Compositional heterogeneity and microstructural diversity of coral skeletons: implications for taxonomy and control on early diagenesis
Résumé
Fine-scale features of scleractinian skeletons were investigated in living colonies, from ultra-thin sections and SEM preparations. In contrast to what is commonly admitted, the coral fiber is a composite structure differing markedly from a simple aragonite crystal unit. The heterogeneity of coral fiber is shown by the occurrence of a micron-scale zonation resulting from incremental growth during elementary cycles of biomineralization. In addition, high magnification SEM reveals that a calcification center corresponds to a particular structural element clearly differentiated from the surrounding fibers and defined by its own crystal arrangement. The duality of this center/fiber arrangement corresponds to a general architecture of scleractinians, resulting from similar processes of skeletogenesis. An additional fine-scale diversity related to taxonomy is reflected by variations in the geometry and crystallinity of centers and also in the strength and regularity of fiber incremental zonation. These initial differences both within the same colony and between distinct taxa would be emphasized during diagenetic history, leading to differential susceptibility of structural elements to diagenetic processes and also to specific behavior of distinct taxa in relation to diagenesis.