Limits on marine carbon dioxide removal potential set by coastal air-sea gas exchange rates

Marine carbon dioxide removal (mCDR) has attracted significant interest as a strategy to mitigate rising atmospheric carbon dioxide (CO2) from fossil fuel burning in order to combat climate change, where a key variable is the gas transfer velocity (k), controlling air-sea CO2 fluxes. This study determined k using the 3He/SF6 dual-tracer technique in Hvalfjörður, Iceland, a potential site for mCDR research. Measured k values were lower than open ocean values at similar wind speeds, reflecting strong wind-fetch limitation in this semi-sheltered fjord. Tracer-based results indicate a short inner-fjord residence time of four days, causing rapid export of potentially perturbed surface water to the North Atlantic Ocean, where open-ocean gas exchange dominates. Consequently, local gas exchange has only a minor impact on the overall CO2 uptake estimates, provided the exported water remains in atmospheric contact. In coastal environments with substantially longer residence times and reduced k’s, or if the surface waters subduct rapidly after export, local gas exchange becomes more critical, as assuming open-ocean gas exchange would overestimate the mCDR CO2 uptake potential.