Résumé
As Archean stromatolites only rarely contain fossil microbes, their biogenicity is tacitly assumed on the basis of macroscopic morphological comparisons with modern structures. We report the occurrence of micron-sized globules of organic carbon intimately associated with the micritic carbonate of the Tumbiana stromatolites (2.72 Ga, Australia, Pilbara Drilling Project). Scanning Transmission X-ray Microscopy (STXM) analysis revealed that these cell-like globules present a complex organic signature. Their association with nano-carbonate spheroids revealed by Transmission Electron Microscopy (TEM) shows striking similarities with nano-carbonate spheroids associated with microbial cells and polymers in modern microbialites. The globules could therefore represent remnants of cells that participated to stromatolite accretion. Moreover, STXM and Scanning-TEM (STEM) analyses show that the organic matter in the stromatolites occurs in two pools that can only be distinguished at the micron scale: (1) the intra-carbonate cell-like globules are rich in aliphatic and carboxylic functional groups as well as abundant organic sulphur in the form of thiophenes, and (2) the intermineral organic matter is sulphur-free and mostly aromatic. Our results suggest that the organic globules represent fossilized microbial cells that were preserved by sulphurization during the earliest stages of diagenesis. In contrast, the intermineral aromatic organic matter could either result from the recondensation of degraded material during early diagenesis or reflect an original heterogeneity of the microbial materials as observed in modern microbialites.