Microbial consortia are found in modern stromatolites. It has been uncertain if such microbial consortia already existed in late Archean stromatolites. Stable carbon and sulfur isotope analyses were performed on organic matter and pyrite in stromatolites, cherts and sandstones of the Jeerinah Formation. Fine-grained microscopic pyrite crystals occur predominantly inside the stromatolites. Sulfur isotope compositions of individual pyrite crystals were determined using the Nd-YAG laser microprobe system. Variable δ34S values (-2.5 to +5.6‰ (CDT)) of pyrite were found in the examined samples, suggesting that pyrite crystals were formed by either abiotic or biotic sulfate reduction in stromatolite. Geological and petrographic evidence suggest that microbial sulfate reduction was the more likely pathway of the pyrite formation. δ13C values of organic matter range from -39.7 to -27.1‰ (PDB). These values indicate that microorganisms capable of photosynthesis were active around stromatolites in addition to the activity of methanotrophic microorganism. These stable isotope data suggest that various type of microorganisms existed during stromatolite formation, although the role of each microorganism in stromatolite accretion is still uncertain.
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