Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy

Sarah P. Slotznick, Nicholas L. Swanson-Hysell, Erik A. Sperling

Research output: Contribution to journalArticlepeer-review

28 Scopus citations

Abstract

Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (Fe HR /Fe T ) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.

Original languageEnglish (US)
Pages (from-to)12938-12943
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number51
DOIs
StatePublished - Dec 18 2018
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2018 National Academy of Sciences. All rights reserved.

Keywords

  • Eukaryotic evolution
  • Iron speciation
  • Lacustrine environments
  • Oxygen
  • Proterozoic

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