Climate–ocean changes across late Eocene-early Oligocene recorded at IODP Site U1553 (Southern Pacific Ocean)

We studied calcareous nannofossil assemblages from sediment cores of IODP Site U1553 (Campbell Plateau) for adding information on the environmental/climate context in this Southern Ocean region during the late Eoceneearly Oligocene. The aim was to refine our understanding of conditions that led to the establishment of the permanent Antarctic ice sheet, through the study of a 5 million-years interval including the critical Eocene - Oligocene Transition (EOT). We performed a PCA to characterize the ecological preferences of counted species and used Temperate-Warm-Water Taxa index (TWWT index) and Eutrophic Index EI to delineate paleoenvironmental variability. Our results provide evidence on the composition of the nannofossil assemblage for the effects of events preceding the EOT, such as a change chronologically coinciding with the effects of the Popigai meteor impact (at 36.63 Ma) and a transitional cooling interval around 34.6 Ma, which we term “Pre-LEE” (Pre-Late Eocene Event). Within the EOT (starting from ~34.46 Ma), the observed changes in nannofossil assemblage delineate a Temperate Transition Interval (TTI), an interstadial lasting from ~34.3 to ~34.2 Ma, during which temperate conditions prevailed. Through the studied interval, species diversity varies in three main phases, and assemblagebased SST and trophic indicators record a general decline in temperate and oligotrophic taxa and an increase in cool-water and eutrophic taxa. The trend as delineated at Site U1553 shows differences with respect to other high southern latitudes sites, that seem to be related to the different paleoceanographic settings and, possibly, different paleobathymetry and paleotopography.