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The long-term cooling trend from middle to late Eocene was punctuated byseveral large-scale climate perturbations that culminated in a shift to"icehouse" climates at the Eocene–Oligocene transition. We presentradiolarian micro-fossil assemblage and foraminiferal oxygen and carbonstable isotope data from Deep Sea Drilling Project (DSDP) sites 277, 280,281, and 283 and Ocean Drilling Project (ODP) Site 1172 to identify significantoceanographic changes in the southwest Pacific through this climatetransition (~ 40–30 Ma). We find that the Middle EoceneClimatic Optimum at ~ 40 Ma, which is truncated but identifiedby a negative shift in foraminiferal δ<sup>18</sup>O values at Site 277,is associated with a small increase in radiolarian taxa with low-latitudeaffinities (5 % of total fauna). In the early late Eocene at~ 37 Ma, a positive oxygen isotope shift at Site 277 iscorrelated with the Priabonian Oxygen Isotope Maximum (PrOM). Radiolarianabundance, diversity, and preservation increase within this cooling event atSite 277 at the same time as diatom abundance. A negative δ<sup>18</sup>Oexcursion above the PrOM is correlated with a late Eocene warming event(~ 36.4 Ma). Radiolarian abundance and diversity declinewithin this event and taxa with low-latitude affinities reappear. Apart fromthis short-lived warming event, the PrOM and latest Eocene radiolarianassemblages are characterised by abundant high-latitude taxa. High-latitudetaxa are also abundant during the late Eocene and early Oligocene(~ 38–30 Ma) at DSDP sites 280, 281, 283 and 1172 and areassociated with very high diatom abundance. We therefore infer a northwardexpansion of high-latitude radiolarian taxa onto the Campbell Plateau in thelatest Eocene. In the early Oligocene there is an overall decrease inradiolarian abundance and diversity at Site 277, and diatoms are scarce.These data indicate that, once the Antarctic Circumpolar Current wasestablished in the early Oligocene (~ 30 Ma), a frontal systemsimilar to present day developed, with nutrient-depleted Subantarcticwaters bathing the area around DSDP Site 277, resulting in a more restrictedsiliceous plankton assemblage

Expansion and diversification of high-latitude radiolarian assemblages in the late Eocene linked to a cooling event in the southwest Pacific