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September 1, 2008
Volume 86, Number 35
p. 12

Atmospheric Chemistry

O2 Requirement For Burning Rises

Experiments suggest Mesozoic oxygen levels were higher than previously expected

Elizabeth Wilson

SOPHISTICATED NEW combustion experiments put some of the strictest lower limits yet on atmospheric oxygen levels needed to allow burning. University College Dublin researchers Claire M. Belcher and Jennifer C. McElwain report the lower limit for atmospheric O2 that will support combustion is higher than previously predicted (Science 2008, 321, 1197).

8633cov3_SiGNacxd2.tif © 2008 Science
Belcher tests the combustion of materials in a chamber that varies atmospheric O2.

© 2008 Science
Thermal image shows sphagnum moss heated in 15% O2.

This finding has implications for the atmospheric composition of the Mesozoic Era, 251 million to 65 million years ago, during which several mass extinction events occurred that could have been caused by low oxygen levels. Because many fire records date back to this period, scientists have proposed that the atmosphere at that time had enough O2 to support combustion. Earlier combustion experiments suggested a lower limit of 12%. The new work finds a lower limit of 15%, indicating that the Mesozoic atmosphere contained more O2 than previously thought. Today's atmospheric O2 hovers around 21%.

"This is the first experiment to really go after the lower O2 limit; other experiments were more interested in general behavior and the upper limit," notes Richard Wildman, a graduate student at Caltech who recently performed burning experiments in the lab of Robert A. Berner, a geology and geophysics professor at Yale University. Berner has developed pioneering models of Mesozoic atmospheres.

Combustion experiments are difficult, and Belcher's lab, with its controlled atmosphere, took three years to set up and cost more than $1.4 million.

Belcher says the lab, in which they burned sphagnum moss, matches, wood, and paper, contains some unique features, such as a nitrogen generation plant to displace O2. "This combination of gas control and gas-level accuracy has not been achieved before," she says.

Belcher and McElwain also included a thermal-imaging camera, a tool not previously used in such experiments, to precisely determine the onset of combustion. "Wood and plant material will char at all ranges of O2, and therefore, charring itself is not a measure of combustion," Belcher says.

Berner notes that his models suggest that very dry wood might still have burned at O2 levels lower than 15%. "Our result—burning of wood at 2% moisture and 12% O2—is not unreasonable for bone-dry dead ground cover, which often acts to initiate a larger scale fire," he says.

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