L’alcalinité et le carbonate de calcium dans les modèles système Terre, et implications pour le cycle du carbone océanique

• Main Author: Planchat, Alban
• Other Authors: Laboratoire de Météorologie Dynamique (UMR 8539) (LMD), Institut national des sciences de l'Univers (INSU - CNRS)-École polytechnique (X)-École des Ponts ParisTech (ENPC)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Département des Géosciences - ENS Paris, École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-École normale supérieure - Paris (ENS-PSL), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL), Université Paris sciences et lettres, Laurent Bopp, Lester Kwiatkowski Co-encadrant
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2023
• Subjects: Alkalinity; Calcium carbonate (CaCO3) cycle; Carbon cycle; Ocean acidification; Carbonate pump; Marine biogeochemistry; Alcalinité; Cycle du carbonate de calcium (CaCO3); Cyle du carbone; Acidification de l'océan; Pompe des carbonates; Biogéochimie marine; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere
• Online Access: https://theses.hal.science/tel-04531034; https://theses.hal.science/tel-04531034/document; https://theses.hal.science/tel-04531034/file/Planchat_2023_These.pdf

Ocean alkalinity (Alk) is critical for the uptake of atmospheric carbon and provides buffering capacity against acidification. Within the context of projections of ocean carbon uptake and potential ecosystem impacts, the representation of Alk and the main driver of its distribution in the ocean interior, the calcium carbonate (CaCO3) cycle, have often been overlooked. This thesis addresses the lack of consideration given to Alk and the CaCO3 cycle in Earth system models (ESMs) and explores the implications for the carbon cycle in a pre-industrial ocean as well as under climate change scenarios. Through an ESM intercomparison, a reduction in simulated Alk biases in the 6th phase of the Coupled Model Intercomparison Project (CMIP6) is reported. This reduction can be partially explained by increased pelagic calcification, redistributing Alk at the surface and strengthening its vertical gradient in the water column. A review of the ocean biogeochemical models used in current ESMs reveals a diverse representation of the CaCO3 cycle and processes affecting Alk. Parameterization schemes for CaCO3 production, export, dissolution, and burial vary substantially, with no benthic calcification and generally only calcite considered. This diversity leads to contrasting projections of carbon export associated with CaCO3 from the surface ocean to the ocean interior in future scenarios. However, sensitivity simulations performed with the NEMO-PISCES ocean biogeochemical model indicate that the feedback of this on anthropogenic carbon fluxes and ocean acidification remains limited. Through an ensemble of NEMO-PISCES simulations, careful consideration of the Alk budget is shown to be critical to estimating pre-industrial ocean carbon outgassing due to riverine discharge and the burial of organic matter and CaCO3. Such estimates are fundamental to assessing anthropogenic air-sea carbon fluxes using observational data and highlight the need for greater constraints on the ocean Alk budget. L’alcalinité de l’océan (Alk) est ...