MODERN HIGH-MAGNESIUM CALCITE ORGANISMS PRODUCE LOW-MAGNESIUM CALCITE IN EXPERIMENTAL MID-CRETACEOUS SEAWATER

Four modern calcareous organisms altered their skeletal chemistry when grown in artificial seawaters mimicking ones which existed at various times during the Mesozoic and Cenozoic.

The Mg/Ca ratio of seawater, controlled primarily by rates of seafloor spreading, has governed the mineralogy of nonskeletal marine carbonate precipitates during the geologic past (Hardie, 1996; Lowenstein et al., 2002). The requisite Mg/Ca mole ratios are < 1.2 for low-Mg calcite (< 4 mole % Mg2+ substituting for Ca2+); 1.2–5.0 for high-Mg calcite; and > 2.0 for aragonite. The calcite-aragonite transitions for non-skeletal CaCO3 are temporally coordinated with similar transitions in the mineralogy of major reef builders and sediment producers throughout the Phanerozoic (Stanley and Hardie, 1998).

Four types of organisms that secrete high-Mg calcite in modern seas (echinoids, crabs, shrimp and serpulid worms) were grown for 160 days in 6 chemically unique artificial seawaters. The artificial seawaters were identical except for their Mg/Ca ratios, which were formulated at 1, 1.5, 2.9, 4.4, 5.4 and 6.7. All four organisms incorporated more Mg into their skeletons as ambient Mg/Ca increased. Specimens grown in the seawater with the lowest Mg/Ca (~ 1) actually changed their mineralogy to low-Mg calcite.

The fractionation curves generated by these experiments were unique for each type of organism, as well as for each skeletal component of the organism. Furthermore, they were all slightly lower than experimentally determined fractionation curves for non-skeletal magnesian calcite (Füchtbauer and Hardie, 1976, 1980). Therefore, while the mineralogies of the organisms are clearly influenced by the ambient seawater chemistry, the severity of the influence varies between the organisms. Such variation does not appear to be linked to the organism’s physiological complexity. It is probably related to the organism’s ability to remove Mg2+ from its skeleton-precipitating solution, or to exclude it directly from its calcite crystals via organic mineral templates.

The experiments suggest that the skeletal chemistry of modern high-Mg calcareous organisms has varied significantly over the Phanerozoic as a function of the Mg/Ca of seawater. During the Mid-Cretaceous, when oceanic Mg/Ca was lowest (~ 1), it is likely such organisms changed their mineralogy to low-Mg calcite.