John H. Oehler's research while affiliated with The Commonwealth Scientific and Industrial Research Organisation and other places

Organically preserved algal microfossils from the Ringwood evaporite deposit in the Gillen Member of the Bitter Springs Formation (late Precambrian of central Australia) are of small size, low diversity, and probable prokaryotic affinities. These rather primitive characteristics appear to reflect the stressful conditions that prevailed in a periodically stagnant, hypersaline lagoon. This assemblage (especially in comparison with the much more diverse assemblages preserved in the Loves Creek Member of the same formation) illustrates the potential utility of Proterozoic microbiotas for basin analysis and local stratigraphic correlation and demonstrates the need to base evolutionary considerations and Precambrian intercontinental biostratigraphy on biotas that inhabited less restricted environments.

Kerogen isolated from stromatolitic, microfossiliferous chert of the Paradise Creek Formation (ca. 1500 m.y. old; Queensland, Australia) was exposed to 14C-labelled organic compounds (hexadecane, heptanoic acid, ribose, or serine) at elevated and room temperatures for periods up to four days. The labelled compounds were then extracted and the kerogen was analyzed to determine the amount of radioactivity remaining. Results show that kerogen can be irreversibly contaminated by younger organic compounds. The level of contamination was highest with the hydrocarbon, followed by the amino acid and the sugar; there was no irreversible contamination by the fatty acid. The maximum observed contamination level was 60 μg hexadecane/g kerogen; calculations suggest that similar contamination levels may occur under natural conditions, especially in clastic sediments. Contamination levels of this magnitude are insufficient to affect isotopic or major elemental analyses of kerogens significantly, but could produce spurious results in analyses (in the ppm range) of the organic chemical composition of kerogens.

The H.Y.C. Pyritic Shale Member of the Barney Creek Formation (ca 1 500 my old; northern Australia) contains several stratiform base metal sulfide deposits of economic significance. Black cherts within these mineral deposits preserve a diverse assemblage of bacterial and algal microfossils. The assemblage differs from most other Precambrian biotas so far described in that it was deposited in deep water, it is not associated with stromatolites or algal mats, and it is dominated by filamentous bacteria, most of which are pyritized. Analysis of the assemblage suggests that the depth of the depositional basin exceeded that of the photic zone, that the bacteria inhabited the basin floor where they maintained anoxic conditions through heterotrophic degradation of detrital organic matter, and that the algae inhabited overlying near surface waters. Most of the algal fossils have been assigned to the Cyanophyta, although two of the described species are potentially referable to the eukaryotic green or red algae. Differences between this assemblage and other biotas described from the McArthur Group suggest that a workable system of biostratigraphic zonation for the Group is feasible.Fossils in the H.Y.C. assemblage are here referred to 21 species and 16 genera, of which 14 species and 6 genera are new. The new taxa are: Bacteria, Biocatenoides incrustata sp. nov., B. pertenuis sp. nov., Ramacia carpentariana gen. et sp. nov., Coleobacter primus gen. et sp. nov., Ferrimonilis variabile gen. et sp. nov.; Chroococcales (Cyanophyta), Nanococcus vulgaris gen. et sp. nov., Bisacculoides tabeoviscus gen. et sp. nov., B. vacua gen. et sp. nov., B. grandis gen. et sp. nov.; Nostocales (Cyanophyta), Oscillatoriopsis schopfii sp. nov., Cyanonema inflatum sp. nov., C. minor sp. nov.; Incertae sedis, Clonophycus elegans gen. et sp. nov., Globophycus minor sp. nov. In addition, the new combination Gunflintia septata (Schopf) is proposed.

Tetrahedral tetrads of coccoid cells are known from two Precambrian localities; one example has been reported from the late Proterozoic Bitter Springs Formation and a second example, morphologically different from the first, is here described from the middle Proterozoic Amelia Dolomite. Among modern algae, sheath- or wall-enclosed tetrahedral tetrads are regularly produced only by eukaryotes, either during meiotic or mitotic reproduction. In evaluating the significance of Precambrian tetrads, consideration should be given to the following: whether the tetrad geometry is truly tetrahedral; if so, whether the tetrahedral configuration is original or artifactual; and if original, whether the tetrad was produced during meiotic or mitotic reproduction. We suggest several kinds of evidence which may assist in making these determinations. Considering the Amelia Dolomite specimen, we conclude that it most closely resembles a mitotically produced green algal autospore tetrad, although this interpretation cannot yet be confirmed because of the difficulty in ascertaining whether the tetrahedral configuration is original. Considering the Bitter Springs specimen, we conclude that the organism probably was eukaryotic but that its mode of origin (whether through meiosis or mitosis) cannot be unequivocally ascertained from data presently available.