MAGNESIUM CONTENT OF AMORPHOUS CALCIUM CARBONATE (ACC) IS REGULATED BY THE INTERPLAY OF PH AND CARBONATE CONCENTRATION

Recent studies in structural biology reveal that diverse calcifying organisms utilize amorphous calcium carbonate (ACC) as an intermediate phase in the biomineralization of calcite. These findings raise the question of whether biogenic calcites that form via this amorphous phase have chemical signatures and properties that differ significantly from calcites that result from traditional step-growth processes. The Mg content of ACC is of particular interest because this IIA ion is a strong inhibitor of calcite growth, and is widely used as a proxy for paleotemperature. Boyle and Erez (2004, Eos Trans. AGU) reported a correlation between the Mg content of foram calcite and bottom water CO₃^2- concentrations, suggesting that foraminiferal Mg/Ca could be controlled by CO₃^2- rather than temperature. Here, we test the hypothesis that the alkalinity of the local growth environment regulates the Mg content of ACC.

To investigate the influence of alkalinity on the Mg content of ACC, a new flow-through reactor procedure was developed to synthesize this phase under solution conditions that maintain a constant supersaturation and temperature. The Mg:Ca of the solution was held at 5:1 to simulate modern seawater, and alkalinity was varied by adjusting the concentration of NaHCO₃ in combination with pH. ACC products were characterized using SEM, Raman Spectroscopy, TGA, and ICP-OES. The data show that ACC with Mg concentrations of 16 mole % to >50 mole % can be produced from solutions with the same Mg:Ca by adjusting the carbonate concentration and the pH of the precipitating solution. Results do not show a simple relationship between the Mg content of ACC and alkalinity, but further analysis reveals a linear correlation between the partitioning of Mg into ACC and the activity product of CO₃^2- and OH^-. This systematic relationship demonstrates the composition of ACC is not solely controlled by solution Mg:Ca and temperature, and provides a predictive means for tuning the Mg content of ACC during controlled synthesis. The findings suggests a chemical basis for how organisms may regulate the composition of this amorphous intermediate through the interplay of the local pH and CO₃^2- concentration at sites of calcification.