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A New Depositional Framework for Massive Iron Formations after The Great Oxidation Event
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  • Athena Eyster,
  • Latisha Brengman,
  • Claire Isobel O'Bryen Nichols,
  • Zoe Levitt,
  • Kristin D Bergmann
Athena Eyster
Massachusetts Institute of Technology

Corresponding Author:[email protected]

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Latisha Brengman
University of Minnesota Duluth
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Claire Isobel O'Bryen Nichols
Massachusetts Institute of Technology
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Zoe Levitt
Massachusetts Institute of Technology
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Kristin D Bergmann
Massachusetts Institute of Technology
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Abstract

The oldest recognized proxies for low atmospheric oxygen are massive iron-rich deposits. Following the rise of oxygen ~2.4 billion years ago, massive iron formations largely disappear from the geologic record, only to reappear in a pulse ~1.88 Ga, which has been attributed to passive margin transgressions, changing ocean chemistry triggered by intense volcanism, or lowered atmospheric oxygen levels. The North American Gogebic Range has exposures of both volcanics and iron formation, providing an ideal field locality to interrogate the relationship between the lithologies and investigate triggers for this pulse of iron formation. To determine the environmental context and key factors driving post-GOE iron formation deposition, we made detailed observations of the stratigraphy and facies relationships and present updated mapping relationshipsof the Gogebic Range Ironwood Iron Formation and the Emperor Volcanics. This work expands existing mine datasets and logs to constrain variations in stratigraphy. Our results are the first to quantitatively constrain thickness variations along the entire Gogebic range and tie them to syn-sedimentary faulting along listric normal faults and half grabens. Furthermore, our datasets suggest that initiation of major local volcanism does not coincide with iron formation deposition, thus, local intense volcanism cannot be invoked as a causal trigger. Finally, the possibility of iron formation deposition in a shallow water environment suggests that the post-GOE iron formation pulse may not reflect global marine transgressions, but instead a chemocline shallowing due to decreased atmospheric oxygen.
Aug 2021Published in Geochemistry, Geophysics, Geosystems volume 22 issue 8. 10.1029/2020GC009113