Using SEM-Based Cathodoluminescence as a Tool for Exploring Diagenesis and Biomineralization in Corals

Fossil coral skeletons are invaluable paleoclimate archives. However, there are two main challenges with using coral skeletal geochemistry for past climate reconstruction: diagenesis and vital effects caused by the coral biomineralization process. Here, we use Scanning Electron Microscopy and Cathodoluminescence (SEM-CL) to explore crystal growth patterns and diagenesis through both intrinsic luminescence of coral skeletons (visualizable for blue wavelengths of emitted light) and extrinsic luminescence (induced by Mn impurities). Cathodoluminescence microscopy is a traditional tool used to characterize alteration in carbonate samples via Mn-induced luminescence, but has been less commonly used to explore the biomineralization process. Moreover, we still lack a complete understanding of the physical and chemical drivers of intrinsic (blue) CL in coral skeletons.

To help identify causes of blue CL luminescence, we examined a set of modern coral samples, and compared CL data with acridine-orange stained sections of the same samples. Preliminary results indicate a possible connection between blue CL and organic matter content, which may be related to organic-matter-driven distortions of crystal lattice structure. To further explore the utility of CL for both diagenesis and biomineralization, we (1) explored variability in Mn-induced and blue CL across a well-characterized sample set of fossil corals from Enewetak Atoll and (2) conducted culture experiments using Mn labeling. Mn-labeled cultured corals display visualizable banding under SEM-CL, suggesting that this labeling technique can be used to trace coral growth patterns in an accessible way for studies of coral growth. We also detect differences in Mn-induced CL and blue-CL patterns for fossil samples with varying preservation quality. Ultimately this work aims to increase our ability to use SEM-CL as a tool for coral-based paleoclimatology.