Risk of Reversal

If sunken carbon makes its way into ocean sediments, either through inorganic or biogeochemical pathways, it will remain out of the fast carbon cycle for geological time scales [Rousselet et al., 2021], with the risk of reversal close to non-existent by design of the system deployed. This design includes selecting only sinking locations that allow for durable storage of accumulated biomass based on depth and local ocean conditions (stability, rates of sedimentation). Selection of sinking locations has a significant impact on mitigating potential reversal risks and is a key component of deployment design; as an example, the movement of deeper water to the surface driven by surface winds (i.e. upwelling) could potentially reduce the durability of storage by recirculating dissolved carbon in the deep ocean back to the surface faster than would occur with typical ocean circulation patterns. However, project activities are conducted far offshore and away from coastal zones where upwelling is prevalent. Non-coastal zones that are associated with upwelling, such as the Equatorial Pacific, would also not be considered ideal sinking locations for that reason.

Once sunk below the ocean’s thermocline, deep-sea ocean conditions and the ocean's natural circulation patterns mitigate virtually all natural reversal risk. Deployment areas must be monitored for potential economic use that could impact the stability of the deep-water area, such as deep-sea mining, trawling, or similar economic activities that could disturb the seabed — but these risks are minimal and can be mitigated through careful site selection and monitoring of deployments.

The exact durability of storage may vary based on site selection, local ocean conditions, and the end fate of sunk biomass, and further research will be conducted to advance a shared understanding of deep ocean circulation patterns.