Ocean alkalinity enhancement in a coastal channel: simulating localised dispersion, carbon sequestration and ecosystem impact

Abstract Ocean Alkalinity Enhancement (OAE) is a proposed marine carbon dioxide removal (mCDR) method that has the potential to sequester CO2 at the gigaton scale. Alkaline substances are added to the surface ocean to strengthen the air-sea pCO2 gradient and induce carbon transfer into the ocean or reduce its outgassing. While global and regional models show that OAE effectively sequesters CO2, their coarse resolution makes them inappropriate for investigating local environmental impacts and sequestration efficiency. Here, we simulate local (200 m horizontal resolution) OAE scenarios at a location where an OAE field trial is planned (Woodbridge, Tasmania) to investigate how simulations can guide the local design of OAE. Our simulations show the importance of seasonality where surface alkalinity retention, plume sizes and carbon sequestration efficiency are primarily controlled by surface stratification with wind stress and tidal currents playing a secondary role. Seasonal stratification variability modulates sequestration efficiency (ΔDIC/ΔTA) within the domain with summer values of 0.6 ± 0.1, and winter values of 0.46 ± 0.11, and no significant differences being driven by wind stress or current strength (p >0.05). In these the simulations, we show a linear scaling between the OAE injection rate and pH change at the addition location and the far field, which allows the injection rate to be modified to meet key environmental targets, for example not exceeding Australian pH guidelines (ΔpH > 0.2). We identify that pH changes return to baseline through mixing, dilution, and carbon uptake. OAE is a multi-scale problem where the impacts and efficiency encompasses the local to global scale responses. The use of local scale modelling (particularly for monitoring, reporting, and verification) is critical to enabling the future deployment of OAE for mCDR, and will need to be coupled to regional and global models to advance OAE safety, efficacy, and use.