(University of Pisa, Italy)
The saturation state control on calcium carbonate pebble reactivity in clastic sea beaches (Marina di Pisa, Italy).
Surface seawater is supersaturated with respect to carbonate minerals, calcite, aragonite and dolomite and it might be expected that seawater cannot dissolve these minerals. In particular calcite, which is the major component of pebbles used as artificial beaches in the Marina di Pisa coastline (Tuscany Region, Italy). Actually, the seawater-carbonate mineral reactivity is described as heterogeneous reactions which are controlled by mixed kinetics between transport-controlled and chemically-controlled. This implies that chemical diffusion through the fluid layer attached to the surface of carbonates affects their dissolution rate, and consequently the relative saturation states. The nature of the seawater-solid interface becomes important in determining the rate of chemical reaction through gradients in chemical potential. In this context, the surface area of reacting solids and the hydrodynamics of the environment become key factors in controlling the dissolution reaction of carbonate phases in contact with seawater. In addition to literature data, simple preliminary experiments on marble pebbles used as clasts for artificial beaches in Marina di Pisa demonstrate their tendency to dissolve in seawater during stirring. It is believed that the complex patterns of heterogeneous dissolution reactions should be carefully addressed before planning coastal management actions.