Oxygen, facies, and secular controls on the appearance of Cryogenian and Ediacaran body and trace fossils in the Mackenzie Mountains of northwestern Canada

Abstract

The causes behind the appearance of abundant macroscopic body and trace fossils at the end of the Neoproterozoic Era remain debated. Iron geochemical data from fossiliferous Ediacaran successions in Newfoundland suggested that the first appearances correlated with an oxygenation event. A similar relationship was claimed to exist in the Mackenzie Mountains, Canada, although later stratigraphic studies indicated that the sections analyzed for geochemistry were incorrectly correlated with those hosting the fossils. To directly connect fossil occurrences with geochemistry in the Mackenzie Mountains, we conducted a multiproxy iron, carbon, sulfur, and trace-element geochemical analysis of stratigraphic sections hosting both the Cryogenian “Twitya discs” at Bluefish Creek as well as Ediacaran fossils and simple bilaterian traces at Sekwi Brook. There is no clear oxygenation event correlated with the appearance of macroscopic body fossils or simple bilaterian burrows; however, some change in environment—a potential partial oxygenation—is correlated with increasing burrow width higher in the Blueflower Formation. Data from Sekwi Brook suggest that these organisms were periodically colonizing a predominantly anoxic and ferruginous basin. This seemingly incongruent observation is accommodated through accounting for differing time scales between the characteristic response time of sedimentary redox proxies versus that for ecological change. Thus, hypotheses directly connecting ocean oxygenation with the appearance of macrofossils need not apply to all areas of a heterogeneous Ediacaran ocean, and stably oxygenated conditions on geological time scales were not required for the appearance of these Avalon-assemblage Ediacaran organisms. At least in the Mackenzie Mountains, the appropriate facies for fossil preservation appears to be the strongest control on the stratigraphic distribution of macrofossils.