SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions

Analyses by secondary ion mass spectroscopy (SIMS) of 11 specimens of five taxa of prokaryotic filamentous kerogenous cellular microfossils permineralized in a petrographic thin section of the similar to 3,465Ma Apex chert of northwestern Western Australia, prepared from the same rock sample from which this earliest known assemblage of cellular fossils was described more than two decades ago, show their delta C-13 compositions to vary systematically taxon to taxon from -31 parts per thousand to -39 parts per thousand. These morphospecies-correlated carbon isotope compositions confirm the biogenicity of the Apex fossils and validate their morphology-based taxonomic assignments. Perhaps most significantly, the delta C-13 values of each of the five taxa are lower than those of bulk samples of Apex kerogen (-27 parts per thousand), those of SIMS-measured fossil-associated dispersed particulate kerogen (-27.6 parts per thousand), and those typical of modern prokaryotic phototrophs (-25 +/- 10 parts per thousand). The SIMS data for the two highest delta C-13 Apex taxa are consistent with those of extant phototrophic bacteria; those for a somewhat lower delta C-13 taxon, with nonbacterial methane-producing Archaea; and those for the two lowest delta C-13 taxa, with methane-metabolizing gamma-proteobacteria. Although the existence of both methanogens and methanotrophs has been inferred from bulk analyses of the carbon isotopic compositions of pre-2,500 Ma kerogens, these in situ SIMS analyses of individual microfossils present data interpretable as evidencing the cellular preservation of such microorganisms and are consistent with the near-basal position of the Archaea in rRNA phylogenies.