Major shifts in low-latitude calcareous nannofossil assemblages across the Early Eocene Climatic Optimum (~5349 Ma)

The Early Eocene Climatic Optimum (EECO) was the warmest sustained episode of global warming during the Cenozoic, accompanied by major alterations in land-based and marine biota. Initially identified through stable oxygen isotope minimum values between ~5250 Ma (herein labelled peak-EECO phase) and later extended to a broader timeframe (5349 Ma) anchored on stable carbon isotope excursions, the EECO provides a crucial window for exploring the long-term, macroevolutionary consequences of warm climates on marine primary producers. The fossil remains of coccolithophores and other calcareous nannoplankton have been studied previously in the mid- and high latitudes, where the EECO is characterized by a transition from assemblages dominated by the genus Toweius (Prinsiaceae) to the enduring presence of the genus Reticulofenestra (Noelaerhabdaceae), as is still the case for their descendants in modern assemblages (Gephyrocapsa spp. and Emiliania huxleyi). Using a newly collected nannofossil record from the equatorial Atlantic (ODP Site 1258), we detail changes in low-latitude calcareous nannofossil assemblages throughout the various stages of the EECO and the subsequent early to middle Eocene cooling transition (EMET). The decline in Toweius spp. occurred in two steps: first, at the start of the peak-EECO phase (~52 Ma), with abundance plummeting to about one-third of previous levels, followed by its final and permanent decline and the first continuous occurrence of Reticulofenestra spp. at the end of the peak-EECO phase (~50 Ma). The EECO is also marked by a broad acme of Discoaster spp., as previously reported at several sites. Here we also report on distinct abundance increases in Campylosphaera, Umbilicosphaera and Calcidiscus. These genera declined in abundance by the conclusion of the EECO (~49 Ma) in conjunction with the rapid and sustained expansion of Reticulofenestra, marking the EMET. Multivariate statistical analysis of nannofossil datasets at Site 1258 and sites from higher latitudes highlights the occurrence and prevalence of specialist taxa exclusively in the tropics, revealing a distinct tropical signature atop the previously identified latitudinal expansion of (sub)tropical taxa during the EECO. Compositional contrasts between the tropical and higher-latitude sites diminished significantly after the EECO, coinciding with the decline of taxa with inferred high thermal optima in the tropics. Our combined results suggest the highest biogeographical differentiation of tropical nannoplankton assemblages from the subtropics (e.g., ODP Sites 1263 and 1210) during the EECO, contrary to some expectations related to a much flatter meridional thermal gradient. The restructuring of the nannoplankton communities after the EECO, however, points to increased connectivity and dispersal between the two regions. It is important to explore the regional driving forcings (e.g., ocean circulation, temperature, nutrient availability, and biotic interactions) on local phytoplankton community structures in the tropics in order to understand broadscale changes in biogeographical and macroevolutionary patterns.