Reducing atmospheric carbon dioxide (CO2) concentrations to combat global warming and associated environmental changes is one of the greatest challenges of humanity (IPCC,2018). In order to prevent temperature rising of more than 1.5°C above pre-industrial levels to achieve Paris Agreement, technical measures have been developed to remove CO2 from the atmosphere (Carbon Dioxide Removal, CDR), and to assure its long-term storage (IPCC, 2018). Augmenting ocean alkalinity by enhanced weathering is one of the measures that has proven the highest CDR potential (e.g. McLaren, 2012). But the very question remains: will this way of CDR be suitable to take urgent action to combat climate change and its impacts (United Nations 2015: Sustainable Development Goal 13) Weathering of alkaline minerals increases the natural alkalinity of seawater by releasing cations, which bind CO2 as bicarbonate and carbonate ions (Feng et al., 2017). This process can be enhanced when the material is spread in coastal environments (e.g. Montserrat et al., 2017). However, the rate of CO2 consumption depends on the type of minerals used, grains sizes, temperature, pH, and other abiotic parameters (Feng et al., 2017). The minerals weathering will also release nutrients or potentially toxic elements on dissolution (Bach et al., 2019), which may affect the environment and the biodiversity (Meysman and Monserrat, 2017). Despite promising results from simulation and laboratory studies, field experiments under natural conditions are urgently needed, to evaluate the effectiveness, biogeochemical and ecological impacts of alkalinity enhancement (e.g. Renforth and Henderson, 2017; Gagern et al., 2019). Its implementation in marginal marine environments will guarantee constant surveillance. PIDALI project strategy to close this gap of knowledge is to implement a novel in-situ experiment in the Ria Formosa Coastal Lagoon (RFCL), southern Portugal. RFCL is a highly dynamic system, with multiple inlets that communicate with the ocean, guarantying daily renewal of water and nutrients. The lagoon is fringed by a succession of salt marshes with different types of vegetation and faunal communities. PIDALI project expected to obtain strategic knowledge on the potential and the risks of alkalinity enhancement, in intertidal environments, for future implementation of upscaling scenarios for decarbonization, contributing to define measures for mitigating climate change by removing CO2 from the atmosphere. To provide this information, data sets will be obtained during a 24-months field experiment where natural alkaline substrates (basalt, olivine) were deployed. The experiment will be installed in a stable and pristine saltmarsh zone with Spartina maritima, comprises three replicates, each with five treatments (coarse olivine, fine olivine, coarse basalt, fine basalt and control). Monthly water samples (supernatant and interstitial) from each treatment and from the water column will be analysed for: temperature, salinity, oxygen, pH, alkalinity, trace metals (Ni, Cu, Cr) and nutrients (NO3, NH4, PO4 and SiO4). Quarterly surface sediment samples from each treatment will be analysed for macrofauna, meiofauna, microfauna, diatoms and bacteria. Statistical analyses and integration of these data will allow: 1) to constrain the effects and environmental impacts of each treatment during two seasonal cycles; 2) to depict changes between treatments and the natural environment and reveal the response of the biological community; 3) to assess the alkalinity enhancement, the toxicity and nutrient release for
each treatment; 4) to constrain the covariation between weathering rates and climatic conditions; 5) to assess positive and negative feedbacks, in particular the response pattern of each organism group; and 6) to identify the treatment with highest efficiency and estimate the potential sequestration of atmospheric CO2 for the entire saltmarsh pioneer zone of RFCL. The results obtained within the project will be compared with a complementary project running at higher latitudes (Germany) to constrain latitudinal variations. PIDALI main findings will be disseminated through the scientific community, students, policymakers, management agencies, and general public. Target actions for the society for public perception about CDR measures will be organized. The expertise, the multidisciplinary background and large experience of PIDALI research team, will guarantee a leading advantage and all requirements to achieve the main scientific goals of the project. During the last decades, the main focus of the research team has been devoted to investigating the ecological, geochemical, oceanographic, sedimentary and hydrodynamic processes, with special focus in the RFCL, largely contributing to the detailed knowledge of this system (e.g. Carrasco et al., 2016; 2018; Cravo et al., 2014; 2019;Schönfeld and Mendes, 2018; accepted).