Focussed ion beam preparation and in situ nanoscopic study of Precambrian acritarchs

The taphonomic nanostructure of acritarch cell walls from the c. 650 million years old Chichkan Formation was studied with optical microscopy (OM), Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The integration of high-resolution methods and classical optical microscopy allows for the assessment of the relationship of the fossil to the embedding chert and of the authenticity of the fossil. Partial etching of paleontological (150 μm) and petrographic (30 μm) thin sections rather than maceration of the semi-stable cell structures served as the preparation method suitable for AFM and SEM studies. Focussed Ion Beam (FIB) preparation of sections normal to cell walls, yielded stable thin foils of the same thin sections and microfossils as investigated by OM, AFM and SEM. Unicells that appeared excellently preserved by optical microscopy standards, consisted of disconnected kerogen particles, dispersed in the cryptocrystalline quartz matrix and arranged in stacks of variable spacing on micro- to nanometer scale. The density of carbon particles was found to be correlative to the stability of cell walls and to inhomogeneities in the chert. SEM and AFM images of cell cross-sections are directly comparable at the same scale of magnification, but AFM offers higher resolution possibilities and 3-D information on the arrangement of particulate carbon within the cell. Whereas the microscopic appearance of cells was highly variable within the same rock unit, from the same locality in the Chichkan Formation, the nanoscopic structure of kerogen was found to be similar in all cells, consisting of multi-laminated (sheeted) amorphous carbon films, built up of layers with measured thickness between 10 and 20 nm, as revealed by TEM and AFM.