PALEOREDOX OF THE MALL BAY FORMATION (EARLY EDIACARAN, CANADA): EVIDENCE FOR OXYGENATION DURING THE PROTRACTED ONSET OF THE GASKIERS GLACIATION?

The Conception and St. John’s groups of the Avalon Basin in Newfoundland, Canada host an important record of early animal evolution and environmental change. Previous Fe speciation studies of this succession suggest a pronounced shift in local water column redox occurred between the deposition of the predominantly anoxic and ferruginous, glaciogenic Gaskiers Formation and the overlying oxic, non-glaciogenic Drook to Fermeuse formations. Ediacara-type fossil impressions of the Mistaken Point biota are known only from above the Gaskiers Formation, and this potentially implies a causal relationship between oxygenation and the appearance of biological complexity in the wake of the Gaskiers glaciation.

The record of marine redox prior to the Gaskiers glaciation and the appearance of Ediacaran fauna in Avalon Basin, however, is unclear. Fe speciation work on the underlying Mall Bay found that the majority of strata from this unit could either reflect deposition under oxic or anoxic, ferruginous water columns. Still, this previous study only sampled the upper 120 m of the formation and our geochemical, sedimentological and petrographic study examines its entire 1000 m thickness. Thus far over 40% of the new Fe speciation data classify as oxic, whereas less than 5% are definitively anoxic, ferruginous. One potential caveat to the interpretation of the Fe speciation data, however, is that highly reactive Fe could have been incorporated into silicate phases during diagenesis and metamorphism, leading to false oxic signals. Yet, total iron-to-aluminum ratios show that most Mall Bay samples do not have total Fe enrichments, suggesting they reflect deposition under an oxic water column. Therefore, based on our data, the first deep-water oxygenation in the Avalon Basin may have preceded the appearance of the Mistaken Point biota by 5-to-25 million years. Moreover, it was recently found that the uppermost Mall Bay Formation exhibits increasing evidence of glaciation (i.e., dropstones, till pellets, and glendonites), implying a protracted onset of the Gaskiers glaciation. If the oxic signals from the Mall Bay Formation are caused by glacially enhanced downwelling, this mechanism for deep-ocean ventilation may apply to other Ediacaran basins and highlights the need for this to be further explored as a mechanism for deep-ocean oxygenation during the Neoproterozoic dawn of animal life.