Several theories have been proposed by scientists to explain the two mass extinction events which took place on the earth 250 and 200 million years ago.

The Permian-Triassic catastrophe (250 million years ago) was the worst of all five of the mass extinction events to ever have befallen the earth. It eradicated almost 95% of all species, 53% of marine families, 84% of marine genera and an approximated 70% of all land species including plants, insects and vertebrate animals.

Many scientists suspect that the event was the result of a comet or an asteroid colliding with the earth. Others believe that flood volcanism from the Siberian Traps and the associated oxygen loss in the seas was the cause. While others continue to investigate the possibility that thinning levels of atmospheric oxygen caused the eradication of so many species at the time.

But new research findings by University College Dublin scientists published in Science question the theory of falling oxygen levels as a mechanism for causing the mass extinction events.

To assess the likely atmospheric oxygen levels at the time of the mass extinction events, using purposefully designed walk-in-plant-growth rooms equipped with thermal imaging system and full atmospheric, temperature and humidity control, Dr Claire Belcher and her University College Dublin colleagues spent several months measuring the lower limits of oxygen at which combustion can occur.

When the measurements were recorded, they compared their results with the charcoal in the fossil record from ancient times because the charcoal that remains in the fossil record reveals the presence of ancient wildfires which require a sufficient level of oxygen in the air for plants to burn.

“By performing experimental burns using pine wood, moss, matches, paper and a candle at 20°C in varying ranges of oxygen concentrations and comparing these results to the occurrences of fossil charcoal throughout the Mesozoic (250-65 million years ago), we were able to identify that prolonged periods of low oxygen are unlikely to have occurred,” says Dr Claire Belcher from the School of Biology and Environmental Science, University College Dublin, the lead author of the report.

“Low oxygen atmospheres, less than 12%, are considered to be the primary driver of at least two of the ‘big five’ mass-extinction events,” explains Dr Belcher. “But our research findings question that hypothesis and highlight the need for more detailed studies of fossil charcoal across these mass extinction events.”

This is the first time that research to identify the lower limit of atmospheric oxygen under which combustion can occur have been conducted within fully controlled and realistic environments. The six walk-in chambers at University College Dublin, funded by EU Marie Curie, enable the realistic reconstruction of environmental conditions from the past.