Life cycle assessment of ocean alkalinity enhancement using hydrated carbonate minerals

The ocean absorbs around one quarter of anthropogenic carbon dioxide (CO₂) emissions, playing a critical role in regulating the global carbon cycle. This uptake, however, has also contributed to a ~30% increase in ocean acidification, with significant ecological consequences. Ocean alkalinity enhancement (OAE) is being investigated as a scalable carbon dioxide removal (CDR) strategy that may also mitigate acidification. Among proposed approaches, the use of hydrated calcium carbonate minerals such as ikaite (CaCO₃·6H₂O) show promise, but their environmental sustainability remains poorly quantified. This study presents a cradle-to-grave life cycle assessment (LCA) of ikaite-based OAE, combining a baseline (traditional LCA) assessment for 2024 with a prospective LCA extending to 2100 across four regions: Norway, Canada, Russia, and USA-Alaska. Baseline results show that ikaite-based OAE is already net-negative, with greenhouse gas emissions ranging from -638 kg CO₂-eq. tCO₂⁻¹ in Norway to -28.31 kg CO₂-eq. tCO₂⁻¹ in Russia. Environmental impacts are dominated by electricity-intensive ikaite production, particularly water circulation and pressure-swing operations, while limestone quarrying and transport had low contributions. Electricity consumption was the main environmental hotspot, especially in fossil-intensive regions, while a sensitivity analysis revealed that under Paris-aligned climate scenarios, net-negativity improves significantly, with all regions converging to -672 to -677 kg CO₂-eq. tCO₂⁻¹ and endpoint scores of -2.40 to -5.08 Pt by 2100. In contrast, a no-policy scenario yields higher impacts and reduced climate benefits. Overall, ikaite-based OAE offers strong net CO₂ removal potential, but its sustainability depends on low-carbon energy supply, efficient process design, and deployment in cold-water regions. • Ikaite-based OAE achieves net CO₂ removal across all regions studied. • Decarbonised electricity is key to maximising OAE climate benefits. • Production process is the main source of environmental impacts. • Paris-aligned scenarios yield strong improvements in all impact categories. • Early deployment should focus on cold-water regions with clean energy.