Episodes 2016; 39(4): 540-555
Published online December 1, 2016
Copyright © International Union of Geological Sciences.
Shuhai Xiao1, Guy M. Narbonne2, Chuanming Zhou3, Marc Laflamme4, Dmitriy V. Grazhdankin5, Malgorzata Moczydlowska-Vidal6, Huan Cui7
1Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA. E-mail: email@example.com
2Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
3Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China
4University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada
5Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, prospekt Akademika Koptyuga 3, Novosibirsk 630090, Russia
6Department of Earth Sciences, Uppsala University, Villavägen 16, SE 752 36 Uppsala, Sweden
7Department of Geoscience and NASAAstrobiology Institute, University of Wisconsin, Madison, Wisconsin 53706, USA
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The Ediacaran Period follows the Cryogenian Period in the wake of a snowball Earth glaciation and precedes the Cambrian Period with its rising tide of animal radiation. It is also the longest among all stratigraphically defined geological periods, lasting 94 million years (635–541 Ma). Hence, a good Ediacaran time scale is essential, not only to elucidate geological time, but also to provide a temporal context for extreme climatic events and transformative evolutionary transitions. Ediacaran fossils are known from many sections and boreholes around the world, permitting ready age recognition and stratigraphic correlation of Ediacaran strata. However, the Ediacaran fossil record is colored by taphonomic biases that variously affect the preservation of the soft-bodied organisms that dominated Ediacaran marine ecosystems, and the Phanerozoic approach of defining stratigraphic boundaries using the first appearance datum (FAD) of widely distributed, rapidly evolving, easily recognizable, and readily preservable species would have limited success in the Ediacaran System. The subdivision of the Ediacaran System must therefore be founded on a holistic approach integrating biostratigraphic, chemostratigraphic, and geochronometric data for correlation. Series-level subdivision of the Ediacaran System is a challenging task, and alternative models subdividing the Ediacaran System into two or three series can be recognized. Resolving these alternatives critically depends on obtaining further data to constrain the age, duration, and global extent of the Shuram negative δ 13C excursion, to calibrate and correlate Ediacaran acanthomorph biozones, and to determine the temporal relationship among the Shuram excursion, the Gaskiers glaciation, and Ediacaran acanthomorph biozones. Stage-level subdivisions at the bottom and top of the Ediacaran System, however, are realistic goals in the near future, and we propose that the subdivision of the Ediacaran System should initially aim at the second Ediacaran stage (SES) and the terminal Ediacaran stage (TES) where stratigraphic information is relatively rich and consensus for stratigraphic correlation is emerging. Potential stratigraphic markers for the definition of the SES include the post-glacial radiation of eukaryotes as represented by the first appearance of acanthomorph acritarchs, the termination of the cap carbonate series, or the end of the negative δ13C excursion (EN1 = Ediacaran negative excursion 1) associated with the cap carbonate. Terminal Ediacaran strata are well dated and host several taxa of skeletal and tubular fossils that postdate the Shuram negative δ13C excursion (or its probable equivalent, EN3 = Ediacaran negative excursion 3) where their stratigraphic relationship can be determined; these biostratigraphic markers may be used to define the TES in a Phanerozoic fashion. Additional Ediacaran stages between the SES and TES can be envisioned. Through collaborative efforts in the Ediacaran community, we hope that the first Precambrian stage will be established in the near future to facilitate a better understanding of the geological aftermath of snowball Earth, the redox history of global oceans, the early evolution of multicellular life, and the evolutionary fuse of the Cambrian explosion.