Developing a robust biogeochemical framework of the coccolith vital effects for more reliable paleoclimatic reconstructions

The major climatic forcing parameters on Earth climate are temperature and the atmospheric concentrations of CO₂. Even if their evolutions covaried to the first-order, the geological record show periods with non-linear evolution between those two parameters. Such delinking requires accurate paleoclimate reconstructions with implications for the modelling studies of our future climate.

pCO₂ and Sea Surface Temperature (SST) reconstructions are usually quantified using proxies relying on both the organic matter produced by coccolithophores (UK37’ index and δ¹³C_alkenones) and calcite of foraminiferal tests (δ¹¹B, δ¹⁸O, Mg/Ca). These proxies have been very useful for a variety of paleoclimatic advances, yet present unresolved and potentially important biases. As an example, alkenone carbon isotopes are not able to register low to moderate pCO₂ levels (Badger et al., 2019). This is notoriously a major issue for paleoclimate reconstructions of the last 6 My (Plio-Pleistocene period).

Our approach is to use a unique archive – the coccoliths – for determination of coeval SST and pCO₂. Coccoliths are small calcite plates produced by unicellular photosynthetic algae called coccolithophores. They are a very promising substrate to analyse for paleoclimate studies because they calcify in the uppermost water column and because their isotopic ratios are sensitive to both photosynthesis and calcification (Hermoso et al. 2020). Therefore, these isotopic ratios provide physiological and metabolic information about coccolithophores of the past. In order to infer paleoclimates from the sedimentary archives, we have to deconvolve the isotopic biological imprint (vital effect) from the environment signal. For the evaluation of the vital effects, we have undertaken a large-scaled culture experiments with various strains of coccolithophore grown under various CO₂ concentrations and pH (Le Guevel et al. in prep). Even if we have managed culture until 1400ppm and 7.55 unit of pH, we were particularly interested in low pCO₂ and high pH conditions because the bibliography is lacking of vital effect for Plio-Pleistocène applications. All the selected strains produce coccoliths within the size range of the one we find predominantly in the marine sediments throughout geological times.

We document a large decrease of the carbon differential vital effect with the CO₂ concentration increase between Gephyrocapsa oceanica and Coccolithus braarudii. This is consistent with previous studies but the absolute values are slightly different and we provide a more precise dataset at low to moderate pCO₂ than the previous ones (Rickaby et al., 2010; Hermoso et al., 2016). We propose the first study of the oxygen and carbon vital effect of the Helicosphaera carteri group with combined CO₂/pH changes. Taken together, the culture data and measurements of the isotopic composition of the calcite biominerals allows better paleoreconstructions of SST and aqueous CO₂.