Phytoplankton Communities across the Eocene-Oligocene Transition: A Paleo-Atlantic Meridional Transect

Marine phytoplankton play a fundamental role in marine ecosystems and are sensitive to changes in ocean temperature and associated ocean properties (such as dissolved CO₂ and nutrient availability). Fossil time series recovered from the deep-sea are unique archives of the long-term adaptation and evolution of marine algae with mineralized parts, such as coccolithophores and diatoms. For example, ample evidence exists for long-term compositional overturn and extinctions in marine plankton communities across the Eocene Oligocene transition (EOT; ~34.5-33.7 Ma), when a globally warm and largely ice-free climate shifted to an overall cooler state with major ice sheets on Antarctica. Early studies already highlighted how coccolithophore species compositions and their latitudinal contrasts drastically changed from the late Eocene to the early Oligocene. Here, we revisit these meridional gradients in species composition across a north-south transect in the Atlantic and Southern Ocean, in order to detail the cell size distribution of ancient coccolithophores and to determine the timing of phytoplankton community shifts on a common age scale. Calcareous nannofossil census counts confirm the existence of distinct regional signatures and ecological gradients between sites. Coccolithophore communities in the Southern Ocean stood out with lowest species richness and largest cells, whereas the Atlantic sites hosted more species with smaller cells. A decrease in mean cell size across the EOT was most pronounced in the Southern Ocean, where communities became dominated by medium-sized Reticulofenestra daviesii during the early Oligocene. In the Atlantic, phylogenetically related taxa (small Reticulofenestra spp. and Cyclicargolithus floridanus) increased in prominence in the cooler and glaciated world. The compositional changes and decrease in mean cell size of common taxa are consistent with increased cellular growth rates, major changes in the mixed layer depth and (seasonally) increased nutrient entrainment into the upper photic zone. This is supported by regional gradients in δ¹³C between surface- and deep-sea carbonates (indicating alleviation of nutrient-limitation) and abrupt increases in siliceous microfossils in the Southern Ocean and equatorial Atlantic sedimentary archives, although the latter may relate to changes in seafloor preservation of silica because of changes in deep water mass properties.