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Keywords:

  • South China;
  • Ediacaran;
  • Doushantuo Formation;
  • Marinoan glaciation;
  • Gaskiers glaciation;
  • acritarchs

Abstract

The Ediacaran Period immediately follows the last Cryogenian glaciation—the ∼635 Ma Marinoan or Nantuo glaciation, and it is also punctuated by another brief glaciation—the ∼582 Ma Gaskiers glaciation. It is possible that these glaciations may have had significant impact on Ediacaran biological evolution (e.g. the appearance or disappearance of Doushantuo-Pertatataka acritarchs). Alternative hypotheses propose that the diversification of Doushantuo-Pertatataka acritarchs was caused by the Acraman bolide impact or by emerging eumetazoans. To test these hypotheses, high-resolution geochronological and biostratigraphic data are required. The Doushantuo Formation in South China, radiometrically constrained between ∼635 and ∼551 Ma, has the potential to clarify the global picture of early-middle Ediacaran evolution. Here we present preliminary biostratigraphic data from the Doushantuo Formation in the East Yangtze Gorges area and new δ13C chemostratigraphic data from the Doushantuo Formation at Weng'an. These and previously published palaeontological data, aided by the tool of δ13C chemostratigraphy, indicate that the biostratigraphic record of the Doushantuo Formation is locally sensitive to the availability of specific taphonomic windows. In the East Yangtze Gorges area, Doushantuo-Pertatataka acritarchs first appeared shortly after the termination of the Nantuo glaciation and gradually diversified throughout the Doushantuo Formation. At Weng'an, however, such acritarchs first appear—abruptly and in much greater diversity—in phosphorite of the upper Doushantuo Formation, immediately above a subaerial exposure surface. Thus, the biostratigraphic pattern in the East Yangtze Gorges area permits, whereas that at Weng'an apparently disallows, a causal relationship between the Nantuo glaciation and the diversification of Doushantuo-Pertatataka acritarchs. We conclude that the biostratigraphic record is incomplete at Weng'an, where the early Ediacaran evolutionary history is not preserved. The South China data indicate that special attention has to be paid to taphonomic and palaeoenvironmental analysis before extrapolating local and regional biostratigraphic ranges to make global interpretations.

It is less clear when Doushantuo-Pertatataka acritarchs disappeared. Previous investigators have variously suggested that they disappeared before, at, or after, the Gaskiers glaciation. These hypotheses are difficult to test because of the lack of sedimentary evidence for the Gaskiers glaciation in South China and other regions (e.g. South Australia) where Doushantuo-Pertatataka acritarchs are abundant. In Australia, Doushantuo-Pertatataka acritarchs are thought to have experienced a sharp decline after the Egan glaciation, which may well be equivalent to the Gaskiers glaciation. If true, then Doushantuo-Pertatataka acritarchs are largely restricted to the interval between the Nantuo and Gaskiers glaciations. This conclusion allows us to place constraints on the possible causes of the diversification and extinction of Doushantuo-Pertatataka acritarchs and has important implications for the biostratigraphic significance of Doushantuo-Pertatataka acritarchs. Copyright © 2006 John Wiley & Sons, Ltd.