Methods

The process of identification of signs of life in rocks of this ancient age commences with detailed field study since it is necessary to be able to place the rock samples with the potential evidence for life in a well-defined geological framework, in order to avoid the first error of finding evidence of "life" in an environmental situation which is not conducive to life. Study of the rock sample and subsamples in the hand specimen, in thin section with a light microscope, as well as with the scanning electron microscope and the microprobe and other chemical methods (neutron activation, particle induced X-ray emission PIXE, etc.) can provide further detailed background information concerning sedimentological/ igenous/metamorphic textures and minerals, thus furnishing a more detailed context for structures that are on the micron scale. These investigations also document the history of the rock, such as any mechanical and chemical changes that have taken place over time through diagenesis and metamorphism.

In this way, selected specimens are chosen for further isotope and microfossil study. Considering the silicified nature of the material, delicate etching with HF is necessary to expose the carbonaceous microfossils (the carbonaceous material being more resistant to the HF compared to pure SiO2) for scanning electron microscope investigations. Contamination in the form of microbial infiltration of cracks and fissures, both on the terrain and in the laboratory, as well as precipitate from the etching process, need to be identified and avoided [61, 78]. The identification of fossil microbial life forms (prokaryotes in this case) is based on three main criteria: (i) the morphology of the structure, including shape, size, cell wall texture (only visible with electron microscope techniques), evidence for cell division, evidence for flexibility (if filamentous), evidence for cell lysis (death); (ii) colonial characteristics including colony formation, association with polymer (extracellular polymeric substances or EPS), biofilm formation, presence of consortia; and (iii) biogeochemical characteristics, such as carbonaceous composition, C, N, S isotope ratios and perhaps the concentration of certain heavy or rare earth elements.

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