nature of these relationships, we generated calcareous nannofossil and geochemical data sets (delta 18O, delta 13C and ÊCO3) over a similar to 5 Myr stratigraphic interval recovered across the EOT from IODP Site U1509 in the Tasman Sea, South Pacific Ocean. Based on trends observed in the calcareous nannofossil assemblages, there was an overall decline of warm-oligotrophic communities, with a shift toward taxa better adapted to cooler more eutrophic conditions. Assemblage changes indicate four distinct phases caused by temperature decrease and variations in paleocurrents: late Eocene warm-oligotrophic phase, precursor diversity-decrease phase, early Oligocene cold-eutrophic phase, and a steady-state cosmopolitan phase. The most prominent shift in the assemblages occurred during the similar to 550 kyr-long precursor diversity-decrease phase, which has relatively high bulk delta 18O and ÊCO3 values, and predates the phase of maximum glacial expansion (Earliest Oligocene Glacial Maximum-EOGM).Around 34 Ma, Earth experienced a major glaciation in the southern hemisphere, leading to the formation of permanent continental-scale ice-sheets on Antarctica. The Antarctic glaciation is supposed to have been triggered by a gradual decline of carbon dioxide with tectonic changes playing a secondary role. The separation of Antarctica from Australia and South America, and thus the development of the cold Antarctic Circumpolar Current, may have further intensified the conditions necessary for glacial expansion. This new scenario had profound consequences on climate, with direct effects on the global oceans and marine biota. In the past decades, geoscientists have tried to understand the glacial, tectonic and climatic history of Antarctica, but many questions remain unresolved. In particular, what were the impacts on the oceans in key areas between the Southern Ocean (high-latitudes) and the Equator (low-latitudes), such as the Tasman Sea? We use changes observed in the fossil record of calcifying unicellular algae (calcareous nannofossils) and stable oxygen and carbon isotopes to reconstruct the past surface-ocean evolution of this area. Our results suggest that biological productivity increased as cooler surface-water conditions developed in the Tasman Sea. This was likely driven by invigorated northward-moving nutrient-rich bottom-waters formed in the colder circum-Antarctic area.Significant changes in calcareous nannofossil assemblages occurred at the onset of the Eocene-Oligocene transition (EOT; similar to 34 Ma) Calcareous nannofossil diversity decreased during the EOT and warm-oligotrophic taxa were replaced by cold-eutrophic taxa Increased paleoproductivity and surface ocean cooling are inferred from the calcareous nannofossil assemblage