BIOTIC DRIVERS OF MARINE BIOGEOCHEMISTRY AND CLIMATE: LESSONS FROM THE END-CRETACEOUS PLANKTON EXTINCTION (Invited Presentation)

Marine plankton regulate the “biological pump”: the process that redistributes carbon and nutrients throughout the ocean via organic matter formed during photosynthesis. As such, plankton ecosystems are critical to the partitioning of carbon dioxide (CO₂) between ocean and the atmosphere and the rate at which it can be geologically removed via burial in marine sediments. However, the links between the structure and function of planktic ecosystems in the ocean and the strength of the biological pump, as well as the resilience of the biological pump to disruption, are poorly understood. The aftermath and recovery following the extinction at the Cretaceous/Palaeogene (K/Pg) boundary provides an excellent opportunity to quantify this link as both ecosystems and the biological pump were fundamentally disrupted. A new record of planktonic foraminifera as a tracer of K/Pg pelagic biotic recovery, combined with the carbon isotope record of the biological pump, shows that the recovery of full ecological function occurred much later (~2-4 m.y.) than the recovery of the biological pump.

We explore this observed decoupling further using a trait-based plankton community model coupled with an Earth System model. The model predicts emergent plankton communities by linking eco-physiological traits of plankton, such as nutrient uptake and grazing rates, to plankton size via fundamental allometric relationships. We impose two key features observed in the fossil record associated with the K-Pg extinction event: a systematic loss of larger cell sizes and disruptions to trophic interactions. The model predicts a similar decoupling between the biological pump and plankton ecosystems, as observed in the K-Pg aftermath. This is robust to a range of assumptions in the model and arises from the characteristics and global distribution of smaller plankton. However, the response of atmospheric CO₂ and marine biogeochemistry to the most extreme extinction scenarios tested varies widely dependent on the impact on grazers, suggesting they may have played a role in the long-term recovery of ecosystems and climate. Our results provide an example of how plankton ecosystem models can be applied to help interpret and reconcile trends in the proxy and fossil records.