The traces of primitive life can be studied in two ways : either by the direct study of preserved fossils or bio-constructions, or by determining isotopic or elemental biosignatures. We discuss these two approaches, beginning with the study of stromatolites and current microbialites (carbonate constructions). These types of samples are formed in quite different deposits and, therefore, probably also by very distinct processes. However, the role of microbial mats in the formation of microbialites is still not understood (e.g., in hyper-alkaline lakes). In an attempt to answer this question, we began a comparative study of current microbialites that have been deposited in different environments (hyper-alkaline, lake, and ocean). We hope to identify potential “biological” criteria in these different contexts, which would allow us to go back in geological time to the construction of stromatolites, whose biogenicity has not been confirmed. The second part of this project will concern the determination of isotopic signatures that are witnesses to biological processes (biosignatures). This task will be achieved by a coupled study of the isotopic signatures of iron and sulphur in potential biominerals, such as pyrites. Indeed, it was shown that the joint action of sulphate-reducing and iron-reducing bacteria produced pyrites with compositions that are very poor in heavy isotopes of sulphur or iron. One of the first envisioned applications will be the study, in collaboration with the team of Abder El Albani, of the first multicellular fossils recently discovered in Gabon and dated at 2.1 Gy. We plan eventually to apply this technique to Archean pyrites whose biogenic origin remains to be determined. ￼
Recent microbialites from hyper-alkaline lakes (Mexico) that are mainly composed of aragonite.