Out of rock: a new look at the morphological and geochemical preservation of microfossils from the 3.46 Gyr-old Strelley Pool Formation

The biogenicity of most of the putative Archean organic-walled microfossils discussed in the literature remains debated. Here, we report morphological and geochemical characterizations of an assemblage of microfossils isolated from the 3.46 Gyr-old Strelley Pool Formation (Western Australia), thereby providing a new set of data on the oldest authentic microfossils on Earth. Isolated microstructures/microfossils were studied by combining scanning electron microscopy imaging and elemental analyses, Raman spectroscopy and nanoscale secondary ion mass spectrometry (NanoSIMS). We identified four morphological types, namely filaments, films, spheroids and lenses. A minority of spheroids, films and lenses exhibits a continuous wall cell surface and high N and P concentrations, i.e. exceptionally high morphological and geochemical preservation levels. In addition to the detection of P within microfossil walls, P was also found within micrometric patches in lenses exhibiting a fusiform shape. These patches may be remains of polyphosphate granules possibly formed during sporulation, suggesting, in turn, that cellular organization appeared as soon as 3.46 Gyr ago. Taken together, the morphological and geochemical properties of the set of isolated Strelley Pool microfossils investigated here provide additional evidences for a widespread biodiversity 3.46 Gyr ago.