Impact of EECO Hyperthermals on Planktic Foraminiferal Communities

Due to their environmental sensitivity, planktic foraminifera are important proxies for reconstructing past climatic and oceanographic shifts, offering critical insights into marine ecosystem responses during intervals of rapid warming. Early Eocene hyperthermal (~50-200kyrs) episodes provide a unique opportunity to explore the effects of extreme heat on planktic foraminifera, a group of marine calcifiers highly sensitive to temperature, pH, and nutrient levels changes that may induce evolutionary modifications and variations in abundance and test size. Here we disentangle the global-scale impacts of Eocene Climatic Optimum (EECO,~53-49 Ma) hyperthermals on planktic foraminiferal communities, offering insights into their adaptability or loss of resilience under climatically related environmental stress.

We reveal significant modifications in PF assemblages during two major hyperthermals of the EECO, the J event (53.26 Ma), which marks the beginning of the EECO, and the K/X event (52.86 Ma). Following the J event, the symbiont-bearing genus Morozovella experienced a permanent decline in abundance and diversity at low-latitude locations while Acarinina thrived globally. At the same time, size in another symbiont bearing taxon Acarinina exhibits a reduction at tropical Pacific sites while Morozovella is becoming larger. Our stable isotope analyses reveal that Acarinina displays lower δ¹³C values than Morozovella, a feature likely linked to a reduction in photosymbiotic activity or change in symbiont and habitat deepening. This ecological strategy probably enabled Acarinina to succeed under the EECO environmental conditions.

Approximately 400kyr later, during the K/X event, a switch in Morozovella coiling direction – a dominance clockwise (dextral) or counterclockwise (sinistral) morphotypes- from dextral to sinistral was observed in the Atlantic, Pacific and Indian Oceans. This coiling shift represents a valuable biostratigraphic tool. It is   linked to an evolutionary turnover in Morozovella species, with M. crater and M. aragonensis becoming dominant. These two morphotypes may represent cryptic species, akin to living taxa, and their change can be interpreted as a further evolutionary change in the taxon. The K/X event is characterized by the disappearance of the Oxygen Deficient Zone (ODZ)-dwelling genus Chiloguembelina, indicating ODZ contraction during the EECO, as also supported by published Foraminifera-bound nitrogen isotope data from the Atlantic and tropical Pacific.

The J and K/X events appear to represent turning points for foraminiferal assemblages. δ¹³C records distinct long-term decrease (J) and increase (K/X), consistent with respectively enhanced and lowered fluxes in carbon inputs

Across the studied hyperthermals PF communities shifted to a new configuration that was never fully recovered, reflecting loss of resilience.

The environmental impacts of these relatively slow, compared to modern warming events underscore the potential for more severe and rapid ecosystem consequences in the modern context.  Our findings provide critical insights into the mechanisms and limits of marine ecosystem resilience.