Calcitic Scleractinian Corals: When, where and why?

It has been proposed that the molar Mg²⁺/Ca²⁺ ratio of seawater may directly control the skeletal mineralogy of corals, favoring an aragonitic mineralogy when the Mg²⁺/Ca²⁺ ratio is above 2 and calcitic mineralogy when the Mg²⁺/Ca²⁺ ratio below 2. In concordance with this hypothesis, calcitic deposits were observed in some zooxanthellate scleractinians growing in artificial seawater with Mg²⁺/Ca²⁺ = 1.0 (Geology 2006 34:525-528). Furthermore, structural and biogeochemical observations indicated that the solitary scleractinian Coelosmilia produced pristine and purely calcite skeleton in the Maastrichtian (ca. 70-65 million years ago) when Mg²⁺/Ca²⁺ ratio was presumably about 1 (Science 2007 318: 92-94).

Here we show that diverse solitary corals with well-preserved calcitic skeletons occur both in Europe in earlier epochs of Late Cretaceous (Campanian-Santonian) and in the Danian (Paleogene) deposits of America. All calcitic skeletons with well-preserved micro-structural features, represent simple "Desmophyllum-like" organization. The mid-septal zone consists of minute calcification centers, with bundles of fibers radiating perpendicular to the skeletal surfaces. Extremely fine-scale microstructural features, bioerosion-activity within the skeleton, occasional silification developed along the original organic-enriched regions, and other geochemical signatures support the conclusion that the original mineralogy of these fossils was purely calcitic. In contrast, some Turonian and Cenomanian solitary scleractinians whose skeletons are preserved in sediments devoid of originally aragonitic fossils, have calcitic coralla affected by diagenesis and are clearly recrystallized. Additionally, throughout the Upper Cretaceous deposits, there are abundant examples of scleractinians that produced aragonite skeletons, including skeletons with more advanced types of microstructural organization.

Based on these observations, we propose that mineralization of the Cretaceous scleractinians might be affected by changes in Mg²⁺/Ca²⁺ sea-water ratio but that the actual skeletogenic response is different among taxonomically different corals and strongly controlled by the biological processes involved in the skeletal formation.