3.4 Ga biostructures from the Barberton greenstone belt of South Africa: new insights into microbial life
Résumé
Raman spectroscopy is a molecule-specific technique allowing the investigation of the chemical structure of organic and inorganic geological materials. Since this is a non-destructive and relatively non-invasive analytical procedure, Raman spectroscopy is ideally suited to palaeontology. Raman spectroscopy is herein applied to the study of carbonaceous chert facies of the ~3.4 Ga old Buck Reef Chert of South Africa, which contain some of the oldest well-preserved evidence of early life. Laminated chert typically consists of microbands composed of microcrystalline quartz (chert) and an association of siderite and carbonaceous material (CM) in the form of mat-like laminations, simple carbonaceous grains, vein infills and diffuse CM. Using Raman spectroscopy the structural characteristics of CM in mat-rich chert have been investigated and compared with grains of CM from the same unit, but wich were deposited as massive layers that bear no evidence of microbial influence. All CM retains a structural organisation consistent with a lower greenschist grade regional metamorphic imprint, however, this detailed study of the CM Raman signals revealed some heterogeneity between the different sedimentary facies, thus pointing to the presence of different CM types. Different CM precursors are indicated and may reflect either different CM sources or different alteration chemistries from various microbial metabolic pathways.