NANOSCALE CHARACTERISATION OF THE 1.9 GA GUNFLINT CHERT MICROFOSSIL ASSEMBLAGE

The Gunflint Chert (GFC) of North America contains some of the best preserved examples of the Gunflint biota, an assemblage of microscopic organisms that appear to have thrived globally ~1.9 Ga ago. Although examples of carbonaceous cells have been fairly well documented, there have been few high-resolution investigations into alternative fossil preservation styles exhibited within the GFC, including pyrite and hematite mineralisation. The precise mechanisms of this Proterozoic microfossil preservation are still not fully understand due to the vastly differing geochemical and environmental conditions of the early Earth, coupled with the partial obliteration of original biological features due to taphonomic and diagenetic processes. Here we concentrate on hematite microfossil mineralisation in the GFC, for which two published theories currently exist: biologically-induced mineralisation via iron-oxidising bacteria, and diagenetic mineralisation from later circulating iron-rich groundwaters.

Correlative in situ techniques (FIB-SEM, TEM, NanoSIMS) were used to examine microfossils from multiple localities within the GFC. STEM images and TEM-EDS elemental mapping of Huroniospora cells show differing iron oxide growth textures, ranging from large (1-2mm) sub-rounded grains with embedded silica, to small (0.2-0.5mm) sub-angular platy crystals, with rare nanoscale (<50nm) rounded particles or thin strands growing between larger grains, forming connective ‘bridges’. Some Huroniospora and Gunflintia show definitive secondary hematite after pyrite, evidenced by pyrite-rich cores. Titanium, barium and aluminium were detected in low concentrations in some hematite fossils walls, plus traces of organic carbon typically associated with the nanoscale bridges. The presence of original cellular material in conjunction with distinctive textures and grain sizes allows us to distinguish fossilised biological cellular remnants from abiotic mineral growths. The occurrence of similar bacterial morphotypes preserved in carbon, pyrite and hematite, in conjunction with a range of nanoscale textural features and varying trace element concentrations, are suggestive that the GFC microfossils represent an assemblage that have been subsequently overprinted by multi-stage diagenetic fluids.