Probing salty waters: re-assessment of the early stages of calcium carbonate formation in seawater

Calcium carbonate formation in the oceans is a major component of the global carbon cycle. Yet, nucleation and crystallisation in ionically-complex seawater are not well understood. Here, we investigate CaCO 3 nucleation and growth using potentiometric titration experiments in model solutions directly relevant to marine calcifying organisms. We develop a new framework for the conceptual interpretation of titration experiments, based on influx and outflux of ions in a dynamic system, and demonstrate the influence of the seawater major ions on CaCO 3 nucleation and precipitation dynamics. Before nucleation, we find a significant underrepresentation of Ca-CO 3 interactions in state-of-the-art ion speciation models, suggesting strong ion association in solution, in seawater at conditions relevant to biomineralisation. After precipitation, structural, microscopic, and isotopic analyses indicate the presence of a nano-scale, disordered phase at precipitate surfaces, potentially contributing to crystal growth through particle attachment. The disordered phase forms at or in vicinity of crystal surfaces, and not as a distinct amorphous precursor in solution. Our findings show that ion association-driven processes during nucleation and crystal growth may be the usual case for inorganic aragonite formation in greatly supersaturated solutions, possibly implying this is the case for all or most (bio)carbonates, which is a significant conceptual shift for the understanding of carbonate formation in marine settings.