If you have a solution of calcium carbonate and magnesium carbonate and you add carbonate ions, you change the concentration of the Ca and Mg ions by an amount which depends on the eqquilibrium constants of the respective precipitation/solution rections.

That is, the solubility of Mg and Ca will change differently as CO2 dissolvess in water, and you have told us that foraminifera amplify the differences in solubility.

Those whom the Gods wish to destroy They first make mad. -- Euripides