Oxygen's role in extinctions queried

UCD researchers raise doubts that low oxygen caused mass extinctions, such as the one that wiped out the dinosaurs writes Claire O'Connell

SOME PERIODS ON EARTH have been tough going. Such as around 250 million years ago, when some 90 per cent of species suddenly died out, prompting scientists to wonder what on earth happened.

One theory suggests that oxygen thinned out to such low levels that it caused trouble for animals. But now a new study by researchers at University College Dublin has fired the debate by burning plants in low-oxygen environments. The results, published this week in the influential journal Science, suggest that oxygen may not have been so scarce after all.

"Low oxygen has been hypothesised as being one of the key driving mechanisms at two big extinction events - the Permian/Triassic event and the Triassic/Jurassic event ," says Dr Claire Belcher from UCD's school of biology and environmental science.

But the fossil record for these periods includes charcoal, which suggests there was enough oxygen in the atmosphere for plant material to burn, says Belcher, whose work focuses on the Greenland area.

Today oxygen makes up around 21 per cent of the air we breathe, but to see how low oxygen could go and still allow plants to burn, she donned protective gear, went into a purpose-built chamber and set fire to a range of plant species and materials under different atmospheric conditions.

The large room-like chamber allows tight control of gases, humidity and temperature and yields more realistic results than smaller desktop boxes used in other combustion studies, according to Belcher. "I set about trying to define what the limits of combustion really were, in as close to a realistic situation as possible," she says.

For her experiments, she chose materials to represent the types of species that would have been present during those periods, and also what could actually light up in practice. "Starting a fire is hard if you don't have an accelerant, as I have found out," says Belcher. "So it was a combination of trying to get something we might find in the fossil record and also something we could ignite even in ambient conditions."

The results of her burning experiments quench previous suggestions that oxygen levels in the atmosphere may have plummeted to around 12 per cent for prolonged periods around the times of mass extinctions.

Instead, Belcher found that in the chamber, paper smouldered briefly at 14 per cent oxygen, a candle and sticks of highly-flammable pine wood sustained a brief flame at 16 per cent and sphagnum moss produced no flame below 15 per cent atmospheric oxygen. Meanwhile, increasing the oxygen levels to more than 17 per cent vastly improved burning across the board.

"Previous work has put the lower limit for combustion at about 12 or 13 per cent," says Belcher, who co-authored the paper with Dr Jennifer McElwain at UCD. "But we have revised this based on what we feel are more realistic situations for burning to being closer to 15 per cent, or perhaps even higher, 17 per cent to get frequently occurring fires."

The new findings don't completely snuff out the possibility of oxygen levels being a problem for species at those times though, notes Belcher. "Even 15 per cent is pretty low, I had to wear breathing apparatus under 18 per cent," she says, adding that other stressors, such as changes in temperature, could have made things even worse. "Some animals, particularly reptiles and amphibians, fare particularly badly in lower-oxygen environments, especially when it's so warm, so it might be a combination associated with a lower oxygen [level]. And even a slight drop in oxygen from our present-day ambient would cause quite a lot of problems for many animals."

The push is on now to scour the fossil record more closely and see whether wildfires stopped happening for shorter periods, suggesting that oxygen could have become even scarcer. "We cant rule out that very low oxygen levels occurred for short periods because that was below the resolution of our study," says Belcher. "So we now need highresolution fire studies across these mass extinction events to try to get [shorter] intervals and really look at whether low oxygen could have affected them."