Ocean alkalinity enhancement in an open-ocean ecosystem: biogeochemical responses and carbon storage durability

Abstract

Abstract. Ocean alkalinity enhancement (OAE) is considered for the long-term removal of gigatonnes of carbon dioxide (CO2) from the atmosphere to achieve our climate goals. Little is known, however, about the ecosystem-level changes in biogeochemical functioning that may result from the chemical sequestration of CO2 in seawater and how stable the sequestration is. We studied these two aspects in natural plankton communities under carbonate-based, CO2-equilibrated OAE of up to a doubling of ambient alkalinity (+2400 µeq kg−1, Ωaragonite∼10) in the nutrient-poor North Atlantic. During our month-long mesocosm experiment, the majority of biogeochemical pools, including inorganic nutrients, particulate organic carbon and phosphorus, and biogenic silica, remained unaltered across all OAE levels. Noticeable exceptions were a minor decrease in particulate organic nitrogen and an increase in the carbon-to-nitrogen ratio (C:N) of particulate organic matter in response to OAE. Thus, in our nitrogen-limited system, nitrogen turnover processes appear more susceptible than those of other elements, which could lead to decreased food quality and increased organic carbon storage. However, alkalinity and chemical CO2 sequestration were not stable at all levels of OAE. Two weeks after alkalinity addition, we measured a loss of added alkalinity and of the initially stored CO2 in the mesocosm where alkalinity was highest. The loss rate in this mesocosm accelerated over time and amounted to ∼10 % of stored CO2 within 4 weeks after alkalinity enhancement. Additional tests showed that such secondary precipitation can be initiated by particles acting as precipitation nuclei and that this process can occur even at lower levels of OAE. In conclusion, in scenarios like our study with carbonate-based OAE, where the carbon is already sequestered, the risk of major and sustained impacts on biogeochemical functioning may be low in the nutrient-poor ocean. However, the durability of carbon sequestration using OAE could be constrained by alkalinity loss in supersaturated waters with precipitation nuclei present. Our study provides an evaluation of the ecosystem impacts of an idealised OAE deployment for monitoring, reporting, and verification in an oligotrophic system. Whether biogeochemical functioning is resilient to more technically simple and economically viable approaches that induce stronger water chemistry perturbations remains to be seen.

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