Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere

Sean McMahon (Corresponding Author), Ashleigh v. S. Hood, John Parnell, Stephen Bowden

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4 Citations (Scopus)
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Life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence. Here we apply the recently developed uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental rocks (red beds). Although these common palaeo-redox features have previously been suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been lacking. Our analyses reveal that the uranium present in reduction spheroids is isotopically heavy, which is most parsimoniously explained as a signal of ancient bacterial uranium reduction, revealing a compelling record of Earth’s deep biosphere.
Original languageEnglish
Article number4505
Number of pages6
JournalNature Communications
Early online date29 Oct 2018
Publication statusPublished - 29 Oct 2018


  • astrobiology
  • element cycles
  • environmental microbiology
  • palaeontology


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