SIMILAR EFFECTS OF CHANGES IN THE MAGNESIUM/CALCIUM RATIO OF SEAWATER OVER GEOLOGICA TIME ON THE CALCIFICATION OF AN ARAGONITIC BENTHIC ALGA (HALIMEDA) AND CALCITIC PLANKTONIC ALGAE (COCCOLITHOPHORES): A TRIBUTE TO LAWRIE HARDIE

The clever analysis by Lawrie Hardie of the history of the Mg/Ca ratio of seawater and the mineralogy of carbonate and evaporate inorganic precipitates triggered the present research, which compares the effects of the ambient Mg/Ca ratio of seawater on the mineralogies and rates of calcification for the aragonitic benthic green alga Halimeda and the planktonic coccolithophores, which produce calcite. Nonskeletal precipitation of calcium carbonate at the salinity of modern seawater is in the form of calcite at Mg/Ca ratios below 2 and aragonite at Mg/Ca ratios above 2. Modern seawater has the very high ratio of 5.2. The work of Hardie and others has shown that the Mg/Ca ratio of seawater was ~0.5 at the start of the Late Cretaceous and rose to only ~1.0 by the end of the Cretaceous. It continued to rise throughout the Cenozoic, crossing the critical level of 2 at about the end of the Eocene.

The equation for precipitation of calcium carbonate can be written as follows:

Ca²⁺ + 2H₂CO₃ = CaCO₃ + H₂O + CO₂

Because of this production of CO₂ it was hypothesized that if the ambient Mg/Ca ratio were shifted in a direction favoring their mineralogy, both kinds of algae would not only calcify at a higher rate but also grow faster. Experiments employing artificial seawater confirmed this hypothesis. Controlled experiments showed that both the Mg/Ca ratio and the absolute concentration of Ca²⁺ influenced growth rates for both kinds of algae. Furthermore, Halimeda produced a substantial amount of calcite at low Mg/Ca ratios. The exponential growth rates of populations of two coccolithophore species themselves increased exponentially as the ambient Mg/Ca ratio was lowered to Cretaceous levels! The results account for the massive chalks deposits of the Late Cretaceous. “Creta” is the Latin word for chalk. This research explains why the Cretaceous has its name.

My colleague and great friend Lawrie Hardie changed my life by urging me to examine relationships between seawater chemistry and biocalcification, and for this I will be forever grateful.