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

Ocean alkalinity enhancement (OAE) is a carbon dioxide removal strategy with high carbon dioxide (CO ₂ ) uptake potential and moderate cost. Long-term implementation has been assessed in modelling studies, but most laboratory experiments have focused on short time frames with strong advection, which may not be representative of natural systems. Hence, the long-term fate of added alkalinity has yet to be fully addressed in laboratory experiments. Furthermore, the role of CO ₂ ingassing on overall efficiency is still largely overlooked. In a new setup, six-month experiments were conducted using seawater amended with solid Ca(OH) ₂ and Mg(OH) ₂ and liquid NaOH, with a constant atmospheric CO ₂ supply. Bottles were either kept stagnant or exposed to slow circular advection, yielding gas-transfer-velocity equivalent wind speed of ∼3 m s ⁻¹ . All experiments revealed smaller measured increases in total alkalinity than expected, but in the bottles subjected to advection, alkalinity generation was nearly twice as high in some cases and CO ₂ ingassing rates were significantly faster. This led to a faster equilibration, decreasing the high pH/low CO ₂ period, recently identified as a potential threat to living organisms. The critical alkalinity period (CAP) is introduced here to represent this critical period of altered carbonate chemistry during which secondary calcium carbonate (CaCO ₃ ) formation may occur, significantly altering the CDR potential of the approach. CaCO ₃ precipitated in most bottles, either as needles when adding Ca(OH) ₂ or as broccoli-like structures when adding Mg(OH) ₂ . These broccoli-like structures most likely resulted from precipitation on slowly dissolving Mg(OH) ₂ particles. After the CAP ended, conditions remained stable and freshly precipitated CaCO ₃ did not redissolve. In addition, when NaOH was used as the alkaline feedstock, Mg(OH) ₂ flakes were observed but these eventually redissolved in the bottles exposed to advection. The study emphasises the role of water advection on OAE's efficiency and the data presented here may be used to derive lower thresholds for OAE implementation in global models.