Paleoenvironmental and taphonomic controls on the occurrence of Paleoproterozoic microbial communities in the 1.88 Ga Ferriman Group, Labrador Trough, Canada

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 μm wide and reach tens of μm 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 δ13C values is also present. Most values vary between ca. −38 and −20‰, 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.

► Multidisciplinary approach to understand paleoenvironmental context of fossil bacterial communities.
► Novel insights into how Precambrian bacterial fossils are preserved.
► New information on iron formation depositional processes.
► Techniques employed yield a more detailed picture of the Precambrian biosphere.