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

• Main Authors: Godbillot, Camille, Minoletti, Fabrice, Bassinot, Franck, Hermoso, Michaël
• Format: Text
• Language: English
• Published: 2021
• Subjects: ice core; North Atlantic
• Online Access: https://doi.org/10.5194/cp-2021-76; https://cp.copernicus.org/preprints/cp-2021-76/

Beyond the pCO 2 records provided by ice core measurements, the quantification of atmospheric CO 2 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 CO 2 and a type of organic compounds that these photosynthetic microalgae synthesize (the alkenones) represents a relatively robust proxy to reconstruct past atmospheric CO 2 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 CO 2 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 CO 2 , and, ultimately, to atmospheric CO 2 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 CO 2 during Termination II ( ca. 130–140 ka). Performing paired measurements of the isotopic composition (δ 13 C and δ 18 O) 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 CO 2 concentrations computed from the ice core CO 2 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 CO 2 in the range of Quaternary CO 2 concentrations. We here consolidate a new coccolith ∆δ 13 C proxy that overtakes the strong assumptions that have to be made pertaining to the chemistry of the carbonate system in seawater, as required in CO 2 proxy methods such as the boron isotope and alkenone proxies.