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Ediacaran iron formations and carbonates from Uruguay: palaeoceanographic, palaeoclimatic and palaeobiologic implications

  • Author / Creator
    Pecoits, Ernesto
  • The Ediacaran in Uruguay preserves a unique record of deposits generated during the assembly of the palaeocontinent Gondwana and concurrent with major changes in the atmosphere and oceans, and the rise of animal life. Recent studies have suggested that the deep oceans remained anoxic and highly ferruginous throughout the Ediacaran and possibly into the Cambrian. Unfortunately, acceptance of this idea has been hindered by the virtual absence of iron formations (IF). Detailed studies of Ediacaran IF in Uruguay confirm that ferruginous conditions dominated the pre-Gaskiers (~580 Ma), and interestingly, they also extended well into the upper Ediacaran before complete ocean ventilation occurred. Significantly, a simple twolayer stratified system that argues for an oxygenated surface layer overlying a suboxic zone is proposed. The association of negative δ13C excursions in Neoproterozoic carbonates and large-scale glaciations has become a tempting explanation for the short-term perturbation of the global carbon cycle. Not surprisingly, negative δ13C shifts in Ediacaran-aged carbonates from Uruguay have been interpreted as recording post-Gaskiers glacial events. New highresolution δ13C-chemostratigraphy of carbonates shows negative fractionations in deep facies with a progressive rise towards shallow-water settings, and suggests a deposition across a stratified ocean. Furthermore, 87Sr/86Sr chemostratigraphy coupled with radiometric data allowed a more precise chronostratigraphy, which supports an age of ~600-575 Ma for the unit, and suggests a deposition concurrent with the Gaskiers glaciation. Notwithstanding whether associated δ13C variations in shallow water facies were produced by glacially-related conditions or by the dynamic of the basin itself remains unresolved. Although these conclusions are particularly valid for these deposits, they carry important implications for the understanding of other negative δ13C excursions recorded in the Precambrian. Finally, bilaterian burrows occur in Gaskiers age glaciomarine rocks in Uruguay implying that these are the oldest definite animal tracks yet reported. Crucially, our new discovery unites the palaeontological and molecular data pertaining to the origin of bilaterians, and brings the origin of animals firmly into the interval of the Neoproterozoic glaciations. It also implies that ancestral bilaterians likely evolved first in relatively shallow seas, and only colonized the deep-sea floor once sufficient bottom water oxygenation had taken place.

  • Subjects / Keywords
  • Graduation date
    2010-11
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R3FK7D
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Doctoral
  • Department
    • Department of Earth and Atmospheric Sciences
  • Supervisor / co-supervisor and their department(s)
    • Konhauser, Kurt (Earth and Atmospheric Sciences)
    • Gingras, Murray (Earth and Atmospheric Sciences)
  • Examining committee members and their departments
    • Pemberton, George (Earth and Atmospheric Sciences)
    • Creaser, Robert (Earth and Atmospheric Sciences)
    • Rosiere, Carlos A. (Universidade Federal of Minas Gerais)
    • Owttrim, George (Biological Sciences)
    • Gingras, Murray K. (Earth and Atmospheric Sciences)
    • Konhauser, Kurt O. (Earth and Atmospheric Sciences)