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Earth's oxygen highs

Fossil charcoal under SEM

Scientists using charcoal preserved in ancient bogs have been able to reconstruct the oxygen content of atmospheres past. Ted Nield reports.

Geoscientist Online 4 August 2010

Variations in the Earth’s atmospheric oxygen levels are thought to be closely linked to the evolution of life. Over the past 400 million years, the level of oxygen has diverged considerably from the 21% value we breathe today – but by how much? Scientists from Royal Holloway, University of London and from The Field Museum in Chicago, have revealed this week (in the journal Nature Geoscience) that the amount of charcoal preserved in ancient peat bogs (now turned to coal) can provide an accurate measure of how much oxygen there was in past atmospheres.

Until now, scientists had relied on geochemical models to estimate atmospheric oxygen. In fact, a number of competing models exist, each with significant discrepancies and no clear way to resolve them. However, all models agree that around 300 million years ago in the Late Palaeozoic, atmospheric oxygen levels were much higher than today. These elevated concentrations have been linked to gigantism in some animal groups, in particular insects like the dragonfly Meganeura monyi with a wingspan of over 60 centimetres. Some scientists think these higher concentrations of atmospheric oxygen may also have allowed vertebrates to colonise the land.

Higher levels of oxygen came about as a direct consequence of the colonisation of the land by plants. When plants photosynthesize they give off oxygen. However, when the carbon stored in plant tissues decays, atmospheric oxygen is used up. To produce a net increase in atmospheric oxygen over time organic matter must become buried. The colonisation of land by plants not only led to new plant growth, but also to a dramatic increase in carbon burial. Such burial was particularly high during the Late Paleozoic when the huge coal deposits accumulated.

Dr Ian J Glasspool (Chicago Field Museum) says: “Atmospheric oxygen concentration is strongly related to flammability. At levels below 15% wildfires could not have spread. However, at levels significantly above 25% even wet plants could have burned, while at levels around 30 to 35%, as have been proposed for the Late Palaeozoic, wildfires would have been frequent and catastrophic”.

Prof Andrew Scott The researchers, including Professor Andrew C Scott (Royal Holloway University of London) have shown that charcoal found in coal has remained at concentrations of around 4-8% over the past 50 million years - indicating levels of atmospheric oxygen close to modern values. However, there were times when charcoal percentage in coals went as high as 70%. This indicates very high levels of atmospheric oxygen, promoting frequent, large, and extremely hot fires. Such oxygen highs include the Carboniferous and Permian Periods from 320-250 million years ago and the Middle Cretaceous Period, approximately 100 million years ago.

“It is interesting”, says Scott “that these were times of major change in the evolution of vegetation on land, with the evolution and spread of new plant groups - conifers in the late Carboniferous and flowering plants in the Cretaceous”. These periods of high fire (resulting from elevated atmospheric oxygen concentration) might have been self-perpetuating; more fire meaning greater plant mortality, more erosion, and thus greater burial of organic carbon - which would have then promoted elevated atmospheric oxygen. “The mystery to us is why oxygen levels appear to have more or less stabilised about 50 million years ago”.


  • Glasspool, I.J. and Scott A.C. 2010. Phanerozoic atmospheric oxygen concentrations reconstructed from sedimentary charcoal. Nature Geoscience DOI: 10.1038/NGEO923