TRACKING THE ONSET OF PHANEROZOIC-STYLE REDOX-SENSITIVE TRACE METAL ENRICHMENTS: NEW DATA FROM NEOPROTEROZOIC POST-GLACIAL STRATA IN NW CANADA

A global rise in oxygen levels is often associated with the radiation of animals in the Ediacaran to Cambrian periods, yet the precise timing and nature of this change remains unresolved. One hypothesis is that the ocean/atmosphere system became temporarily well-oxygenated in the earliest Ediacaran, directly following the Marinoan Glaciation (~635Ma). The evidence for oxygenation is based on large enrichments of Redox Sensitive Elements (RSEs) in black shale deposited under an interpreted euxinic water column from South China—reaching or exceeding enrichments observed in Phanerozoic shale deposited under similar conditions. Here we test the early Ediacaran oxygenation hypothesis with new data from a high-resolution multi-proxy geochemical and sedimentological study of three stratigraphic sections through the earliest Ediacaran black shale of the Sheepbed Formation, NW Canada. These sections span the paleo-shelf (Stoneknife Creek and Sheepbed Creek) and slope (Rackla belt) along a marine margin. Iron speciation data from all sections suggest that the local water column was dominantly ferruginous, with a notable exception of probable euxinic conditions recorded in the basal Sheepbed Formation at Stoneknife Creek. RSEs in all basal Sheepbed Formation samples do not show appreciable enrichments, with maximums of Mo (~9ppm), V (~395ppm), U (~7ppm), and Cr (~112ppm) that are only slightly above average shale values. The lack of substantial RSE enrichment within Sheepbed Formation shale is consistent with results from coeval ferruginous shale from Svalbard, but contrasts with euxinic shale data from South China. The dominance of local ferruginous conditions in NW Canada alone cannot account for the muted RSE enrichments as V, U, and Cr are thought to be enriched under these conditions. Furthermore, RSEs are not enriched in samples deposited under an arguably euxinic water column from Stoneknife creek. These contrasting data highlight a need to further investigate the veracity of trace metal enrichments as representing global ocean redox conditions in multiple basins, in order to firmly establish the temporal record of ocean oxygenation as expressed through large Phanerozoic-style RSE enrichments.