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

• One of the longest on site ocean alkalinization experiments. • pH-equilibrated ocean alkalinization is tested at mesocosm scale. • Moderate alkalinization (≤1000 µmol C/L) is stable over long periods. • Oversaturation (>10 Ω Ar ) causes rapid precipitation and CO 2 degassing. • Temperature and salinity shifts affect stability through saturation changes. • Seasonal and site-specific seawater conditions determine optimal alkalinity addition. 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 CO 2 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 µ mol C/L above natural levels. The stability of chemical parameters in the mesocosms was monitored over a 76-day period. Under moderate alkalinization (≤1000 µ mol 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 (Ω Ar ) exceeding 10, exhibited rapid declines in stability due to secondary carbonate precipitation and CO 2 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 Ω Ar 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.