A tracer study for the development of in-water monitoring, reporting, and verification (MRV) of ship-based ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is a marine carbon dioxide removal (mCDR) approach that relies on the addition of liquid or solid alkalinity into seawater to take up and neutralize carbon dioxide (CO2) from the atmosphere. Documenting the effectiveness of OAE for carbon removal requires research and development of measurement, reporting, and verification (MRV) frameworks. Specifically, direct observations of carbon uptake via OAE will be critical to constrain the total carbon dioxide removal (CDR) and to validate the model-based MRV approaches currently in use. In September 2023, we conducted a ship-based rhodamine water tracer (RT) release in United States federal waters south of Martha's Vineyard, MA, followed by a 36 h tracking and monitoring campaign. We collected RT fluorescence data and a suite of physical and chemical parameters at the sea surface and through the upper water column using the ship's underway system, a conductivity–temperature–depth (CTD) rosette, and Lagrangian drifters. We developed an OAE analytical framework that explicitly references the OAE intervention and the resulting CDR to the baseline ocean state using these in situ observations. We evaluated the effectiveness of defining a “dynamic” baseline, in which the carbonate chemistry was continuously constrained spatially and temporally using the shipboard data outside of the tracer patch. This approach reduced the influence of baseline variability by 25 % for CO2 fugacity (fCO2) and 60 % for TA. We then constructed a hypothetical alkalinity release experiment using RT as a proxy for OAE. With appropriate sampling, and with suitable ocean conditions, OAE signals were predicted to be detectable in total alkalinity (TA > 10 µmol kg−1), pH (> 0.01), and CO2 fugacity (fCO2 > 10 µatm). Over 36 h, an ensuing additional CO2 uptake was driven by this persistent gradient in surface fCO2. The calculated CDR signal was detectable as a 4 µatm surface fCO2 increase, a pH decrease of 0.004 units, and a dissolved inorganic carbon (DIC) increase of 1.8 µmol kg−1, translating to 10 % of the total potential CDR. This signal, and the CDR itself, would continue to grow as long as an fCO2 gradient persisted at the sea surface. Climatological results from a regional physical circulation model supported these findings and indicated that models and in-water measurements can be used in concert to develop a comprehensive MRV framework for OAE-based mCDR.