Biogeochemical response of coccoliths from the Pleistocene to the variations of pCO2

• Main Author: Godbillot, Camille
• Other Authors: Institut des Sciences de la Terre de Paris (iSTeP), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, Fabrice Minoletti, Michaël Hermoso
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2022
• Subjects: Coccolithes; PCO2 proxies; Glacial-interglacial cycles; Proxies de pCO2; Cycles glaciaires-interglaciaires; Terminaison II; Transition mi-Pléistocène; Biogéochimie; [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Antarctic; Antarc*
• Online Access: https://theses.hal.science/tel-04241625; https://theses.hal.science/tel-04241625/document; https://theses.hal.science/tel-04241625/file/GODBILLOT_Camille_these_2022.pdf

Results from both in vivo cultures and cell modelling biogeochemical studies have demonstrated a link between the biological fractionation of coccoliths and the CO2 concentration of the living environment of their producers, the coccolithophores. These results have encouraged the use of coccolith vital effects as proxies for the levels of CO2 in the medium. However, a number of biases hinder the application of the empirical calibrations from culture experiments to natural coccolith populations. This work aims at formalizing the transfer function linking the vital effects to the [CO2aq] in natural environments, to be used for the reconstruction of past pCO2. To this end, we use the records from Antarctic ice cores to quantify the forcing of pCO2 on the magnitude of the coccoliths’ vital effects. We evidence, and discuss, a control of CO2 concentrations on the isotopic difference (Δδ18O, Δδ13C) between coccoliths of different sizes produced during the penultimate glacial termination (130-140 ka). The second part of this thesis is dedicated to applying the transfer function to coccoliths from the Mid-Pleistocene Transition (800-1250 ka), a key period of climate change for which pCO2 variations are not documented. The pCO2 record we obtain, which covers the entire transition, exhibits self-consistent amplitudes and variations, thus giving credit to the calibration we present. Our record supports the hypothesis of an increased sensitivity of ice sheets to the radiative forcing of CO2 since the MPT. Des résultats croisés d’études biogéochimiques de cultures in vivo et de modélisations cellulaires ont mis en évidence un lien entre le fractionnement biologique des coccolithes et la concentration en CO2 du milieu de vie de leurs producteurs, les coccolithophoridés. Ces résultats ont ouvert la voie à l’utilisation des effets vitaux des coccolithes comme proxy du CO2 dans le milieu. Toutefois, des biais affectent l’application des calibrations empiriques de culture à des populations naturelles de coccolithes. Ainsi, ce ...