Résumé
Metamorphism has greatly complicated the interpretation of morphological, chemical and isotopic indicators of Early Archean life. Most importantly, however, metamorphism has complicated the reconstruction of environmental setting and habitat of early life. A now classical controversy highlights the importance of this latter problem. Carbonaceous microfilaments in the 3.5 Ga Apex Chert (Pilbara, Western Australia) were originally thought to represent remnants of photosynthesizing organisms that were preserved in a seafloor sedimentary environment. It was subsequently observed that the microfilaments occur in a part of the formation that actually represents a hydrothermal chert feeder dike. The inferred sub-seafloor hydrothermal setting suggests that the carbonaceous microfilaments could be the result of Fischer-Tropsch type synthesis associated with serpentinization of ultramafic crust. It has later been suggested that carbonaceous matter in such chert feeder dikes can actually represent the remnants of chemolithoautotrophic organisms such as methanogens that lived in seafloor hydrothermal environments. It thus appears to be difficult, if not impossible, to distinguish potential remnants of photosynthesis from other pools of carbonaceous matter in hydrothermally influenced Early Archean seafloor settings. Here we report a detailed study of carbonaceous matter in two drill cores that transect the 3.49 Ga Chert Barite Unit of the Dresser Formation at North Pole (Pilbara, Western Australia). This unit represents a shallow water depositional environment within a volcanic caldera, and has experienced lower greenschist-facies metamorphism. It consists from bottom to top of bedded cherts, barite-sulfide beds, and bedded Fe-rich carbonates. An underlying network of barite- and silica-feeder dikes suggests synsedimentary hydrothermal activity. Towards the top of the unit, a succession of thin alternating bands of jasper and Fe-rich carbonate occurs, that includes carbonaceous laminations. Within the carbonate bands carbonaceous matter appears to be closely associated with hematite microcrystal inclusions. Laser Raman spectroscopy confirms that the carbonaceous matter in these laminations has experienced greenschist facies metamorphism, but that it is less altered than carbonaceous matter occurring in the underlying chert-feeder dikes. The carbon isotope ratio of these laminations (delta (super 13) C -30 to -18 ppm) is distinctly different from that of carbonaceous matter in the underlying silica dikes (delta (super 13) C -34 to -31 ppm). The hematite bands display a strong positive delta (super 56) Fe (+0.1 to +1 ppm), compared to the Fe-rich carbonates (-0.6 to +0.4 ppm) and underlying barite-sulfide beds (-0.7 to -0.4 ppm). These petrographic, Raman spectroscopic and isotopic observations exclude a direct hydrothermal origin of the carbonaceous matter and associated hematite-rich bands and suggest that oxygenic and/or anoxygenic photosynthesizers were directly involved in the oxidation of Fe(II)aq.