LAUSR

The fossil record reveals large changes in marine plankton ecosystems linked with both environmental and ecological change across the Cenozoic. An understanding of the drivers of these changes is key to understanding the marine carbon cycle. The response of plankton ecosystems in past warm climates also provides a key analogue for current climate change. While models are employed to quantify interactions between the environment and the biota, current Earth system models strongly encode our understanding of modern marine ecosystems. By contrast, trait-based models aim to describe the marine plankton ecosystem in terms of fundamental ecological and physiological rules that are less likely to change through time. This provides a unique opportunity to assess the interactions between marine ecosystem and paleoclimate. For the first time, we apply a size-structured trait-based plankton ecosystem model embedded in the Earth system model of intermediate complexity, cGENIE, to model plankton communities for the warm climate of the early Eocene. Compared to modern, we find the warm climate is associated with an increase in the mean cell size of plankton communities and export production, particularly in the southern high latitudes, along with lower total phytoplankton biomass. Paleogeography has an important role in regulating the effect of ecosystem structure via changes in ocean circulation and nutrient cycling. Warmer temperatures also drive changes due to enhanced zooplankton grazing. An integration of the fossil record with plankton ecosystem models will provide a powerful tool to assess the impacts of warm climates on marine systems.