Carbonaceous microstructures resembling micro-organisms have been reported from numerous Archean formations (the Archean period ranges between 2.5 and 4.0 billion years ago). However, unequivocally identifying microfossils and their associated metabolism is not trivial in ancient rocks, notably because billions of years of thermal alteration tend to severely modify their morphological and geochemical features.
In a new paper led by Frédéric Delarue from the Muséum National d’Histoire Naturelle in Paris, we present the results of our investigation of the geochemical composition of some exceptionally well-preserved microfossils from the 3 billion-year-old Farrel Quartzite formation, a unit that is part of the Pilbara Craton in Western Australia. To characterise the geochemical composition of these microfossils that are only a few tens of microns in size, we have used an instrument hosted at the Muséum in Paris called the NanoSIMS, which allows imaging the distribution of selected species such as carbon and nitrogen with a spatial resolution of a few 100’s of nanometres (as you can see in the photo below there is also a NanoSIMS instrument installed at the University of Manchester).
Two of the molecular species we mapped at high resolution were 12C2 and 12C14N since they serve as a proxy to image the distribution of C and N, respectively. The figure below shows the distribution of these two species obtained on both the body and the flange of a very well preserved lenticular microfossil. These NanoSIMS maps show that if the C and N abundances are generally correlated, some micron-scale areas appear to be enriched in N and/or C independently of each other.
One of the major outcomes of our study is the identification of various degrees of preservation for different types of microfossils, which we have interpreted as resulting from different modes of fossilisation. Therefore, focussing future investigations on encapsulated microfossils in old Archean rocks should provide the best chances to improve our knowledge on the earliest forms of terrestrial life.
Full citation: Delarue, F., Robert, F., Sugitani, K., Tartèse, R., Duhamel, R. & Derenne, S. (2017) Investigation of the Geochemical Preservation of ca. 3.0 Ga Permineralized and Encapsulated Microfossils by Nanoscale Secondary Ion Mass Spectrometry. Astrobiology, doi: 10.1089/ast.2016.1531.