Impacts of water advection and CO 2 exchange on the carbon dioxide removal potential of ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement is a carbon dioxide removal strategy with high CO2 uptake potential and rather low cost. Long term modelling studies have focused on this strategy, but most laboratory experiments focus on shorter term with strong advection, which may not be representative of natural systems. Hence, the long-term fate of alkalinity is yet to be addressed. Also, the role of CO2 ingassing is still largely overlooked. In a new setup, 6-month experiments using solid Ca(OH)2 and Mg(OH)2, and liquid NaOH have been conducted with a constant supply of CO2. Bottles were either kept stagnant or exposed to slow advection. All experiments revealed lower measured TA compared to expected, but bottles subjected to advection led to nearly twice as much TA generation in some cases and significantly faster CO2 ingassing rates. This led to a quicker equilibration, decreasing the low CO2 period, lately identified as a potential threat to living organisms, and a decreased critical alkalinity period (CAP) for CaCO3 formation. Yet, CaCO3 precipitated in most bottles, either as needles with Ca(OH)2 or as “broccolis” with Mg(OH)2. Such broccoli resulted most likely from precipitation on slowly dissolving Mg(OH)2 particles. Once the CAP was over, conditions remained stable with no redissolution of the CaCO3 precipitates. Using NaOH revealed buffering by Mg(OH)2 precipitation, which ultimately redissolved in the bottles exposed to advection. Finally, the bottles exposed to advection represented open ocean conditions with wind speeds of ~3 m s-1, which, given the calm state of the ocean, may serve as a lower threshold for implementation in global models.