Reticulofenestra daviesii: Biostratigraphy and paleogeographic distribution across the Eocene–Oligocene boundary

Improving the biostratigraphy across the Eocene–Oligocene is fundamental to better constrain thetiming and causes of an important global climate change of the Cenozoic, the Eocene–OligoceneTransition (EOT; 34–33.5 Ma). Across the EOT, only few nannofossil bioevents are considered globallysynchronous and reliable. One of these is the first common occurrence (Bc) of the species Reticulofenestradaviesii that has been proven to be useful for biostratigraphical correlations in the Southern Ocean, butthe potential of R. daviesii as a biostratigraphical marker at mid- and low latitudes has not been exploredyet in detail. We investigate three deep-sea drill sites located across a N-S transect at mid-low latitudesof the Atlantic Ocean spanning from 34.4 to 33 Ma, reviewing the temporal and geographical distributiontogether with the intraspecific variability of R. daviesii. Our data quantify the occurrence of R. daviesii in(sub)tropical regions of the Atlantic Ocean, although with lower abundances ( 4–12%) compared toSouthern Ocean assemblages (40–95%). This suggests that R. daviesii was a cosmopolitan species capableto adapt to a wider range of sea surface temperatures and environmental conditions than previouslythought. However, the temporal distributions of R. daviesii at the three studied sites are not comparableto the trends recorded in the Southern Ocean. Its Bc is clearly identifiable only at the equatorial site,occurring 500,000 years before the age estimated in the Southern Ocean (33.705 Ma). Thus, we suggestcaution when using the Bc of R. daviesii as a reliable biostratigraphical event at mid- and low latitudes. Inaddition, our biometrical data reveal that up to 38% of R. daviesii coccoliths is > 8–10 mm in major axissize, thus bigger than the medium size range (5–8 mm) originally described. Refining the size range ofR. daviesii is important for estimating its cell volume versus surface area and coccolith carbonate mass.