Assessing the effective settling of mineral particles in the ocean with application to ocean-based carbon-dioxide removal

Abstract

Ocean alkalinity enhancement (OAE), a potential approach for atmospheric carbon dioxide removal (CDR), can involve introducing milled mineral particles into the ocean to promote carbon dioxide uptake. The effectiveness of this method relies on particles remaining in the ocean mixed layer while dissolution takes place, which depends on particle settling rates. Conventionally, particle settling rates are assessed using the Stokes settling velocity in stagnant conditions. However, recent numerical modeling reveals that in dynamic, stratified ocean environments, sediment vertical transport can be up to an order of magnitude faster than Stokes settling because of two types of fluid instabilities that can take place at the mixed layer base. Here, we estimate effective settling velocities in the presence of these instabilities and assess the implications for the efficacy of this particular OAE approach for CDR. The new effective settling rate estimates are sufficiently rapid that there is negligible particle dissolution before particles settle out of the mixed layer. This result is independent of initial particle size for the range of sizes considered here. Findings underscore the importance of considering ocean dynamics and stratification in assessing particle settling rates and provide valuable insights for optimizing OAE applications in diverse marine settings.