Interactions of CO2 with sedimentary blue carbon: the fate of leaked CO2 from a geological storage site

One of the main considerations in terms of carbon dioxide removal techniques is the net carbon storage potential, i.e. whether the amount of carbon that can be stored, significantly outweighs the carbon footprint of the process of storage. Here we assess the potential for disruption of blue carbon stored in sediments, and interactions with leaked CO₂ above a sub-seafloor geological storage site. Blue carbon is carbon stored within the ocean which, in this study, is in the form of sedimentary calcium carbonate, or organic carbon. CO₂ injected into sub-sea bed geological storage sites can be effectively trapped and retained. Despite the perceived safety of geological CO₂ storage, it is imperative to thoroughly evaluate and address the risks of carbon loss either through disturbance of sea bed carbon with infrastructure, or through the impacts of potential CO₂ leakage from the storage reservoir. This study aims to quantify the amount of blue carbon, and its different components, which is at risk of loss above a proposed CO₂ reservoir in the North Sea. Second we investigate the impact of CO₂ leakage from the reservoir on sediment-stored blue carbon through laboratory based experiments. The sediments in the North Sea were found to contain minimal organic carbon but a significant variable fraction of biogenic calcite in the form of shells. The leaked CO₂ was found to act as an acid titrating away the CO₃^2- ion in seawater to drive undersaturation with respect to calcite until equilibration occurs between the CO₂ stream and the calcite. The study infers that sites abundant in particulate inorganic carbon (PIC) face a heightened risk of blue carbon depletion in the event of leakage, but with considerable potential for re-sequestration of the escaped CO₂ into solution as the HCO₃^- ion, as a result of enhanced dissolution of seafloor calcium carbonates and release of buffering alkalinity. This aqueous storage of any released CO₂ will be limited under low rates of release, due to the titration of the carbon dioxide with the alkalinity already present in the sediment pore water. Conversely, at higher release rates the re-sequestration of the escaped CO₂ is controlled by the solubility of the calcium carbonate under near CO₂ saturated conditions, such that the proportion of resequestered CO₂ can be quantified through a thermodynamic framework. Locales rich in particulate organic carbon (POC) tend to exhibit a reduced susceptibility to blue carbon loss, yet have a reduced neutralization potential for CO₂ leakage.