Evaluating ocean alkalinity enhancement for carbon dioxide removal: evidence from a one-year saltmarsh field experiment

Abstract. Ocean alkalinity enhancement is a promising carbon dioxide removal (CDR) strategy aimed at reducing atmospheric carbon dioxide (CO2) concentrations. To evaluate its effectiveness and potential biogeochemical impacts, field experiments under natural conditions are essential. We report results from a one-year in-situ experiment conducted in the saltmarsh pioneer vegetation zone at Ria Formosa coastal lagoon, Portugal. The experiment comprised replicate deployments of olivine and basalt (treatments), and untreated control sites. Total alkalinity (TA) responded immediately to the treatments, with pore water 1.5 to 2.3 mM higher than the control. High concentrations of CO2 in pore water led to an increase of dissolved inorganic carbon (DIC) higher than TA. Continuous CO2 degassing from the saltmarsh soil was observed, with the treatments prompting higher CO2 fluxes than control. Carbon was laterally exported to the ocean (outwelling), following the trend of excess TA production. This effect was most pronounced during the first seven months after deployment, with basalt producing the best results. These findings provide critical insights into the temporal dynamics and efficacy of alkalinity enhancement in coastal vegetated systems.