The oldest fossil micrometeorites yet to be recovered - 2.7 billion years old - are analysed in a paper published in Nature this week. The chemical composition of these micrometeorites, which measure just 8.6 to 50 micrometres in diameter, suggests that they may potentially have been oxidized in an oxygen-rich upper atmosphere in the Archaean era (3.9 to 2.5 billion years ago).

Although it is widely accepted that oxygen levels were extremely low during the Archaean (Earth’s early atmosphere contained less than 0.001 per cent of the present-day atmospheric oxygen), most evidence relates to the lower atmosphere and no method has been developed to sample the Archaean upper atmosphere until now.

Andrew Tomkins and colleagues extracted and examined 60 fossil micrometeorites from limestone sedimentary rocks in Australia's Pilbara region. The sampled meteorites are cosmic spherules - material that melted completely during atmospheric entry - that are proposed to have entered Earth’s atmosphere and melted at altitudes of 75 to 90 kilometres. The authors analyse the chemical composition of the meteorites and model how oxidation may have occurred. They suggest that oxygen concentrations in the upper atmosphere in the Archaean may have been close to those of the modern Earth. On the basis of chemical evidence preserved in sediments from the Archaean, the authors further propose that there was minimal mixing between the upper and lower atmospheres during this period.