To understand how Earth’s climate will respond to increasing amounts of carbon dioxide in the future atmosphere, it helps to look at the past, when the world faced similar situations. Improvements in the accuracy of both carbon dioxide levels and sea surface temperature indicators are needed, particularly in tropical oceans. Scientists need information not just on the shifting trends of ocean temperatures, but also on accurate estimates of absolute values in the distant past. Current proxies have limitations that make these unreliable. The EU-funded EPISODE project aimed to test a new proxy for past surface temperatures of surface oceans, focusing on the Cenozoic period of geological time, from 66 million years ago to the present day. The research was undertaken with the support of the Marie Skłodowska-Curie Actions. Coccolithophores are one of the main forms of phytoplankton in the ocean. They account for half of the carbonate production in the ocean today, making them key climate modulators. They fix carbon dioxide into organic matter through photosynthesis, a process that needs light. This limits their habitat to the surface of the ocean. “Applying geochemistry to fossil coccoliths therefore provides information about surface oceans in the past, which is key for climate models,” explains Luz María Mejía, postdoc at ETH Zurich and lead researcher on the EPISODE project.
Coccolithophores produce calcite plates called coccoliths as an exoskeleton. The project analysed these exoskeletons using a relatively novel technique known as clumped isotope thermometry. The technique allows researchers to estimate surface ocean temperatures by measuring the excess of 13C-18O bonds in the coccolith carbonate exoskeleton relative to a random distribution. These bonds are more stable at colder temperatures, so measuring the amount of bonds present in fossil coccoliths can reveal the temperatures they faced. “Crucially, this is entirely independent of the isotopic composition and chemistry of seawater, as opposed to other widely used temperature proxies,” adds Mejía.
“During this project we showed that coccolith clumped isotopes have huge potential to reconstruct mixed layer depth temperatures,” says Mejía. The team observed similar temperatures between different sizes of coccoliths, boosting confidence in the feasibility of the applied technique. Comparisons between coccolith-derived temperatures in the modern Iceland Sea and temperatures taken from satellites, showed the proxy to be accurate. “Most importantly, coccolith clumped isotopes suggest that temperatures in the North Atlantic 15 million years ago were warmer than modern temperatures, but nowhere near as warm as suggested by other proxies. This indicates more modest polar amplification during warm climate states, and also that the future high latitude warming response to anthropogenic CO2 might not be as extreme as some previous estimates implied,” notes Mejía. Mejía will now continue the research at the University of Bremen. “Application of this proxy to both high-latitude and low-latitude coccoliths further in the past can provide more information on mixed layer temperatures of past warm worlds, and the sensitivity with which Earth’s climate responds to CO2 forcing during warmer climate states,” she explains.
EPISODE, coccolith, exoskeleton, isotope, climate change, proxy