Regional seawater-chemistry controls on ocean alkalinity enhancement efficiency: Carbonate precipitation risk under pH-equilibrated OAE

Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal strategy that increases seawater alkalinity and promotes atmospheric CO2 uptake. However, the stability of stored carbon remains uncertain, particularly when carbonate saturation favors secondary calcium carbonate precipitation. In this study, we evaluate how regional seawater chemistry controls aragonite saturation state (ΩAr) under pH-equilibrated OAE conditions, in which seawater is enriched in dissolved inorganic carbon (DIC) while pH remains close to natural seawater value. Near-surface seawater conditions were selected from ocean regions using GLODAPv2.2023 data, and ΩAr was calculated for baseline conditions and scenarios of +500, +1000, and +2000 µmol kg-1 added DIC. The data show that the carbonate chemistry is not uniform among regions. Thus, although the same DIC additions were adopted in all regions, the post-treatment ΩAr differed because of the non-uniform initial seawater chemistry. High temperature, high salinity, and high pH regions feature higher aragonite supersaturation and move towards the ΩAr = 7 threshold more easily, whereas colder or lower salinity regions remain far from this level. After a DIC addition of 1000 µmol kg-1, most regional median ΩAr values remain below ΩAr = 7. In contrast, the after a DIC addition of 2000 µmol kg-1 several sites fall into dangerously higher supersaturation conditions. These findings indicate that carbonate supersaturation and secondary precipitation risk depend strongly on the receiving seawater chemistry. Therefore, OAE should not be evaluated using a universal alkalinity dosage. An assessment of the region-specific carbonate chemistry is necessary before selecting the treatments and deployment sites.