Potential impact of CO₂ emissions on the seafloor

Carbon Capture and Storage (CCS) aims to capture CO₂ from fossil-fuel sources, transport and store it in suitable geological formations where it will be confined for extremely prolonged times. This is considered a feasible way of mitigating global warming. The seafloor is one of the target areas to safely store large volumes of anthropogenic CO₂ through the development of sub-seabed CO₂ geological storage. For the UK and large parts of Europe, the North Sea is potentially the most suitable location for CCS projects required to achieve planned targets in the reduction of CO₂ emission. Other sub-seabed areas are studied for their storage potential in America and Asia. Even if the technology is considered proven, following the experience gained from the oil industry in using CO₂ for enhanced oil recovery (EOR), and a few CO₂ storage pilot projects, the risk of CO₂ seepage must be addressed considering potential consequences on the marine environment. CO₂ emissions may affect the seafloor with physical (e.g. sediments displacement), chemical (e.g. acidification) and biological (e.g. toxicity) effects. Laboratory experiments and the study of natural submarine CO₂ vents are contributing to the development of a solid body of knowledge on these effects. The research is largely multidisciplinary, requiring expertise in natural sciences, engineering, chemistry and physics. The main observed consequences of CO₂ emissions are a quick and strong acidification of the environment which may lead to the mobilization of pollutants, such as heavy metals, within the sedimentary cover of the seafloor diffusing within the overlying water column. Increased weathering of calcareous substrata is also likely following exposure to CO₂- enriched fluids. Potential impacts upon local biodiversity would include displaced dissolved oxygen leading to anoxic regions, and localised ocean acidification. Consequences of increased CO₂ would be a reduction in local seawater pH, and in the availability of carbonate ions required by calcifying organisms to grow. Predicted CO₂ levels from potential seeps would lead to dissolution of exposed calcium carbonate shells and skeletons of local organisms. While some species would be able to tolerate and survive in such situations, others would die or suffer reduced growth rates. This is particularly pertinent for sessile benthic organisms which could not migrate following the onset of seepage. Naturally occurring submarine CO₂ vents located along active tectonic and volcanic areas provide a good example of how organism communities change depending on their proximity to released CO₂. For example, coral survival and growth decreases the closer they are to the CO₂ vents, with macroalgae dominating the habitat instead. The timing and duration of potential industrial CO₂ seeps would preclude the possibility of organism genetic adaptation over successive generations, and as such, possible impacts would depend upon organism acclimation potential and resilience. This chapter reviews the physical, chemical and biological implications of subseabed storage of CO₂, and the consequences of potential seep events.