Isometric recently issued the world’s first verified Ocean Alkalinity Enhancement (OAE) carbon dioxide removal credits to Planetary. This represents the culmination not only of Planetary’s or Isometric’s efforts, but also of the broader scientific community working to advance OAE as a promising carbon removal solution.
These credits are an important step towards scaling marine carbon removal pathways responsibly, and we are excited to share a behind the scenes look at how the credits were quantified. Additional information on safety can be found in the Operations Report.
Background
Planetary’s Tufts Cove OAE Project is located in Halifax Harbor—an industrialized estuary, home to a busy commercial port and significant wastewater discharge. Water circulation in Halifax Harbor is predominantly seaward on the surface with a subsurface return current into Bedford Basin, although the specific water flows vary based on season and wind patterns (Burt et al., 2013).
In this project, alkaline feedstock is added to an existing outfall at the Tufts Cove power generation station, which discharges into a tidal channel. The channel is 20 meters deep, vertically well-mixed due to tidal mixing, and has a mean residence time of 3-13 days (Wang et al., 2025). For more details on the project site, see Section 1.1 of the Project Design Document.
The alkaline feedstock primarily consists of magnesium oxide (MgO) and magnesium carbonate (MgCO3). Before the feedstock was added, measurements were conducted in accordance with Isometric’s Rock and Mineral Feedstock Characterization Module to assess the feedstock’s safety and alkalinity content. More information on the Feedstock Characterization can be found in Appendix 2 of the Project Design Document (PDD).
Quantification approach
The Ocean Alkalinity Enhancement from Coastal Outfalls Protocol outlines three steps to quantify gross carbon removal. Below is an overview of how these steps were applied to Planetary’s project.
Step One: Outfall measurements
In Planetary’s project, the feedstock was combined with seawater in a mixing tank before being discharged through the existing outfall. The rate of this alkalinity addition was monitored and controlled by Planetary’s dosing system. The dosing was gradually increased over time, starting with an initial low-dose testing phase, followed by a period of gradually ramping-up the dosing, before reaching a continuous dosing phase.
The amount of alkalinity dosed was quantified by measuring the alkalinity content of the feedstock and the total mass of feedstock that was dosed. Continuous measurements of the seawater influent and effluent—representing conditions upstream and downstream of dosing—were conducted to monitor for regulatory compliance. The following parameters were continuously monitored: pH, turbidity, total suspended solids (TSS), temperature, and salinity. In addition, weekly samples were taken to measure trace metal content, salinity, total alkalinity, and pH. See the Operations Report for further monitoring details and results.
Step Two: Alkalinity upscaling
The dispersion and mixing of the alkaline plume from the outfall were characterized by two dye-tracer experiments led by Dalhousie University, which identified a conservative dilution factor of 10x within 50-100 meters of the outfall.
The feedstock was demonstrated to be fast dissolving— achieving 95% dissolution within 21 hours and full dissolution within 4 days—based on laboratory dissolution experiments combined with a particle sinking and dissolution model that was applied to realistic particle size distributions (PDD pp. 17-19, 91-92). The sinking and dissolution model generates a time series of vertical alkalinity release profiles, which are used as an input to the near-field model described in Step 3 below. The fast dissolution rate is further supported by field measurements showing no feedstock accumulation in sediments near the discharge site (Operations Report, p. 17).
The following potential alkalinity loss processes were avoided using mitigation strategies, which are detailed in pp. 48-49 of the PDD and the Operations Report:
- Secondary precipitation was avoided by maintaining pH below 9 at the end of the pipe and leveraging rapid dilution with seawater upon discharge.
- Changes in biotic calcification were avoided by adhering to thresholds of pH < 9 and ΔTA<1000 μmol/kg—which is lower than in previous studies that demonstrated no significant change in biotic calcification (Gately et al, 2023).
- Changes in natural alkalinity fluxes in sediments were avoided by limiting alkalinity accumulation on the seabed. This was achieved by controlling the dosing rate, leveraging a fast dissolving feedstock, and full water column mixing and resuspension. Ongoing sediment sampling was conducted to monitor accumulation.
Step Three: Air-sea carbon dioxide uptake
Air-sea CO2 uptake was quantified using numerical ocean models that simulate the ocean’s physics, chemistry and biology. These models reflect the latest scientific understanding of ocean dynamics relevant to OAE quantification and have been validated against observational data.
The CO2 uptake attributable to the project was determined by comparing the cumulative air-sea CO2 flux between two simulations: one representing baseline conditions and one representing the OAE project. Specific configurations of these models were developed to quantify the air-sea CO2 uptake attributable to this project.
For this project, air-sea CO2 uptake was first quantified in the near-field domain using a regional model. Any unequilibrated alkalinity that leaves the near-field domain was used as an input to a global model to quantify further CO2 uptake in the far-field domain.
This first credit issuance includes all air-sea equilibration that occurred up to March 31, 2025—representing over 50% of the total expected uptake upon complete equilibration.
Near-field
The near-field region was modeled using Halifax ROMS (Regional Ocean Modeling System) for the physical model, coupled with a biogeochemical model (Laurent et al., in prep) developed by the Fennel Lab at Dalhousie University. The modeled domain includes the outfall and extends across Bedford Basin, Halifax Harbor, and the continental shelf. The model is 3-level nested, with horizontal resolution ranging from 61 meters to 1 kilometer, which enables high-resolution simulation of smaller scale dynamics that occur near the coast.
Alkalinity dosing into the model was determined based on the measured dosing time series from Step One and the vertical alkalinity profile modeled using the sinking and dissolution model. The OAE simulation was run multiple times under different historical conditions to assess the interannual variability of CO2 uptake. Years with extreme weather (e.g., hurricanes) were included in the ensemble to capture a wider range of environmental conditions to ensure conservatism. Any unequilibrated alkalinity leaving the near-field domain was tracked to be used as an input to the global model.
Far-field
The far-field domain was modeled using ECCO (Estimating the Circulation and Climate of the Ocean) for the physical dynamics, with biogeochemistry based on Dutkiewicz et al. (2005). The ECCO domain is global, with a horizontal resolution of ⅓ degree (~30 kilometers), and has previously been used to study air-sea equilibration from OAE. Interannual variability in the far-field model was similarly quantified using an ensemble of simulations based on different historical conditions.
See PDD pp. 16-17, for the full suite of sensitivity studies conducted using ROMS and ECCO.
Project emissions
Emissions related to project activities were quantified according to Section 7.4.4 of the Protocol—which aligns with best practices from ISO standards—and subtracted from the gross carbon removal. This includes emissions related to feedstock production and transport, site operations and monitoring, personnel transportation and accommodation, and project establishment and closure. Further details can be found in the Life Cycle Analysis (LCA) Emissions Statement.
Uncertainty
The following sources of uncertainty were quantified and propagated into an overall uncertainty value, which is subtracted from the mean carbon removal quantification for conservatism:
- Uncertainty in mass of feedstock dosed
- Uncertainty in the feedstock alkalinity content
- Interannual variability in the near-field model
- Interannual variability in the far-field model
- Uncertainty in the air-sea CO2 flux parameterization
- Alkalinity production and transportation emissions
- Operational energy use emissions
- Project establishment emissions
- End-of-life emissions
See the Quantification Report for further detail on the uncertainty values and their propagation.
Next steps
We are excited to share the learnings from the first issuance of OAE credits to support continued progress and innovation in OAE.
You can explore the credits on the Isometric Registry, along with associated source documents including the Quantification Report, Emissions Statement, Operations Report, and PDD, for more details on this project.
We look forward to continued discussion and welcome any questions, comments, or suggestions for future improvements. Please get in touch at contact@isometric.com.
