Response of nannoplankton to early Eocene ocean destratification

Nannoplankton play a significant role in the global carbon cycle; changes in the composition of the assemblage can influence primary productivity, carbon burial, and ultimately climate. Here counts of nannofossils in Eocene samples document one of the largest nannoplankton assemblage turnovers of the Cenozoic Era. This event is marked by the global expansion of Reticulofenestra at the termination of the early Eocene climatic optimum (EECO), and continues through the progressive cooling to the earliest Oligocene glaciation. To understand the mechanisms driving this assemblage shift we use multivariate statistical techniques and carbon and oxygen isotope records from localities worldwide. We find that the turnover in the global nannofossil assemblage is tied to thermal destratification of the ocean and a coincident decrease in the efficiency of the oceanic biologic pump at the height of the EECO. We propose that increased vertical mixing and increased nutrient supply to surface waters during the EECO caused a shift in the trophic structure towards enhanced productivity, influencing the global nannoplankton population.

▶ We use calcareous nannofossil counts to track assemblage change during the Eocene. ▶ The early Eocene was marked by a large shift in the nannoplankton community. ▶ The genus Reticulofenestra rapidly expands during the early Eocene. ▶ Isotopes show a parallel decline in biological pumping efficiency and stratification.