Ultrastructural controls on diagenetic patterns of scleractinian skeletons: evidence at the scale of colony lifetime
Abstract
The earliest fine-scale diagenetic changes were investigated by comparing microstructural and ultrastructural features from the uppermost skeletal parts of living colonies with those obtained from the older parts of the same specimen. In addition to its organo-mineral duality, the initial heterogeneity of scleractinian skeleton is shown by the occurrence of a micron-scale zonation of fibers resulting from incremental growth during elementary cycles of biomineralization, and also by the presence of two basic structural features, fibers and calcification centers, clearly differentiated from each other. Within the species analyzed, micro- and ultrastructural data reveal an additional fine-scale diversity related to taxonomy. At the timescale of colony life, the earliest processes of diagenesis produce a thin fringe of syntaxial aragonite cements, alteration of the incremental zonation of scleractinian fibers and also preferential diagenetic changes within calcification centers. These first modifications of coral skeletons are obviously controlled by the biological ultrastructural characteristics of scleractinian taxa and also suggest that early diagenesis does not necessarily imply drastic changes of environmental conditions.