The response of the ocean carbon cycle to artificial upwelling, ocean iron fertilization and the combination of both

Artificial upwelling (AU) and ocean iron fertilization (OIF) both have been proposed as marine carbon dioxide removal methods to enhance ocean carbon uptake by stimulating the biological carbon pump. We simulate global and regional AU and OIF individually and the combination of both methods between the years 2025 and 2100 in ocean-atmosphere model experiments under the moderate RCP 4.5 CO _2 -emission pathway and show that the combination of globally applied AU + OIF yields the greatest ocean carbon uptake potential of +103 Pg C until year 2100. Regional OIF simulated by itself poleward of 45° North and South is responsible for +86.9 Pg C additional ocean carbon uptake. AU-only experiments do not significantly enhance ocean carbon uptake due to the lack of iron in the upwelled waters. We find no consistent relationship between enhanced cumulative export production and changes in the ocean carbon inventory attributable to the biological carbon pump, which makes export production a poor indicator for additional ocean carbon uptake. We identified a strong decrease in the global ocean nitrate inventory (−567 Tmol N) until year 2100 as a consequence of globally applied AU + OIF due to an interrupted balance between N _2 -fixation and denitrification.