Review and syntheses: Ocean alkalinity enhancement and carbon dioxide removal through coastal enhanced silicate weathering with olivine

Abstract. Coastal enhanced silicate weathering (CESW) is increasingly receiving attention as a marine-based carbon dioxide removal (CDR) technology. The method aims to achieve ocean alkalinity enhancement (OAE) by introducing fast-weathering silicate minerals into coastal systems. The latter is envisioned to act as a large natural biogeochemical reactor, where ambient physical and biological processes can stimulate silicate dissolution, thus generating a concomitant alkalinity release and increasing the seawater’s capacity to sequester CO₂. Olivine has been forwarded as the prime candidate mineral for CESW, but to the present, no results from larger-scale field studies in actual coastal systems are available, so all information is exclusively derived from idealized laboratory experiments. As a result, key uncertainties remain concerning the efficiency, CO₂ sequestration potential, and impact of olivine-based CESW under relevant field conditions. In this review, we summarize recent research advancements to bridge the gap between existing laboratory results and the real-world environment in which CESW is intended to take place. To this end, we identify the key parameters that govern the dissolution kinetics of olivine in coastal sediments, and the associated CO₂ sequestration potential, which enable us to identify a number of uncertainties that are outstanding with respect to the implementation and upscaling of olivine-based CESW, as well as the monitoring, reporting, and verification (MRV). From our analysis, we conclude that the current knowledge base is not sufficient to predict the outcome of in situ CESW applications. Particularly, the impact of pore water saturation on the olivine dissolution rate and the question of the additionality of alkalinity generation remain critical unknowns. To more confidently assess the potential and impact of olivine-based CESW, dedicated pilot studies under filed conditions are needed, which should be conducted at a sufficiently large spatial scale and monitored for a long enough time with sufficient temporal resolution. Additionally, our analysis indicates that the specific sediment type of the application site (e.g. cohesive versus permeable) will be a critical factor for olivine-based CESW applications, as it will significantly impact the dissolution rate by influencing the ambient pore water pH, saturation dynamics, and natural alkalinity generation. Therefore, future field studies should also target different coastal sediment types.