The 5th International Conference of Geobiology

Profound evolutionary advancements in eukaryotes occurred in the Ediacaran Period (635-538 Ma). It is marked by drastic large-scale environmental changes across Earth's spheres, which may serve as potential trigger mechanisms for these evolutionary developments. The reported appearance of the Ediacara biota has been temporally linked to the Shuram Excursion event (the largest negative carbon isotope excursion recorded in Earth’s history), a putative oxygenation event, and the meltdown of the Gaskiers glaciation (~ 580 Ma; the first icehouse after the Snowball Earth episodes), suggesting possible causal relationships. The Conception Group of the Avalon Basin in Newfoundland, Canada, is uniquely suitable for testing and dating relationships between early macroevolution and environmental change because the same volcanic-rich succession preserves evidence of glaciation, deep ocean oxygenation, carbon isotope excursions, and eukaryotic evolution.

Paleontological, geochronological, and geochemical studies of the fossil-rich strata of the Ediacaran Conception Group in Newfoundland have focused on the Mistaken Point Ecological Reserve, a UNESCO World Heritage Site. This location contains the oldest reported Ediacara biota worldwide, which also represents the first large (centimeter-to-meter) and biologically complex organisms. A prior iron speciation study of this succession indicates a significant shift in the redox state of the local water column between the deposition of the predominantly anoxic and ferruginous glaciogenic Gaskiers Formation (Gaskiers glaciation) and the subsequent oxic, non-glaciogenic Drook to Fermeuse formations. The well-described, soft-bodied, leaf-shaped fronds of the Avalon Basin (574 Ma) first appear 150 m below the top of the Drook Formation, suggesting a possible connection between increased oxygen levels and the emergence of biological complexity following the Gaskiers glaciation. The record of marine redox, climate, and life in the Avalon Basin prior to the Gaskiers glaciation, however, is unclear. Importantly, it has been suggested that the complexity of the oldest known Ediacaran fronds in the Avalon Basin implies that they likely evolved earlier.

The previous iron speciation study on the pre-Gaskiers Mall Bay found that most strata from this unit reflect deposition under either oxic or anoxic, ferruginous water columns. However, this earlier study only sampled the upper 120 m of the formation, while our geochemical, sedimentological, and petrographic studies examine its entire 800 m thickness. Firstly, one of our studies revealed previously unrecognized evidence of glacial (i.e., dropstones and till pellets) and cold waters (glendonites) in the Mall Bay Formation. These new observations elucidate a progressive buildup of ice in the prelude to the Gaskiers glacial maximum (i.e., the Gaskiers Formation), paralleling younger paraglacial features and at least doubling its previously reported stratigraphic thickness. Secondly, thus far over 40% of the new Fe speciation data classify as oxic, whereas less than 5% are definitively anoxic and 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 do 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. 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, and thus habitability, during the Neoproterozoic dawn of animal life.

Ediacaran,oxygenation,glaciation,Avalonia,Glaciomarine