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

Type Article
Acceptance Date 2021-01 IN PRESS
Language English
Author(s) Godbillot Camille1, Minoletti Fabrice1, Bassinot Franck2, Hermoso MichaëlORCID3
Affiliation(s) 1 : Institut des Sciences de la Terre de Paris (UMR 7193 ISTeP), CNRS, Sorbonne Université, 75005 Paris, France
2 : Laboratoire des Sciences de l’Environnement et du Climat (UMR 8212 LSCE), CEA, CNRS, Université Versailles Saint Quentin, 91191, Gif sur Yvette, France
3 : Laboratoire d’Océanologie et de Géosciences (UMR 8187 LOG), Université du Littoral Côte d’Opale, CNRS, Université de Lille, 62930 Wimereux, France
Source Climate of the Past (1814-9332) (Copernicus GmbH) In Press
DOI 10.5194/cp-2021-76
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.

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Godbillot Camille, Minoletti Fabrice, Bassinot Franck, Hermoso Michaël Parallel between the isotopic composition of coccolith calcite and carbon levels across Termination II: Developing a new paleo-CO2 probe. Climate of the Past IN PRESS. Publisher's official version : https://doi.org/10.5194/cp-2021-76 , Open Access version : https://archimer.ifremer.fr/doc/00703/81525/