The influence of the carbonate pump on the ocean carbon cycle - natural air-sea carbon fluxes, inter-hemispheric carbon transport and anthropogenic perturbations

To what extent does the representation of the carbonate pump and its evolution in a transient climate influence the ocean carbon cycle? The carbonate pump, the amplitude of which can be estimated through the CaCO₃ export flux, is sometimes qualified as a counter-pump. Indeed, by inducing an imbalance between total alkalinity (Alk) and dissolved inorganic carbon (DIC) in the surface ocean, calcification induces relative degassing and acidification. However, it also drives vertical gradients of Alk and DIC in the ocean in response to CaCO₃ dissolution at depth. Using equilibrium simulations of the NEMO-PISCES marine biogeochemical model, we investigate the influence of the carbonate pump on natural air-sea carbon fluxes and inter-hemispheric carbon transport within the ocean. We further show that despite diverse twenty-first century changes in the carbonate pump in CMIP6 models, its impact on the projected upper ocean carbon cycle is minimal compared to other drivers. Interpreting our results, we rely on the pair of conservative variables Alk and DIC, central to the representation of the ocean carbon cycle in models. In particular, we show that the usefulness of the classically represented Alk/DIC diagram can be extended to derive quantitative and comparative visual information on the processes driving the upper ocean carbon cycle.