Investigating the texture, structure and composition of putative Archean microfossils and associated kerogen using petrographic and chemical analysis with high spatial resolution

Investigations led on early life traces are a fundamental research topic concerning Life and Earth Sciences. Implications of multiple expertise to decipher life signatures on rock materi-als remains at the pinnacle of numerous unanswered questions: What was early microscopic life looked like? What metabolism early life performed first? How such ancient microorgan-isms may be preserved through time?
Despite the harsh conditions of the early Earth, e.g. intense volcanism, hydrothermalism, meteoritic bombardment, traces of benthic microbial communities called stromatolites have been discovered in Archean (4 - 2.5Ga) rocks. Chert deposits embedding carbonaceous ma-terial may contain putative microfossils. No clear association between those putative micro-fossils and stromatolites has been established so far. Comparison made with younger averred microfossils and biomorphs question that some of the Palaeo-Mesoarchean may be considered as genuine microfossils. The biogenicity of these carbonaceous structures re-mains doubtful.
This thesis investigates through high resolution techniques these putative microfossils to clarify whether their morphogenesis is best explained by biogenic or abiogenic processes. Among these carbonaceous microstructures, diverse morphologies are putative microfossils, including filaments, spheroids, and lenses. These microstructures are reviewed critically for the whole Archean age in various types of rocks, including evidence for syngenicity and cel-lularity. Cherts are well known to promote production of abiotic objects, possibly associated here with hydrothermal input of exogeneous mineral or organic constituents. Considering this, a clear revision of proposed features to define if a microstructure is a fossil is herein suggested. In turn, spheroidal and lenticular morphologies from cherts of the 3.4 Ga Strelley Pool Formation and the 3.0 Ga Farrel Quartzite from the Pilbara Craton (Western Australia) have been targeted to find out and test new significant features to determine their origin. We used a combination a scanning transmission electron microscopy down to the atomic scale, electron backscattered diffraction, and confocal laser scanning microscopy. The bulk carbonaceous matter of a newly collected rock of the Farrel Quartzite was characterized with laser-assisted mass spectrometry with the aim of targeting single putative microfossils in the future.
Spheroidal microstructures observed herein present evidence of abiotic signature with coa-lescent and nanoparticulate carbonaceous wall-like structures that are included and cross-cutting multiple quartz crystals.
Lenticular specimens remain more enigmatic, as they do not present such intracrystalline features. Instead, only hollow specimens show sub-continuous, deformed walls located at grain boundaries. Other lenses may commonly include spheroid sub-structures, sometimes coalescent, like isolated coalescent spheroids, likely formed abiotically. The combination of flange, sub-continuous walls remain best reconciled with known biogenic origins rather than known abiotic biomorphs. However, the putative taphonomy of those structures remains difficult to address.
Lenses exposed a heterogeneity of the carbonaceous matter texture at atomic scale. At this stage, heterogeneity does not help discriminate biogenicity and abiogenicity. However, the observed atomic scale organization is consistent with the dominance of carbon clusters in laser-assisted mass spectrometry of the bulk insoluble organic matter, along with possibly bound or adsorbed polyaromatic hydrocarbons.
Strengths of this study reside on interconnected methodologies of material sciences and ge-ochemistry that investigate down to the nanoscale the carbonaceous matter structure and texture of putative microfossils. At last, this study provides new insights, based upon petro-graphic and chemical methodology, regarding biogenicity of the putative microfossils and their associated carbonaceous matter.