pH-equilibrated ocean alkalinization: Mesoscale evaluation of long-term stability

A new pH-equilibrated ocean alkalinization method was evaluated at the mesoscale level to assess the long-term stability of carbon retained as bicarbonate in seawater. Natural seawater was enriched in bicarbonate by reacting Ca(OH)2 with CO2 in natural seawater, adjusted to match ambient pH, and introduced into controlled mesocosms to increase the Dissolved Inorganic Carbon (DIC) content by 250 to 1990 µmol C/L above natural levels. The stability of chemical parameters in the mesocosms was monitored over a 76-day period. Under moderate alkalinization (≤1000 µmol C/L of added DIC), >90% of the added inorganic carbon remained stable for nearly two months. In contrast, treatments leading to an aragonite saturation state (ΩAr) exceeding 10, exhibited rapid declines in stability due to secondary carbonate precipitation and CO2 degassing, particularly at high temperatures. While natural seawater salinity and pH did not directly induce instability, they modulated the carbonate saturation state and therefore they must be considered for a correct prediction of the system behavior. Seasonal fluctuations in seawater characteristics, namely salinity, temperature, and pH, were found to influence theoretical ΩAr and should be considered for alkalinity dosing and site selection. These findings underscore the importance of assessing real-time, site-specific conditions for effective and safe implementation.