Stromatolite recognition in ancient rocks: an appraisal of irregularly laminated structures in an Early Archaean chert-barite unit from North Pole, Western Australia

Abstract

The word ‘stromatolite’ should only be applied to organosedimentary structures predominantly accreted by sediment trapping, binding and/or in situ precipitation as a result of the growth and metabolic activities of benthic, principally prokaryotic, micro-organisms. Structures of uncertain origin that resemble stromatolites should be called ‘stromatoloids’. This cautious approach would eliminate the currently common assumption that structures with mesoscopic morphological similarities to microbially accreted sedimentary structures must be biogenic, a misconception that hampers investigations into the antiquity of life. A hierarchical series of meso- and microstructural attributes of stromatolites can be used to assign gradually increasing probabilities of biogenicity to stromatoloids. This method is particularly useful for interpreting ancient noncolumnar stromatoloids with poor microstructural preservation. In a range of Early Archaean pseudocolumnar, nodular and stratiform stromatoloids from North Pole studied using this method, none could be proved to be stromatolites and only a few are probable or possible stromatolites. As these stromatoloids closely resemble previously reported structures from North Pole interpreted as stromatolites, we consider that the evidence for the existence of life c. 3500 my ago at North Pole is less definitive than previously supposed.