UK-US: New technique estimates past oxygen levels
Currently 21% of the atmosphere is oxygen and past variations in that proportion are thought to be linked to evolution.
For example, it is believed that high concentrations of oxygen about 300 million years ago enabled the evolution of gigantic forms of some animals, such as dragonflies with two-foot (0.6-metre) wingspans.
Scientists at Royal Holloway, University of London, and The Field Museum in Chicago have found that during the past 50 million years charcoal appears in coal in concentrations of about 4% to 8%.
But in older coals - from the Carboniferous and Permian Periods 320 to 250 million years ago and the Middle Cretaceous Period about 100 million years ago - the proportion of charcoal is as high as 70%.
The researchers say this indicates very high levels of atmospheric oxygen that would have promoted many frequent, large and extremely hot fires that created the charcoal, which later became part of the coal.
Dr Ian J Glasspool from The Field Museum explains: "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 Paleozoic, wildfires would have been frequent and catastrophic."
Professor Andrew C Scott from Royal Holloway says the periods of high oxygen concentration were times of major change in the evolution of vegetation on land, with the evolution and spread of new plant groups - the 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, with more fire meaning greater plant mortality, and in turn more erosion and therefore greater burial of organic carbon which would have then promoted elevated atmospheric oxygen concentrations.
"The mystery to us is why oxygen levels appear to have more or less stabilised about 50 million years ago," Scott says.
The research is published in the journal Nature Geoscience.