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Chert and iron formation from the Ferriman Group (ca. 1.88 Ga), Labrador Trough, Canada, contain an exceptional assemblage of fossil bacteria and biofilms preserved within a well-defined stratigraphic framework. Lithofacies associations suggest that microbes were restricted to suboxic, shallow-water environments through three sea-level cycles. Microfossils are preserved as chert or sedimentary apatite (francolite) in hematite-rich, peritidal facies. Nearshore evaporation and Fe-redox pumping of silica drove rapid penecontemporaneous silicification of microbial mats. In some communities, breakdown of organic matter and Fe-redox pumping of pore water phosphate caused their syndepositional phosphatization. Preservation of bacteria was precluded in deeper settings because these processes could not operate at an anoxic seafloor.

Filamentous forms dominate microbial morphologies. Secondary electron imaging of freshly broken surfaces shows filaments that are similar in size and shape to modern bacteria; filaments vary from 0.5 and 1 mu m wide and reach tens of mu m in length. They commonly envelop chert and iron oxide grains, which stabilized the seafloor and contributed to firmground development. The filamentous morphology, similar mat-forming behavior, and suboxic paleoenvironments where these fossils lived are consistent to those of modern Fe-oxidizing bacteria and some cyanobacteria. Associated organic matter with low delta C-13 values is also present. Most values vary between ca. -38 and -20 parts per thousand, which is consistent with isotopic fractionation by such bacteria. Unlike modern settings, however, their distribution was restricted to shallow marine and peritidal environments where photosynthetically produced oxygen oases likely impinged on the seafloor. Although Fe-oxidizing bacteria were possibly important in forming some iron formation facies in the Ferriman Group, physiochemical precipitation of Fe-(oxyhydr)oxides caused by photosynthetic oxygen was also probably widespread in the photic zone across the shelf. This study demonstrates that a combined microanalytical, sedimentologic, and stratigraphic analysis of fossil bacteria not only yields a more detailed picture of the Precambrian biosphere, but also highlights the importance of taphonomy in the search for early life. (C) 2012 Elsevier B.V. All rights reserved.