CL microscopy

Fossil corals were analyzed using CL microscopy. Thin sections of corallites cross-sectioned in the transverse plane were polished and coated with carbon. CL analysis was performed using a hot cathode microscope, HC1-LM, at the ZPAL, with the following parameters: an electron energy of 14 keV and a beam current density of 0.1 μA mm⁻². CL is a simple method to determine the spatial distribution of primary (aragonite) and secondary (calcite) features in coral skeleton (20). Diagenetic calcite typically contains a high concentration of Mn²⁺ [the main activator of luminescence in carbonates (39)] and exhibits strong orange-to-red luminescence. In contrast, in original skeletal aragonite, the abundance of Mn is much lower than that in diagenetic calcite, especially because of (i) a higher partition coefficient for Mn in calcite than in aragonite and (ii) higher concentrations of Mn in the reducing waters (relative to seawater) from which secondary cements are often formed.