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

Abstract. Marine carbon dioxide removal (mCDR) is starting to supplement large-scale emissions reductions to meet internationally recognized climate targets. Ocean alkalinity enhancement (OAE) is an mCDR approach that relies on the addition of dispersed 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 federal waters south of Martha’s Vineyard, MA followed by a 36-hour 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 CTD rosette, and Lagrangian drifters. We developed an MRV 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 umol kg^-1), pH (>0.01) and CO₂ fugacity (fCO₂ >10 μatm). Over 36 hours, the ensuing CDR signal, driven by the gradient in surface fCO₂, grew to greater than 3 μatm in fCO₂, 0.003 pH units, and 1.4 μmol kg^-1 in dissolved inorganic carbon (DIC), translating to 8 % 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.