A NEW APPROACH TO UNDERSTANDING MG-CALCITES AND IMPLICATIONS FOR THEIR OCEANIC RESPONSE TO INCREASING ATMOSPHERIC CARBON DIOXIDE

During the recent decade, considerable attention has been focused on the problem of declining surface seawater carbonate saturation state due to past and future rising atmospheric CO2 concentrations and the effect on marine calcifying organisms. It has been shown in experimental situations for both single calcifying species of corals, foraminifera, coccolithophorids, and coralline algae and carbonate communities that as seawater carbonate saturation decreases, the rate of calcification decreases. This along with the predicted rising temperatures of the future and other stresses could have devastating effects on coral reef and other carbonate ecosystems. Our knowledge of the effect on the magnesian calcite calcifiers is limited because of the problems involving the basic thermodynamic and kinetic properties of these phases. For the naturally occurring biogenic phases, only a kinetic "equilibrium", expressed as log IAP, obtained from dissolution experiments has formed the foundation for the "solubilities" of these phases.

Here we show that the only valid curve that one can provisionally use for the metastable stabilities for the magnesian calcite compositions as a function of magnesium content is that for the synthetically produced phases. In this case metastable equilibrium has been achieved from both under- and over-saturation. An equation has been derived that couples the thermodynamic solubility product to the thermodynamic distribution coefficient, which can be approximated as being close to constant up to about 20 mol % Mg. This overcomes a primary weakness in use of stoichiometric solubility products by confining the Mg to Ca ratio of the solution to a single value for a given Mg concentration in calcite. Surprisingly, the same equation works well for many, but not all, biogenic calcites if the solubility product of Mg-free biogenic calcite is set at about 1.3 times that of pure abiotic calcite.