Planktic foraminiferal abundance and test-size record across the Early Eocene Climatic Optimum (EECO, ~53- 49 Ma) at Shatsky Rise (Pacific Ocean)

The dynamic early Paleogene climate presents the crucial opportunity to detect relationships among calcareous plankton productivity, past carbon cycle perturbations and climate. We focus on the Early Eocene Climatic Optimum (EECO, ~53-49 Ma) as it is associated with peak temperature and pCO2 of the Cenozoic and therefore offers a longterm perspective of global warming impacts on marine biota. We investigate the Pacific sites 1209-1210, complementing evidence that the EECO markedly impacted planktic foraminiferal assemblages at the Atlantic Oceans. The selected sites have an excellent age model and stable isotope constraints that document the carbon isotope excursions that are the expression of the several hyperthermals events superimposed on the long-term warming. We record an abrupt and permanent abundance decline of more than one-third for the symbiont-bearing genus, Morozovella, from the beginning of the EECO (J event, ~53 Ma) at sites 1209-1210, whereas Acarinina concomitantly increased in agreement with Atlantic sites. One possible cause of the morozovellid abundance decline is a negative impact on their photosymbiotic relationships with algae, as suggested by the lower 13C values exhibited by survivors. Symbiosis is advantageous in the oligotrophic mixed-layer habitat and increases growth and final size. Therefore, we hypothesise that the morozovellid decline may have reduced planktic foraminiferal productivity and test-size. We evaluated the Coarse Accumulation Rate (CFAR) as an approximation of the planktic foraminiferal accumulation rate. The >38 m dominantly consists of planktic foraminifera in pelagic sediments and here is not significantly affected by dissolution which would reduce CFAR. In addition, we perform a test-size analysis on planktic foraminiferal assemblages across the EECO. Changes in the size of marine calcifiers can be caused by evolutionary mechanisms or environmental perturbance, both of which represent potential consequences of extreme warmth. Earlier work has shown that morozovellids are generally larger than acarininids, hence suggesting that size should decrease when morozovellid abundance drops. Samples were split into aliquots of 1000–1500 specimens and imaged at 160x magnification. The morphological parameters were analysed in Olympus Stream Motion, and the 95th percentile of the maximum diameter was calculated. Unexpectedly, our results show that the planktic foraminiferal assemblages do not display test-size reduction. Overimposed to a long-term general test size increase trend, we recorded test-size fluctuations across the short-term hyperthermals. The increase of planktic foraminiferal sizes across the hyperthermals may be linked to increases in the abundance of acarininids and decreases of morozovellids, which is the common pattern across these events. CFAR decreases across the EECO during the morozovellid abundance reduction, suggesting that this decline impacted overall planktic foraminiferal productivity and was seemingly not balanced by the acarininids. Future test-size analysis of the two groups involved will help to unravel a more exhaustive response to the EECO perturbance of planktic foraminiferal assemblage