A GEOCHEMICAL INVESTIGATION OF OXYGENATION IN THE EARLY EDIACARAN OCEANS AT THE TYPE SECTION OF THE SHEEPED FORMATION, MACKENZIE MOUNTAINS, NORTHWEST TERRITORIES, CANADA

Atmospheric oxygen rise has been commonly linked to the advent of animal life near the Ediacaran-Cambrian boundary, yet the nature and timing of this oxygenation event have remained unclear. One proposed oxygenation event occurred in the earliest Ediacaran following the Marinoan glaciation. To test this hypothesis, we conducted a sedimentological and multi-proxy geochemical study of black shale from well-exposed sections of the post-glacial Sheepbed Formation at its type locality in the southern Mackenzie Mountains, Northwest Territories, Canada. High-resolution iron speciation data reveal a transition from ferruginous to oxygenated bottom waters approximately 200 meters above the Ravensthroat cap carbonate; however, ferruginous conditions return episodically at higher stratigraphic levels in the Sheepbed Formation. Therefore, these data do not support a model of stable basinal oxygenation in the early Ediacaran at this locality. In conjunction with geochemical studies at other localities in northwest Canada, these data suggest widespread redox instability in the early Ediacaran. Redox-sensitive trace metal abundances in strata deposited under ferruginous bottom waters directly above the Ravensthroat cap carbonate are not elevated with respect to crustal levels and do not provide support for global ocean oxygenation directly following the Marinoan Glaciation. This is also consistent with trace metal data from the basal Sheepbed Formation in the Wernecke Mountains of Yukon, Canada, but contrasts with data from the basal Doushantuo Formation in South China. More redox-sensitive trace metal studies in post-Marinoan transgressive units are necessary to elucidate a clear understanding of global ocean redox conditions in the early Ediacaran. Nonetheless, in northwest Canada, the early Ediacaran does not appear to be a time period of stable widespread ocean oxygenation.