CALCIUM DYNAMICS IN MARINE ENVIRONMENTS: CALCIUM SPECIATION IN SEAWATER AND TRANSFORMATION OF AMORPHOUS CALCIUM CARBONATES

Abstract

This dissertation examines two aspects of the calcium dynamics in marine environments: the speciation of calcium in seawater and the transformation of amorphous calcium carbonate. These aspects are crucial for understanding the marine carbon reservoir, which is particularly important in the context of global warming.The speciation of calcium and magnesium in surface seawater was performed using a combination of theoretical and experimental methods. The findings indicate that approximately half of the dissolved calcium and magnesium exist as free ions, while the remainder is complexed with major anions such as chloride and sulfate. This updated speciation indicates that the supersaturation of calcite and aragonite has been overestimated in surface seawater, and vaterite might be close to saturation. The results may change perspectives of the ocean’s capacity as a carbon sink, and the mineral that limits the concentration of calcium and carbonate in seawater. A comprehensive study on the structure, stability, and transformation of amorphous calcium carbonate (ACC) is also presented. ACC is a common precursor in the biomineralization process of calcium carbonate. Two ACC phases with distinct structural features were synthesized from solutions with varying supersaturation levels. Structural characterization shows that the distortion of carbonate ions, stabilized by hydrogen bonding with structural water, plays a crucial role in the stability of ACC. Seawater ions, such as magnesium and barium, could enhance the stability of ACC by strengthening the hydrogen bonds and inducing strain in ACC structure, due to mismatches between metal-oxygen polyhedrons. Magnesium also affects the transformation selectivity of ACC by changing the surface chemistry of particles, while strontium and barium have negligible impact at seawater concentrations and substitute for calcium in the transformation product. The molecular properties of dissolved O2 in aqueous solutions compared to the gaseous O2 are discussed. The results highlight the impact of hydration on the reactivity of O2, particularly for redox reactions in aqueous environments. The dissertation provides new insights into the role of seawater ions in calcium dynamics in marine environments. The findings are crucial for a deeper understanding of the marine carbonate system and its response to potential climate changes.