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System Overview

With this scientific foundation, interventions to amplify the fast-to-slow carbon pathways in the Earth’s natural carbon cycle can be conducted safely, in line with the application of the precautionary principle considered against a baseline of rapidly deteriorating ocean and planetary health.
This protocol describes a system in which responsibly sourced, carbon-rich terrestrial biomass (forestry residues) and carbonate materials (CaCO3 and/or CaO) are processed and combined to make “carbon buoys.” These buoys act as a substrate that may be seeded with marine macroalgae, such as Saccharina latissima or Ulva, and are deployed in the surface ocean to be distributed by ocean currents. As the buoys float and disperse, macroalgae grow, biologically fixing carbon over a period of weeks to months, while the carbonate minerals dissolve, sequestering CO₂ and combatting ocean acidification via ocean alkalinity enhancement. After a calibrated period of time during which the buoy has absorbed sufficient seawater, it flips from positive to negative buoyancy, thus sinking and carrying the embodied organic carbon to the deep ocean for durable storage.
In this context, ‘durable storage’ is synonymous with the movement of carbon from the fast carbon cycle to the slow cycle with negligible reversal risk. Exact durability will vary based on deployment location (targeting sinking sites at a minimum of 1,000 meters in depth), local ocean conditions, and the fate of sunk biomass — but, critically, the sunk carbon will remain out of contact with fast carbon reservoirs because of the slow-moving ocean overturning circulation [Rousselet et al., 2021] and suppressed remineralization of organic carbon at depth [Siegel et al., 2021].
Currently, the benchmark established by Siegel et al. [2021] is used for assessing durability from a specific deployment, wherein the retention of CO₂ injected and dissolved into the ocean interior is sensitive to its location, given general overturning circulation. It’s important to note that the approach described in this protocol does not rely on the injection of gaseous carbon at depth, but on the sinking of intact biomass. Durability is thus likely to be longer than the centennial timeframes predicted by Siegel et al., given that the biomass will first need to be remineralized from its solid form prior to being subject to overturning circulation in an aqueous form. Furthermore, biomass that is buried in marine sediments and avoids remineralization will be removed from the fast cycle for even longer (hundred of thousands to millions of years).
Buoy composition may vary from deployment to deployment and may not necessarily utilize all of the carbon removal pathways outlined in this protocol. For instance, it might consist of the deployment of terrestrial biomass coated with alkaline materials, but not seeded with macroalgae. The composition will depend on the local ocean and operational factors, such as the availability and carbon content of the biomass, to increase the efficiency of carbon removal. This process's organic components are intended to amplify the ocean's biological pump.