The scientific validation is widely recognized as a critical step towards scale-up deployment of any marine carbon dioxide removal (mCDR) pathways, and field experiments constitute the heart to achieve this validation. The Locking Ocean Carbon in the Northeast Shelf and Slope (LOC-NESS) Project was established to evaluate the effectiveness and environmental impacts of ocean alkalinity enhancement (OAE). The project is funded by private and public funds, and has no financial stake in the development of mCDR.

In August 2025, after securing an EPA permit through the Marine Protection, Research, and Sanctuaries Act, the LOC-NESS team conducted LOC-02, an engineered dispersal of 16,500 gallons 50 wt% sodium hydroxide (NaOH) and 250 gallons of 20% Rhodamine Water Tracer dye in Federal waters in the Wilkinson Basin area of the Gulf of Maine. The engineered dispersal occurred over a period of 6 hours from a supply vessel into the ship’s wake, with the R/V Connecticut conducting surface underway measurements of the carbonate system (pH, pCO₂, and total alkalinity (TA)), rhodamine fluorescence, oxygen, and photosynthetic health (Fv/Fm) in real time directly behind the dispersal vessel. Thereafter, to constrain the CO₂ uptake and the impacts and fate of the dispersal, the research team conducted a round-the-clock, comprehensive monitoring campaign, tracking the TA-rhodamine patch for four days on the R/V Connecticut. A fleet of three Spray2 gliders and one long-range autonomous underwater vehicle equipped with pH, fluorometer, and other biogeochemical and physical sensors were deployed, providing additional monitoring assets before, during, and after dispersal. During the monitoring phase of the trial, complemented to the continuous underway monitoring, vertical CTD profiles with discrete sampling, primary production and calcification incubations, and plankton net tows were conducted inside and outside the TA-rhodamine patch. Satellite imaging was also used to evaluate the patch dynamics and CO₂ uptake. This field campaign aimed to establish and compare multiple monitoring approaches for OAE Monitoring, Reporting, and Verification (MRV).

We present the results from this comprehensive OAE-MRV data set, which allows the team to assess and validate the engineered dispersal of alkalinity and tracer; estimate alkalinity loss due to secondary mineral precipitation from OAE deployment; document the environmental impacts of NaOH-based OAE on the local ecosystem; and use multiple state of the art techniques to quantify the CDR efficiency of the OAE dispersal. The data analysis reveals that OAE deployment and CDR signals are detectable and verifiable using different monitoring approaches during the field trial; a physical tracer, such as rhodamine, is a fundamental component for early OAE field trials; secondary precipitation can be a significant loss term for OAE deployment; and initial OAE effects on plankton and fish species were minimum.

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