Influences of Alkalinity and pCO2 on CaCO3 Nucleation from Seawater

Observations on ancient sedimentary carbonates indicate that during the Phanerozoic Eon oscillations between “aragonite seas” and “calcite seas” took place. These changes in carbonate deposition have been attributed to variations in seawater Mg2+:Ca2+ ratios. In this study we investigated the influences of alkalinity and pCO2 on CaCO3 nucleation from synthetic seawater of estimated Albian Cretaceous composition (Mg2+:Ca2+ = 1.2 and diminished sulfate).

Experiments were conducted in which slow degassing of CO2 resulted in an increase of saturation state with respect to CaCO3 ¬and eventually nucleation. pH was monitored during the experiments. From the pH at nucleation and the known initial total alkalinity the pCO2 and degree of supersaturation at which nucleation occurred were calculated. Precipitates were examined by SEM and mineralogy determined by both crystal morphology and XRD analyses.

It was observed that the initial alkalinity greatly affects the pCO2 and supersaturation at which nucleation of CaCO3 occurs. At high (~40 mM) alkalinities calcite commonly formed. At what we deemed a reasonable alkalinity (~10 mM) and pCO2 values (<~2500 µatm) calcite was also observed to nucleate, but at higher pCO2 values only aragonite nucleated. As alkalinity was lowered below 10 mM the upper pCO2 value at which calcite could nucleate also decreased. These results point to the need for the consideration of factors other than just the Mg2+:Ca2+ ratio, such as alkalinity and pCO2 values, in CaCO3 nucleation experiments and in trying to explain the reasons for “aragonite seas” and “calcite seas” at various times during the Phanerozoic Eon.