Alkalinity enhancement in intertidal environments: preliminary results of a field experiment

Reducing atmospheric carbon dioxide (CO₂) concentrations to combat global warming is one of the greatest challenges of humanity.

Marine alkalinity enhancement is a promising carbon dioxide removal measure with high potential to increase oceanic carbon uptake and storage. The natural processes of weathering on land sustains the alkalinity of the ocean and thereby removes CO₂ from the atmosphere on geological time scales. The weathering can be enhanced by deploying fine-grained alkaline minerals to coastal areas, to directly supply more alkalinity to near-coastal waters. Nevertheless, the rate of CO₂ consumption depends on the minerals used, grains size, temperature, pH, and salinity. During mineral dissolution, nutrients and trace elements are also released, which may affect marine biota. In order to evaluate the CO₂ sequestration potential, ensuing biogeochemical and ecological impacts of alkalinity enhancement in intertidal environments, a novel in-situ experiment was installed in the Ria Formosa Coastal Lagoon, southern Portugal.

The Ria Formosa is a highly dynamic lagoon system, with daily renewal of water and nutrients through multiple tidal inlets. A succession of salt marshes with varying zonation and faunal communities fringes the lagoon. The experiment was installed in an undisturbed zone colonized with Spartina maritima, in September 2022. The experimental set-up includes three replicate treatments with coarse olivine, fine olivine, coarse basalt, fine basalt, and an untreated control. Lagoonal, supernatant, and porewater waters are sampled from each treatment every month and analysed for temperature, salinity, oxygen concentration, pH, total alkalinity, nutrients, and trace metals. Preliminary data show an increase in total alkalinity in the supernatant and porewaters shortly after minerals deployment, by 0.36 and 2.05 mmol kg^-1 on average, relative to the control. Lower values of total alkalinity were recorded in December 2022, followed by markedly lower salinities after heavy rainfall in the study area. The experiment will run over two years and monthly sampled for water properties. For monitoring potential biodiversity changes, sediment samples are analysed for faunal and floral composition. Results of this novel field experiment will provide strategic knowledge on the benefits and risks of alkalinity enhancement in intertidal environments.

Acknowledgement. Research supported by the Portuguese Science Foundation, with the projects PTDC/CTA-CLI/1065/2021, UID/00350/2020CIMA and contracts DL57/2016/CP1361/CT0009, DL57/2016/CP1361/CT0002 and CEECINST/00146/2018/CP1493/CT0002.