Contrasting response in test size and abundance of planktic foraminifera Morozovella and Acarinina to the EECO in the subtropical Pacific Ocean

The EECO (~53–49 Ma) is characterized by the highest global average temperature and CO2 levels of the Cenozoic, providing the opportunity to explore the interplay between prolonged warmth, potential physiological stress, and marine planktic ecosystems, of which planktic foraminifera are a component. Previous studies have documented a dramatic decline in the dominant planktic foraminiferal genus Morozovella and a concurrent increase in Acarinina abundances at the onset of the EECO in the Atlantic, Tethys and Pacific Oceans. This study aims to investigate how extreme climatic conditions during the EECO influenced the abundance, size, and ecological strategies of planktic foraminifera, focusing on the response of the symbiont-bearing mixed-layer dweller Morozovella and Acarinina at Ocean Drilling Program (ODP) sites 1209–1210 Shatsky Rise, in the subtropical Pacific Ocean. We present species abundances, test size and δ13C data of Morozovella. Acarinina test size and δ13C data were quantified to assess whether observed changes affected all mixed-layer dwellers or were specific to Morozovella. Our results show a temporary increase in Morozovella test size at the EECO onset, partially linked to the dominance of larger species such as M. crater and M. aragonensis. In contrast, Acarinina displays a reduction in test size. The generally lower δ13C values across the EECO in Acarinina suggest a deeper habitat in the mixedlayer and/or reduced symbiosis relative to Morozovella. We speculate that Acarinina evaded the high temperatures by moving deeper in the mixed layer. The reduced symbiosis relationship due to diminished light in the deeper water column might have led to a smaller test size. In contrast, Morozovella maintained a shallower position in the mixed layer allowing it to sustain efficient symbiosis and larger size. However, the limited ecological flexibility of this taxon may have impeded its ability to adapt and maintain high abundance across the EECO.