Letter | Published:

Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere

Nature 533, 235–238 (12 May 2016) | Download Citation

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Abstract

It is widely accepted that Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen (O2) level, until the Great Oxidation Event resulted in a major rise in O2 concentration about 2.4 billion years ago1. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere2 in the Archaean era; to date no method has been developed to sample the Archaean upper atmosphere. We have extracted fossil micrometeorites from limestone sedimentary rock that had accumulated slowly 2.7 billion years ago before being preserved in Australia’s Pilbara region. We propose that these micrometeorites formed when sand-sized particles entered Earth’s atmosphere and melted at altitudes of about 75 to 90 kilometres (given an atmospheric density similar to that of today3). Here we show that the FeNi metal in the resulting cosmic spherules was oxidized while molten, and quench-crystallized to form spheres of interlocking dendritic crystals primarily of magnetite (Fe3O4), with wüstite (FeO)+metal preserved in a few particles. Our model of atmospheric micrometeorite oxidation suggests that Archaean upper-atmosphere oxygen concentrations may have been close to those of the present-day Earth, and that the ratio of oxygen to carbon monoxide was sufficiently high to prevent noticeable inhibition of oxidation by carbon monoxide. The anomalous sulfur isotope (Δ33S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment4, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.

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Acknowledgements

We thank N. Wilson and A. Langendam for assistance with electron microprobe work and electron microscopy, respectively. The authors acknowledge use of the Monash Centre for Electron Microscopy, and use of the CSIRO Microbeam Laboratory. Part of this research was undertaken on the powder diffraction beamline at the Australian Synchrotron, Victoria, Australia. M.G. acknowledges STFC grant number ST/J001260/1.

Author information

Affiliations

  1. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia

    • Andrew G. Tomkins
    • , Lara Bowlt
    • , Siobhan A. Wilson
    •  & Jeremy L. Wykes

  2. Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

    • Matthew Genge

  3. Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 2BT, UK

    • Matthew Genge

  4. Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia

    • Helen E. A. Brand
    •  & Jeremy L. Wykes

  5. Department of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2113, Australia

    • Jeremy L. Wykes

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Contributions

A.G.T. conceptualized the project, conducted fieldwork and EMP analysis, and wrote the paper. L.B. conducted fieldwork, micrometeorite separation and SEM analysis. M.G. generated the micrometeorite oxidation model. S.A.W. advised on micrometeorite separation, conducted Raman spectroscopy and interpreted synchrotron results. H.E.A.B. conducted the synchrotron analysis. J.L.W. modelled the oxidizing conditions imposed at equilibrium by different atmospheres. All authors reviewed the paper prior to submission.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Andrew G. Tomkins.

Extended data

Extended data figures

  1. 1.

    Geological map showing the context of the sampling location (red star), and examples of the samples used in the study.

  2. 2.

    Examples of modern iron-type micrometeorites collected from the Antarctic ice sheet.

  3. 3.

    Results of laser Raman spectroscopy confirming the identity of wüstite.

  4. 4.

    Two synchrotron powder X-ray diffraction patterns, each collected from a single micrometeorite, and reference positions of magnetite peaks.

  5. 5.

    Equilibrium modelling of the oxidizing conditions imposed by different atmospheres relative to the stability fields of haematite, magnetite and wüstite.

  6. 6.

    Results of experiments on oxidation of low carbon steel in different gas mixes.

  7. 7.

    Results of atmospheric entry modelling.

Extended data tables

  1. 1.

    Representative analyses of micrometeorite magnetite, wüstite and metal

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DOI

https://doi.org/10.1038/nature17678

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