Microscopy

Scanning electron microscopy was undertaken on selected samples, in order to assess whether the original biomineral crystal habits are preserved, and to identify the best-preserved regions within the belemnite rostra. Analyses were performed using secondary electrons on an FEI Quanta Inspect 250 Scanning Electron Microscope under a high vacuum of 2.40 to 2.93·10^–4 Pa and an electron beam of 95–97 μA at the Geological Museum in Copenhagen, out on selected aragonites from phragmocones, ammonites and bivalves analysed in this study.

For SEM–EDS and EBSD analysis, cross- and longitudinal- sections of a selected rostra were mounted in epoxy and mechanically polished down to a 0.25 µm diamond solution grain size, followed by chemical–mechanical polishing using an alkaline solution of colloidal silica in a neoprene substrate. EBSD orientation mapping was performed on the coated sample (~ 2.5 nm carbon) in a Thermo Fischer—FEI Quanta 200F equipped with an EDAX Hikari EBSD camera and TEAM software for data acquisition at the Scientific Center for Optical and Electron Microscopy (ScopeM) at ETH Zurich. Acquisition was performed with an accelerating voltage of 20 kV, beam current of 8 nA, working distance of 17 mm, and mapping step size of 1 µm. Post-acquisition cleaning included grain confidence index (CI) standardization followed by one step of grain CI correlation. All points with CI < 0.1 and grains with less than 10 pixels were removed to prevent artifacts in the calculations.

The SEM–EDS element maps were undertaken at the SEM laboratory at the Geological Survey of Denmark and Greenland (GEUS), which hosts a ZEISS sigma 300VP field emission scanning electron microscope that is equipped with 2 Bruker Xflash 6|30 129 eV EDS detectors and a Bruker e-Flash FS EBSD detector. Element maps were acquired from infilled apical area to the outermost pyritised rim of the rostrum, covering the changes of growth ring density and mineralogical changes. The sets of transect section were obtained for the cross- and the longitudinal-section. Elements mapped (Ca, Mg, Fe, Mn, O) cover the range of possible carbonates, with aragonite distinguished by trace Sr; the potential occurrence of apatite (P), pyrite (Fe, S), clays (Al, Si, Ba, K) and quartz (Si) was also tested.