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Nature. 2016 May 12;533(7602):235-8. doi: 10.1038/nature17678.

Ancient micrometeorites suggestive of an oxygen-rich Archaean upper atmosphere.

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School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia.
Impact and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK.
Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 2BT, UK.
Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia.
Department of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2113, Australia.


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 ago. There are multiple lines of evidence for low O2 concentrations on early Earth, but all previous observations relate to the composition of the lower atmosphere 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 today). 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 (Δ(33)S) signature of pyrite (FeS2) in seafloor sediments from this period, which requires an anoxic surface environment, implies that there may have been minimal mixing between the upper and lower atmosphere during the Archaean.


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