Novel Field Experiment on Alkalinity Enhancement in Intertidal Environments—A Trailblazer for Natural Climate Solutions

Abstract

One recently proposed approach to reduce atmospheric CO₂ concentrations is marine alkalinity enhancement. This technique increases the CO₂ uptake capacity of seawater through weathering of fine-grained (mafic) rocks and minerals in marine environments. The weathering process has been extensively tested in laboratory studies and verified by numerical models. Field experiments scaling the CO₂ uptake under natural conditions are still lacking. In a methodological approach, a novel in situ experiment was designed and installed in a salt marsh at Ria Formosa coastal lagoon, southern Portugal. The experiment comprised deployments of different sizes of olivine and basalt substrates, and a control site, which were tidally submerged twice a day. A monthly monitoring scheme of supernatant and porewater properties from each deployment and control site was established, and procedures for temperature, salinity, oxygen, pH, total alkalinity, nutrient, and trace metal analyses were defined. This paper is devoted to the methods and describes the design, a protocol for the analyses, and an evaluation of experimental performance and reliability. Data from the first 6 months are presented for validation of the experiment. They demonstrated elevated total alkalinity in water samples, mostly in porewater after the deployments, while salinity, oxygen, and pH reflect the control conditions. Significant alkalinity differences were observed between the treatments and the natural background conditions monitored at the control site, during the 6 months of the experiment. The methodological approach is presented with strengths, limitations, and recommendations for an upscaling as CO₂ removal measure, servicing, and subsequent investigations.