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A Geochemical Investigation of Oxygenation in the Early Ediacaran Oceans at the Type Section of the Sheepbed Formation, Mackenzie Mountains, Northwest Territories, Canada

by Miller, Austin J

Abstract

The timing of oxygenation events throughout Earth’s history remains controversial. Oceans and the atmosphere were essentially devoid of oxygen until the ‘great oxygenation event’ around 2.4 billion years ago that raised oxygen levels to 1-10% of todays present atmospheric levels (Holland, 2006). Earth’s second canonical oxygenation event, that raised atmospheric levels to near present, is thought to have occurred in the Neoproterozoic prior to the diversification of animal life. Using geochemical proxies, two frameworks of oxygenation have been developed for the timing of the Neoproterozoic event. The first suggests oxygenation of the deep oceans early in the Ediacaran succeeding the Marinoan Glaciation (~635Ma) (Shen et al., 2008; Sahoo et al., 2012), whereas the second places the event roughly coincident with the first large multicellular eukaryotes following the Gaskiers Glaciation (~580 Ma) (Canfield et al., 2007; Fike et al., 2006). Understanding the timing of oxygenation is critical in understanding the mechanisms and feedbacks associated with the diversification of animal life. A multi-proxy geochemical approach on black shale is useful to investigate the redox conditions in the Early Ediacaran following the Marinoan glaciation. Black shale samples are from well-exposed sections of the Sheepbed Formation, of known early Ediacaran age, at its type locality in the Mackenzie Mountains, Northwest Territories, Canada. Shen et al. (2008), Canfield et al. (2008), and Johnston et al. (2013) all conducted geochemical redox studies on the formation in different localities with mixed results. The current study extends the geologic coverage of redox data in the complex and laterally variable Northwest Canada Neoproterozoic strata. High-resolution iron speciation data in this study suggests a transition from ferruginous to oxygenated ocean waters with episodic ferruginous conditions. These data support the conclusions of Shen et al. (2008) that oxygenation is recorded within the Sheepbed. They do not suggest a “great event” occurred due to fluctuations in the redox stat following the oxygenation signal. Furthermore, in light of the studies at other localities, these data suggest heterogeneity in redox conditions across the early Ediacaran basin strata of Northwest Canada. Redox sensitive trace element data in ferruginous bottom waters of the basal Sheepbed Formation do not support the conclusion of Sahoo et al. (2012) that global ocean oxygenation directly followed the Marinoan Glaciation. Rather, moderate enrichments later in the section could suggest a possible global ocean oxygenation signal. More redox sensitive geochemical studies in post- Marinoan transgressive units are necessary to elucidate a clear understanding of global ocean redox conditions in the early Ediacaran and overcome the noise associated with localized effects on redox proxies.

Note

The author has given permission for this work to be deposited in the Digital Archive of Colorado College.

Colorado College Honor Code upheld.

Includes bibliographical references.

Administrative Notes

The author has given permission for this work to be deposited in the Digital Archive of Colorado College.

Colorado College Honor Code upheld.

Copyright
Copyright restrictions apply.
Publisher
Colorado College
PID
coccc:11279
Digital Origin
born digital
Extent
153 pages : illustrations, map(s)
Thesis
Senior Thesis -- Colorado College
Thesis Advisor
Erik Sperling Myrow, Paul
Department/Program
Geology
Degree Name
Bachelor of Arts
Degree Type
bachelor
Degree Grantor
Colorado College
Date Issued
2015-05