Global Coiling Shifts in Morozovella and Ecological Resilience of Acarinina during the EECO

The Early Eocene Climatic Optimum (EECO; ~53–49 Ma) represents the prolonged interval with the highest temperatures and CO₂ levels of the Cenozoic with superimposed transient peak warming events (hyperthermals). The geological record provides a long-term perspective to current observations of marine ecosystem response to global warming. The EECO interval offers the opportunity to evaluate how global climatic shifts have influenced the resilience of planktic foraminifera, a key component of marine ecosystem.

Planktic foraminifera morphologic traits, including shifts in coiling direction - the ability to grow their chambers either clockwise (dextral) or counterclockwise (sinistral) - serve as highly sensitive indicators of environmental changes. This underscores their pivotal role in the study of past climate conditions.  Previous studies highlighted a permanent decline in the symbiont-bearing Morozovella abundance and diversity near the EECO onset and a coiling shift from dextral to sinistral during the K/X event in the Atlantic Ocean.

Here, we extend the coiling direction record to tropical Pacific (Shatsky Rise, Sites 1209–1210), southern Pacific (Tasman Sea, Site U1510), and Indian Ocean locations (Exmouth Plateau, Hole 762C). Our results reveal that the switch to sinistral coiling in Morozovella occurred at all the studied sites thus it appears globally recorded within the last ~200 kyrs after the K/X event. This evidence emphasizes the utility of this coiling shift as a valuable biostratigraphic tool. The Morozovella species-specific analysis discloses that the dominant M. aragonensis and M. crater significantly contributed to the coiling switch in the Atlantic, tropical Pacific, and Indian Oceans. Regardless sinistral and dextral Morozovella forms indicate cryptic speciation or morphotypes within the same species, our record implies that this interval favoured sinistral forms, so that he morozovellid decline in abundance can be largely read as the decline of dextral morphotypes. Notably, Acarinina exhibits no coiling preference.

Stable isotope analysis on dextral and sinistral Acarinina and Morozovella morphotypes can shed light on the intricate ecological dynamics of planktic foraminifera during the EECO. Sinistral Morozovella have lower δ ¹³C values across the EECO with respect to the pre-EECO interval, with both dextral and sinistral Acarinina showing even lower values. This suggests that Acarinina occupied a deeper habitat within the mixed layer and/or had reduced symbiotic activity. This ecological strategy may have ensured the Acarinina success, allowing it to thrive during the EECO, but only partially advantaged the sinistral morozovellids forms, which survived with respect to dextral morphotypes but only in small abundance.

Within the first ~600 kyr of the EECO, morozovellids declined in abundance and changed their coiling direction. The scenario recorded in this research delineates on how planktic foraminifera adapted—or struggled— in response to extreme warmth, a crucial result for a future climatic perspective.