High frequency in situ total alkalinity measurement for monitoring ocean alkalinity enhancement field trials

Ocean alkalinity enhancement increases seawater alkalinity to boost carbon dioxide uptake. We report the first field deployment of an autonomous Lab-on-a-Chip total alkalinity analyzer during an ocean alkalinity enhancement trial using magnesium hydroxide slurry. In 2023, the analyzer-co-deployed with pH, salinity, and temperature sensors 60 m from the discharge-performed 314 total alkalinity and 52 onboard certified reference material measurements over 40 days, totaling ~3300 optical readings. High-frequency alkalinity measurements revealed stronger semi-diurnal tidal coherence prior to dosing, followed by reduced coherence and more variable phase relationships as dosing progressed. Over the deployment, total alkalinity relative to a baseline alkalinity-salinity relationship significantly increased by ~40 µmol/kg after ~210 tonnes of alkaline addition and did not return to baseline between dosing intervals, indicating a system memory effect with cumulative alkalinity retention. This autonomous in situ approach captures high-resolution variability relevant to monitoring and verifying alkalinity-based carbon dioxide removal, which is challenging to achieve using discrete bottle sampling.