Short water residence times on the global continental shelves support an efficient cross-shelf transfer of organic matter: Was the pre-industrial coastal ocean already a global CO2 sink?

Carbon fluxes on continental shelves are still poorly quantified at the global scale. Notably, the pre-industrial net ecosystem production (NEP) and air-sea CO₂ (FCO₂) fluxes on the global continental shelf, which is needed to estimate its anthropogenic perturbation, have only been assessed using conceptual box models. These models, however, do not represent the complex physical and biogeochemical spatiotemporal dynamics prevailing in coastal regions. Analysis using global ocean general circulation models was also limited in the past due to model resolution constraints and the omission of riverine inputs, while regional models that account for the highly heterogeneous dynamics of a single coastal system lack the global perspective. We applied the global ocean biogeochemistry model HAMOCC coupled to the general circulation model MIPOM at a resolution of ~0.4 degrees. HAMOCC was extended to include riverine loads (carbon and nutrients) and improved organic matter dynamics in the ocean to estimate the pre-industrial NEP and FCO₂ of the global continental shelf. The mean shelf water residence time of 14-16 months computed by the model is substantially shorter than what was assumed in conceptual box models (>4 yr). This short residence time, combined with tDOM kinetics evaluated against observational data, induce a substantial cross-shelf export of organic carbon (0.43 Pg C yr^-1) and a positive NEP on the global continental shelves (0.25 Pg C yr^-1). Model results also suggest a slight pre-industrial sink of CO₂ (0.04 Pg C yr^-1), with regions dominated by open ocean inflows and positive NEPs being the largest contributors to this sink (i.e. the Patagonian Shelf, the Barents Sea, the East China Sea). Our results are in stark contrast to the conceptual view of a pre-industrial heterotrophic state and FCO₂ on continental shelves. They also imply that the contribution of continental shelves to the oceanic anthropogenic carbon sink could be significantly smaller than was previously assumed.