A new pathway to enhance the oceanic carbon sink: inorganic carbonate precipitation driven by calcium-alkali coupling

Anthropogenic CO₂ emissions are intensifying climate change, creating an urgent need for scalable and efficient strategies to enhance the oceanic carbon sink. This study proposes and evaluates a new calcium-alkali coupling pathway in which the co-addition of Ca²⁺ and OH^- promotes the conversion of seawater dissolved inorganic carbon (DIC) into solid CaCO₃. This process perturbs carbonate-system equilibrium and subsequently drives additional atmospheric CO₂ uptake during system re-equilibration. Unlike conventional alkalinity-enhancement approaches that mainly retain absorbed CO₂ as dissolved bicarbonate and carbonate species, the pathway examined here emphasizes carbonate precipitation as a major mode of DIC removal. Laboratory experiments, mesocosm simulations, and a 1000 m³ offshore field demonstration in the southern Yellow Sea were conducted to assess carbon-removal efficiency and short-term environmental responses. Under a dosing scheme of 1.79 mmol Ca²⁺ and 3.58 mmol OH^- per litre of seawater, DIC in the 1000 m³ field system decreased by 1763 μmol kg^-1 and was converted into CaCO₃, corresponding to an estimated uptake of 77.6 kg atmospheric CO₂. During the observation period, the monitored physicochemical parameters and trace-metal indicators showed only limited and controllable perturbations. A first-order scaling estimate based on this single-application, dose-dependent capacity suggests a theoretical carbon-sink enhancement potential of 2.48 Pg C on the Chinese continental shelf and 74 Pg C across global continental shelves. Because the carbonate system tended to relax toward its near-initial state after intervention, repeated applications may be feasible in principle and could provide considerable carbon-sink enhancement potential, although their long-term effectiveness, ecological consequences, and practical scalability require further evaluation. Overall, these results indicate that calcium-alkali coupling provides an efficient and mechanistically distinct pathway for engineered enhancement of the oceanic carbon sink.