Using Coccolith clumped isotopes to reconstruct Cenozoic absolute sea surface temperatures

In light of the mounting evidence of the imminent anthropogenically-caused warming, it is urgent to reach a consensus of the climate sensitivity to greenhouse forcing, for which both reliable temperature and CO₂ records are required. Currently available paleothermometers (i.e. UK’37, TEX86 and foraminiferal δ¹⁸O and Mg/Ca) often fail to produce reliable estimates of absolute sea surface temperatures (SST), especially for the vast warm tropical oceans of the high-CO₂ world before the late Miocene.

The carbonate clumped isotope paleothermometer appears to overcome several of the limitations of the commonly used SST proxies. Coccolitho-phores are chronologically/geographically ubiquitous and dissolution/recrystallization-resistant unicellular calcifying phytoplankton. Therefore, the analysis of their calcite assures a surface signal. The main goal of this project is to apply clumped isotopes to coccolithophorid calcite to achieve reliable estimates of absolute tropical SST and concomitant latitudinal SST gradients during the Cenozoic to improve climate sensitivity evaluation.

For this, we will assess potential vital effects in coccolith clumped isotopes, produce a robust calibration and validate the proxy for deep-time application. The relative effects of temperature and potential vital effects associated to carbon limitation will be isolated using both cultured and core top samples. Potential size effects will be tested by separating size fractions using microfiltration and settling techniques. The unique methodology developed by the Climate Geology group will allow obtaining exceptionally good precision with very small sample sizes.

The appropriateness of the corrections to isolate the temperature effect of the proxy will be tested by applying it to reconstruct SSTs from downcore samples for which there are reliable published alkenone-derived SST estimates, with the ultimate aim of reconstructing long-term paleotemperatures of Cenozoic tropical oceans and the respective latitudinal SST gradients. This will allow a better understanding of climate sensitivity during periods of CO₂ concentrations similar to those projected for the upcoming centuries, and to improve climate prediction models.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska -Curie grant agreement 795053.