“Ghost” nannofossils reveal nannoplankton more resilient than thought to Earth’s previous warm ocean periods

Predictions of how marine organisms like mollusks, sea urchins and coccolithophores will continue to form their calcium carbonate components as climate change impacts the oceans rely heavily on the fossil record of nannoplankton during past warming events. Now, an unconventional approach to studying fossils of marine coccolithophores discovered in black shale deposits worldwide reveal plankton were more resilient to Earth’s past periods of prolonged ocean warming than traditional fossil evidence suggests. As the planet warms and levels of atmospheric carbon dioxide (CO₂) rise, ocean acidification and declining concentrations of seawater carbonite will likely impact marine organisms that form calcium carbonate shells and skeletons. This is particularly true for coccolithophores, or nannoplankton, which are the most productive marine calcifiers, and which play a central role in global biogeochemical processes; even so, predicting their response to future environmental change remains a challenge. Declines in nannoplankton fossils during previous global warming events, such as the Toarcian OAE, which occurred roughly 183 million years ago in the early Jurassic, have been interpreted as a “biocalcification crisis,” whereby ocean acidification and related environmental factors compromised calcium carbonate production. However, some have argued that these declines in calcium carbonate are caused by dissolution of carbonate at the seafloor during these warming events and that independent evidence for nannoplankton responses to ocean acidification must be better demonstrated before biocalcification crises are invoked. Using scanning electron microscopy on samples of Toarcian rock from the UK, Germany, Japan, and New Zealand, Sam Slater and colleagues investigated types of constituent organic matter. Moving beyond conventional nannofossil analyses, Slater and team focused on an overlooked form of preservation – namely, imprint (or “ghost”) nannofossils – which provide critical information that may be lost from the more routinely studied body fossil records. In doing so, Slater et al. unexpectedly discovered abundant ghost nannofossils of coccolithophores pressed into the surfaces of larger organic particles, including plankton, spores and seeds. The authors found flawlessly preserved nannoplankton imprint fossils throughout several inferred biocalcification crises during the Jurassic and Cretaceous, suggesting that nannoplankton was more resilient to past warming events than traditional fossil record analyses suggest. These findings indicate no evidence of biocalcification crises during the periods studied, at least for plankton that form calcite, and they highlight how a literal reading of the fossil record can mislead interpretations, say the authors. In a related Perspective, Jorijntje Henderiks discusses the findings in greater detail.