Reconstructing the Preindustrial Coastal Carbon Cycle Through a Global Ocean Circulation Model: Was the Global Continental Shelf Already Both Autotrophic and a CO2 Sink?

The contribution of continental shelves to the marine carbon cycle is still poorly understood. Their preindustrial state is, for one, essentially unknown, which strongly limits the quantitative assessment of their anthropogenic perturbation. To date, approaches developed to investigate and quantify carbon fluxes on continental shelves have strongly simplified their physical and biogeochemical features. In this study, we enhance the global ocean biogeochemistry model HAMburg Ocean Carbon Cycle by explicitly representing riverine loads of carbon and nutrients, as well as improving the representation of organic matter dynamics in the coastal ocean. Our simulations, at a resolution of similar to 0.4 degrees, reveal a globally averaged shelf water residence time (RT) of 12-17 months, which is much shorter than the global RTs previously assumed in benchmark studies (>4 years). This shorter global RT, induced primarily through outer shelf regions with large oceanic inflows, promotes an efficient offshore transport of terrestrial and marine organic carbon (0.44 PgCyr(-1)) and a dissolved inorganic carbon sink from the organic cycling of carbon on the global shelf (net ecosystem productivity [NEP] equal to +0.20 PgCyr(-1)). In turn, this autotrophic state of continental shelves contributes to a weak global preindustrial sink of atmospheric CO2 (0.04 PgCyr(-1)), dominated by extensive regions with large oceanic inflows and positive NEPs, such as the Patagonian shelf, the East China Sea and the outer North Sea. The contemporary global shelf CO2 uptake of 0.15 PgCyr(-1) furthermoresuggests that the anthropogenic CO2 uptake (0.11 PgCyr(-1)) on the global continental shelf is less efficient with respect to the open ocean.

Automated summary

The contribution of continental shelves to the marine carbon cycle is still poorly understood, with limited knowledge of their preindustrial state. This study enhanced a global ocean biogeochemistry model to investigate carbon fluxes on continental shelves, revealing a shorter water residence time than previously assumed. The findings suggest that continental shelves play a significant role in the global carbon cycle, promoting efficient transport of organic carbon and contributing to a weak preindustrial sink of atmospheric CO2.