Chemical stability of bicarbonate-enriched solution in seawater for ocean-based carbon dioxide storage: A mesocosm study

Ocean alkalinization is increasingly investigated as an ocean-based carbon dioxide storage approach, yet its effectiveness depends on the chemical stability of alkalinity-enhanced seawater after discharge into the marine environment. Here, we assess the post-discharge stability of a bicarbonate-enriched seawater system using a 76-day mesocosm experiment. A near-ambient-pH bicarbonate-rich solution was produced by reacting Ca(OH)2 with CO2 in natural seawater and subsequently introduced into replicated mesocosms, increasing dissolved inorganic carbon (DIC) by 250–1990 µmol C/L above natural levels. Temporal changes in DIC, total alkalinity, pH, and saturation state of the seawater were monitored to evaluate carbon retention. Moderate alkalinity additions (≤1000 µmol C/L of added DIC) exhibited high chemical stability, retaining more than 90% of the added inorganic carbon for nearly two months. In contrast, higher alkalinity treatments experienced rapid losses associated with secondary carbonate precipitation and CO2 degassing, particularly under elevated temperature conditions. While variations in natural seawater salinity and pH did not independently trigger instability, they strongly modulated carbonate supersaturation and the system’s sensitivity to precipitation and degassing. These results demonstrate that alkalinity-enhanced seawater can provide effective marine carbon storage within a constrained chemical and environmental window, highlighting the importance of site-specific seawater characterization for the safe deployment of ocean alkalinization strategies.