MODERN SCLERACTINIAN CORALS PRODUCE CALCITE IN EXPERIMENTAL MID-PALEOZOIC/CRETACEOUS SEAWATER

Two species of modern scleractinian corals, Porites cylindrica and Montipora digitata, which produce aragonite in modern seawater, produced calcite in experimental seawater formulated at Mg/Ca ratios which existed during the Ordovician through Mississippian and the Cretaceous.

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 molar ratios are < 1.2 for low-Mg calcite (< 4 mole % MgCO₃); 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).

Two species of corals were grown for 11 months in 5 artificial seawaters formulated at molar Mg/Ca ratios of 1.0, 1.5, 2.5, 5.2 and 7.0, encompassing the range believed to have existed over the Phanerozoic (Hardie, 1996). TEM powder ring diffraction patterns revealed that P. cylindrica and M. digitata grown in artificial Cretaceous seawaters (Mg/Ca < 2) precipitated calcite, while corals grown in the artificial seawaters with Mg/Ca > 2 precipitated aragonite. The aragonite- and calcite-precipitating corals both produced acicular crystals.

EDS electron microprobe analysis revealed that the Mg/Ca and Sr/Ca of the calcitic corals increased from .030 to .060 and .005 to .006, respectively, between seawaters of Mg/Ca ~1.0 and 1.5, thereby mimicking Mg fractionation exhibited by non-skeletal calcite (Füchtbauer and Hardie, 1976) and coralline algae (Stanley et al., 2002). The Mg/Ca and Sr/Ca of the aragonitic corals increased from .003 to .010 and .010 to .016 between seawaters of Mg/Ca ~ 2.5 and 7.0.

During the Ordovician through Mississippian, when Mg/Ca ratios were below 2, Rugose, Tabulate and Heliozoan corals produced low-Mg calcite. It now seems plausible that the Scleractinia precipitated low-Mg calcite during the Cretaceous, when oceanic Mg/Ca ratios were again below 2. Determination of the original mineralogy of Cretaceous corals is required to test this hypothesis. Future experiments, of similar design, will include additional coral genera (Acropora, Astrangia) and will evaluate growth rates.