Trial by Fire

What makes a fire burn? In addition to fuel (such as wood or paper) and heat, fires need oxygen. If there isn’t enough oxygen in the atmosphere, combustion simply won’t happen.

That was as true hundreds of millions of years ago as it is today. So wildfires, scientists say, can provide a unique way to estimate how much oxygen was in Earth’s atmosphere throughout its history.

Traces of charcoal that appear periodically in the fossil record are evidence of when ancient wildfires raged — and when there was at least enough oxygen in the atmosphere for those fires to ignite.

But how much oxygen there was in Earth’s past has been hotly debated. Some models of Earth’s atmosphere about 190 million years ago — the start of the Jurassic age of dinosaurs — suggest that oxygen content may have dipped to as low as 10 percent for many years. For comparison, oxygen today is about 21 percent.

But 10 percent oxygen seems too low, because there were wildfires during that time, says Claire Belcher of University College Dublin. So Belcher and colleague Jennifer McElwain created a carefully constructed experiment to try to find out just how little oxygen is needed for these fires to combust.

They placed five different sources of fuel, including paper, moss and a resinous, flammable wood called “pinus caribaea” in a chamber, and altered the oxygen content in the room. Using thermal imaging, they waited to see how much oxygen was needed for these materials to combust.

Based on these experiments, the scientists determined a new lower limit for oxygen: below 15 percent oxygen, nothing would ignite, Belcher says. And only limited combustion would happen below 17 percent oxygen, the scientists in an article published Friday, Aug. 28, in the journal Science.

Since there were wildfires in the Jurassic, that means that the dinosaurs probably breathed a little easier than has been thought.

Carolyn Gramling
Thursday, September 4, 2008 - 18:05