Abstract
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). This assessment is based on single species culture experiments that are now revealing complex, synergistic, and adaptive responses to such environmental change. Despite this complexity, there is still a widespread perception that coccolithophore calcification will be inhibited by OA. These plankton have an excellent fossil record, and so 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. Here we target fossil coccolithophore groups (holococcoliths and braarudosphaerids) expected to exhibit greatest sensitivity to acidification because of their reliance on extracellular calcification. Across the Paleocene-Eocene Thermal Maximum (56 Ma) rapid warming event, the biogeography and abundance of these extracellular calcifiers shifted dramatically, disappearing entirely from low latitudes to become limited to cooler, lower saturation-state areas. By comparing these range shift data with the environmental parameters from an Earth system model, we show that the principal control on these range retractions was temperature, with survival maintained in high-latitude refugia, despite more adverse ocean chemistry conditions. Deleterious effects of OA were only evidenced when twinned with elevated temperatures.
Highlights
Increasing atmospheric CO2 is currently driving a decrease in surface ocean pH and carbonate saturation, a phenomena termed ocean acidification (OA; Royal Society, 2005)
By assessing the relative prevalence of extracellular coccolithophore calcifiers (ECCs) across the PETM and comparing this with environmental changes predicted by an Earth system model, we provide a unique test for whether OA may pose a threat to coccolithophores in the near future
Coarser resolution bins were used for comparing distribution data and modeled environmental parameters (Figs. 2 and 3): pre-PETM–background (0 yr, metadata from intervals 1, 5, and 6), characterized by the widespread occurrences of ECCs in the background normal conditions; carbon isotope excursion (CIE) onset (+6000 yr, metadata from interval 2), where there is the most dramatic range contraction of the ECCs; and plateau (+40,000 yr, including metadata from intervals 3 and 4), where environmental conditions are still perturbed but there is some degree of recovery of ECC distribution
Summary
Increasing atmospheric CO2 is currently driving a decrease in surface ocean pH and carbonate saturation, a phenomena termed ocean acidification (OA; Royal Society, 2005). To generate a scenario for PETM warming and recovery that could be related to specific carbon isotopic features (e.g., pre-CIE, onset, and plateau of Fig. DR2), we used the time history of carbon release devised by Zeebe et al (2009), comprising an initial pulse of highly isotopically depleted carbon followed by an extended and slower rate leak of less isotopically depleted carbon.
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