Subdivision of the Ediacaran System

Since the Ediacaran System was formally ratified in 2004, its subdivision into multiple series and stages has become a primary task of the ICS Ediacaran Subcommission. Currently, the Ediacaran Subcommission has established the “Final Ediacaran Stage Working Group” and the “Ediacaran Series Working Group”, which focus on the definition of stages in the upper Ediacaran and the series-level subdivision of the Ediacaran System, respectively. Although constrained by lithofacies and preservation conditions, various features, including the occurrences of microscopic acanthomorphic acritarchs in the lower Ediacaran and macroscopic Ediacara-type fossils in the upper Ediacaran, changes in seawater carbon and strontium isotopic compositions recorded in carbonate facies, regional glacial events developed on some blocks, and a series of high-precision radioisotopic ages obtained in recent years, provide potential tools for the subdivision of the Ediacaran System. Before the ~580 Ma Gaskiers glaciation, the marine biosphere was dominated by microscopic eukaryotes, whereas afterwards, macroscopic Ediacaran organisms became dominant. The Gaskiers glaciation event thus has the potential to serve as a series-level boundary within the Ediacaran System. However, due to its limited occurrences, the possible extisence of cold climate as suggested by the occurrence of glendonites in the early Ediacaran, and the presence of confirmed regional glaciations in the late Ediacaran, the utility of Gaskiers glacial deposits as a global correlation marker is challenging. The middle Ediacaran Shuram excursion (~574—567 Ma) – the largest negative carbon-isotope excursion in geological history – has been documented in many sections on multiple blocks and is widely considered a potential marker for the subdivision of the Ediacaran System. The onset of the Shuram excursion occurred shortly after the Gaskiers glaciation and before the first appearance of the Avalon assemblage of the Ediacaran biota. Given its global occurrence and the marked changes in biotic assemblages before and after the Shuram excursion, it is reasonable to use the Shuram excursion as a series-level boundary marker within the Ediacaran System. Above the Shuram excursion, three assemblages of the Ediacaran biota – the Avalon, White Sea, and Nama assemblages – provide a basic framework for the stage-level subdivision of the Upper Ediacaran Series. In particular, the widespread global appearance of tubular skeletal fossils (e.g., Cloudina, Sinotubulites) and non-mineralized tubular fossils (e.g., Conotubus, Shaanxilithes) offers globally correlatable biostratigraphic markers for defining the base of the Final Ediacaran Stage. Because the biostratigraphic zonation of large acanthomorphic acritarchs in the early Ediacaran is still under active investigation, the stage-level subdivision of the Lower Ediacaran Series below the Shuram excursion remains highly uncertain. At present, the significant carbon and strontium isotope excursions, as well as the presence and absence of some acanthomorphic acritarchs, have the potential to serve as criteria for defining stages within the lower Ediacaran System.