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Shuhai Xiao, Virginia Polytechnic Institute and State University (United States)
Chuanming Zhou, Nanjing Institute of Geology and Paleontology (China)
Xunlai Yuan, Nanjing Institute of Geology and Paleontology (China)
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The global boundary stratotype section and point (GSSP) for the base of the Ediacaran System has been officially ratified, and we now face the challenge of subdividing and correlating the Ediacaran System using biostratigraphic, chemostratigraphic, and physical stratigraphic tools. Recent studies show that stromatolites, macroalgal fossils, acanthomorphic acritarchs, rangeomorph fossils, and cloudinid tubular fossils are useful for regional or interregional correlation. Of these, the latter three fossil groups may have potential for global correlation. However, their biostratigraphic significance remains to be independently verified, apparent discrepancy between South China and South Australia acritarch biostratigraphy needs to be resolved, and species-level taxonomy of Ediacaran acanthomorphic acritarchs has to be clarified.
Carbon isotope chemostratigraphy is also useful in regional and interregional correlation. The occurrence of extremely negative carbon isotope excursions (-10 per mil) in the Shuram Formation, Wonoka Formation, upper Doushantuo Formation, Krol B-C, Rainstorm Member, and possibly Shuiquan Formation may record a global disturbance of the carbon cycle and thus may have chemostratigraphic significance. Although currently available evidence indicates that the upper Doushantuo and Wonoka negative excursions predate a 551 Ma ash bed and postdate the disappearance of most (if not all) Doushantuo-Pertatataka acanthomorphic acritarchs, the exact age and duration of these excursions are uncertain, limiting their chronostratigraphic value as representing a unique and globally synchronous event. In addition, the application of the Shuram excursion as an exclusive criterion in Ediacaran subdivision and correlation is met with the following challenges:
(1) diagenetic alteration has to be carefully evaluated and conclusively eliminated, (2) carbon isotope excursions of similar magnitude have not been identified in some apparently continuous Ediacaran successions (e.g., upper Windermere Supergroup), and (3) possible cm- to dm-scale stratigraphic fluctuations and spatial heterogeneity makes it difficult to precisely define a GSSP on the basis of carbon isotopes alone.
Climatic and physical events (e.g., Gaskiers glaciation, Acraman impact ejecta layer) have also been successfully used in regional correlation. The Gaskiers glaciation has been precisely dated, but the absence of Ediacaran glacial deposits in many regions and the possibility of multiple regional glaciations in the Ediacaran Period limit its potential as a global stratigraphic marker. Similarly, the Acraman impact ejecta layer is unlikely to have a global distribution.
It seems that the best available option is to unambiguously define the Ediacaran subdivisions in biostratigraphic terms and select a GSSP where multiple biostratigraphic, chemostratigraphic, and physical stratigraphic markers are available or potentially available, so that the value of the GSSP can be maximized.
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