Biogenic calcite and aragonite are major constituents of many marine calcifying organisms such as foraminifera, mollusks, corals, echinoderms, and bryozoa [ 1]. The mineralized hard parts of these organisms commonly exhibit exceptional complexity in morphological architecture and composition, significantly differing from those found in the inorganic world. Most notably, biogenic calcium carbonates contain a significant fraction of organic material (up to 4 wt%) that is heterogeneously intermingled with the mineral moiety at the nanoscale and is responsible for the unique mechanical properties of biominerals [ 2]. The microstructural and compositional analysis of biogenic CaCO3 presents unique challenges, particularly when employing advanced microanalytical techniques like Electron Probe Microanalysis (EPMA), and Electron Backscatter Diffraction (EBSD), Photo-induced Force Microscopy (PiFM), Atomic Force Microscopy (AFM), micro-Raman spectroscopy etc.
In this study, we analyzed skeletal hard parts of bryozoa, a diverse group of predominantly marine, colonial suspension-feeding animals. Bryozoa skeletons consist of calcite and/or aragonite [ 3]. This group is of interest as potential paleo-environmental and climatic proxy archives, as they can be traced back to the Ordovician period in earth history [ 3].
The major challenges for the microanalysis of biominerals in general and bryozoan skeletal parts in particular are caused by the intricate organic-inorganic intergrowth in the skeleton, where occluded organic macromolecules impact sample preparation and cause the material to be more sensitive to electron and laser beam damage than geological calcite. Furthermore, bicarbonates are well-known to have heterogeneous crystallinity, as they can contain amorphous calcium carbonate as well as metastable crystalline CaCO3 polymorphs (e.g. vaterite) in addition to calcite and /or aragonite [ 4].