Preprints
https://doi.org/10.5194/cp-2021-76
https://doi.org/10.5194/cp-2021-76

  28 Jun 2021

28 Jun 2021

Review status: this preprint is currently under review for the journal CP.

Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: Developing a new paleo-CO2 probe

Camille Godbillot1, Fabrice Minoletti1, Franck Bassinot2, and Michaël Hermoso3 Camille Godbillot et al.
  • 1Institut des Sciences de la Terre de Paris (UMR 7193 ISTeP), CNRS, Sorbonne Université, 75005 Paris, France
  • 2Laboratoire des Sciences de l’Environnement et du Climat (UMR 8212 LSCE), CEA, CNRS, Université Versailles Saint Quentin, 91191, Gif sur Yvette, France
  • 3Laboratoire d’Océanologie et de Géosciences (UMR 8187 LOG), Université du Littoral Côte d’Opale, CNRS, Université de Lille, 62930 Wimereux, France

Abstract. Beyond the pCO2 records provided by ice core measurements, the quantification of atmospheric CO2 concentrations and changes thereof relies on proxy data, the development of which represents a foremost challenge in paleoceanography. In the paleoceanographic toolbox, the coccolithophores occupy a notable place, as the magnitude of the carbon isotopic fractionation between ambient CO2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO2 concentrations during the Cenozoic. The isotopic composition of coeval calcite biominerals found in the sediments and also produced by the coccolithophores (the coccoliths) have been found to record an ambient CO2 signal through culture and sediment analyses. These studies have, however, not yet formalized a transfer function that quantitatively ties the isotopic composition of coccolith calcite to the concentrations of aqueous CO2, and, ultimately, to atmospheric CO2 levels. Here, we make use of a micro-separation protocol to compare the isotopic response of two size-restricted coccolith assemblages from the North Atlantic to changes in surface ocean CO2 during Termination II (ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ13C and δ18O) of relatively large and small coccoliths provides an isotopic offset that can be designated as a “differential vital effect”. We find that the evolution of this offset follows that of aqueous CO2 concentrations computed from the ice core CO2 curve and an independent temperature signal. We interpret this biogeochemical feature to be the result of converging carbon fixation strategies between large and small cells as the degree of carbon limitation for cellular growth decreases across the deglaciation. We are therefore able to determine a transfer function between the coccolith differential vital effects and aqueous CO2 in the range of Quaternary CO2 concentrations. We here consolidate a new coccolith ∆δ13C proxy that overtakes the strong assumptions that have to be made pertaining to the chemistry of the carbonate system in seawater, as required in CO2 proxy methods such as the boron isotope and alkenone proxies.

Camille Godbillot et al.

Status: open (until 23 Aug 2021)

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Camille Godbillot et al.

Camille Godbillot et al.

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Short summary
We develop a new method to reconstruct past atmospheric CO2 levels based on the inorganic geochemistry of coccolith calcite, which recent coccolithophore culture and numerical experiments have related to ambient CO2 concentrations. By comparing the isotopic composition of fossil coccoliths to the inferred surface ocean CO2 level at the time they calcified, we derive a transfer function and argue that coccolith vital effects can be used to reconstruct geological pCO2 beyond the ice core record.