Strong evidence for a weakly oxygenated ocean-atmosphere system during the Proterozoic

Changle Wang; Maxwell A Lechte; Christopher T Reinhard; Dan Asael; Devon B Cole; Galen P Halverson; Susannah M Porter; Nir Galili; Itay Halevy; Robert H Rainbird; Timothy W Lyons; Noah J Planavsky

doi:10.1073/pnas.2116101119

Strong evidence for a weakly oxygenated ocean-atmosphere system during the Proterozoic

Proc Natl Acad Sci U S A. 2022 Feb 8;119(6):e2116101119. doi: 10.1073/pnas.2116101119.

Authors

Changle Wang^{1

2

3}, Maxwell A Lechte⁴, Christopher T Reinhard⁵, Dan Asael², Devon B Cole⁵, Galen P Halverson⁶, Susannah M Porter⁷, Nir Galili⁸, Itay Halevy⁸, Robert H Rainbird⁹, Timothy W Lyons¹⁰, Noah J Planavsky²

Affiliations

¹ Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; wangcl@mail.iggcas.ac.cn maxwell.lechte@mail.mcgill.ca.
² Department of Geology and Geophysics, Yale University, New Haven, CT 06511.
³ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.
⁴ Department of Earth and Planetary Sciences, McGill University, Montréal, QC H3A 0E8, Canada; wangcl@mail.iggcas.ac.cn maxwell.lechte@mail.mcgill.ca.
⁵ School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30318.
⁶ Department of Earth and Planetary Sciences, McGill University, Montréal, QC H3A 0E8, Canada.
⁷ Department of Earth Science, University of California, Santa Barbara, CA 93106.
⁸ Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 761001, Israel.
⁹ Geological Survey of Canada, Ottawa, ON K1A 0E8, Canada.
¹⁰ Department of Earth Sciences, University of California, Riverside, CA 92521.

Abstract

Earth's surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debated-foremost oxygen (O₂) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstones-Fe-rich sedimentary rocks deposited in nearshore marine settings-as a proxy for O₂ levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O₂ in the shallow oceans, settings crucial to eukaryotic evolution. Low O₂ in surface waters can be linked to markedly low atmospheric O₂-likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O₂ levels between ca 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O₂ levels during this interval may have selected for life modes adapted to more oxygenated habitats.

Keywords: Proterozoic; iron; ironstone; isotopes; oxygenation.

Publication types

Research Support, Non-U.S. Gov't