INVESTIGATING THE EFFECTS OF LOCAL REDOX ON THE PRESERVATION OF ORGANIC MICROFOSSIL FROM THE LATE TONIAN CHUAR AND UINTA MOUNTAIN GROUPS OF WESTERN LAURENTIA

Our understanding of early eukaryotic diversity relies heavily on the record of organic-walled microfossils— the organic (non-mineralized) remains of microorganisms. However, changes in the conditions that preserve organic remains as fossils can greatly bias fossil diversity trends. It is a long-held assumption that bottom water anoxia in a necessary condition for organic fossilization. Although this relationship is well-documented in the Phanerozoic record of soft-tissue animal fossils, it is unclear if the same conditions apply to the record of Proterozoic organic-walled microfossils. Here we test the role of local redox conditions on organic-walled microfossil preservation using shales from the ca.766–729 Ma Chuar Group, Arizona, and Uinta Mountain Group, Utah. The Chuar and Uinta Mountain groups were deposited in coeval intracratonic basins on the western margin of Laurentia and both contain diverse assemblages of organic-walled microfossils. To test the relationship between redox conditions and the occurrence of organic-walled microfossils at the scale of individual shale laminae, we utilized synchrotron-based X-ray spectroscopy. The concentrations of major elements were mapped at 75mm resolution using X-ray fluorescence imaging; we observe that the finer-grained laminae are relatively higher in potassium and lower in iron. As iron is redox sensitive, we used X-ray absorption spectroscopy at energies near the K-absorption edge of iron (7112 eV) to identify and quantify oxidized and reduced iron phases. We find that both oxidized and reduced phases are present at the cm scale. The presence of oxidized iron along small cracks and on the outer margins of some laminae are likely the result of secondary oxidation from surface weathering. However, some regions are primarily composed of reduced iron phases, which are likely a primary signal. Microfossils appear non-uniformly across laminae, and thus their occurrence can be mapped with respect to signals of iron reduction.