MIND THE GAP: TARGETING BIOMINERALISATION IMPACT OF CLIMATE CHANGE ON COCCOLITHOPHORES

Current carbon dioxide emissions are an assumed threat to oceanic calcifying plankton (coccolithophores) not just due to rising sea surface temperatures but also because of ocean acidification (OA). Despite the complexity of response being revealed by culture experiments, there is still a widespread perception that coccolithophore calcification will be inhibited by OA. As these plankton have an excellent fossil record, we can test for the impact of OA during geological carbon cycle events, providing the added advantages of exploring entire communities, across real-world major climate perturbation and recovery. We have targeted the Paleocene-Eocene Thermal Maximum (PETM, 56 million years ago), a transient carbon release event characterized by 4-5°C of surface ocean warming with a ~0.3 pH unit decrease and our closest geological analogue to modern fossil fuel burning. There is currently little support for significant OA effects during this event, with some evidence of coccolith thinning and putative skeletal malformation, but plentiful evidence for temperature and nutrient-availability controlled migration and population composition changes. There is thus a need to identify and obtain data that provide a direct test for OA response but this is particularly challenging because of the difficulty in identifying diagnostic indicators that are sufficiently sensitive and selective to OA. Here we assess novel indicators of biomineralization function, focusing on the distribution of holococcoliths and braarudosphaerids - fossil coccolithophore groups expected to exhibit greatest sensitivity to acidification because of their reliance on extracellular calcification. Across the PETM, the biogeography and abundance of these extracellular calcifiers shifted dramatically, disappearing entirely from low latitudes to become limited to cooler, but lower saturation-state areas. By comparing distribution data with environmental parameters from an Earth system model we see that the principal control on these changes was temperature, with survival enabled by shifting populations to high latitude refugia, despite more adverse ocean chemistry conditions. The deleterious effects of OA were only evidenced when twinned with elevated temperatures, in conditions unlikely to develop in future ocean scenarios.