By Sid Perkins
Normal 0 false false false MicrosoftInternetExplorer4 Earth’s atmosphere during some past geological ages wasn’t as oxygen-deprived as previously thought, new experiments suggest.
Results of the new lab work — conducted in climate-controlled chambers large enough to walk inside — indicate that combustion of natural materials such as moss and wood takes place only when atmospheric concentrations of oxygen exceed 15 percent. Some previous experiments suggested that combustion could take place at oxygen levels as low as 12 percent, says Claire Belcher, a biogeochemist at University College Dublin in Ireland. Those tests, however, used unnaturally dry materials such as paper, or were performed in small cabinets where oxygen could have infiltrated, she and colleague Jennifer McElwain note in the Aug. 29 Science.
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The concentration of oxygen in Earth’s atmosphere, which today sits just under 21 percent, has fluctuated significantly during the last 600 million years (SN: 12/17/05, p. 395). Although there’s no direct way to measure ancient oxygen levels, some geochemical data suggest that levels have risen to as high as 35 percent and dropped to as low as 10 or 12 percent for significant intervals. Often, says Belcher, values at the lower end of that range are inferred by the absence of charcoal from the fossil record: When the atmosphere is so oxygen-poor that fires can’t start or spread, sediments deposited during that time contain little, if any, naturally produced charcoal.
Belcher and McElwain conducted their experiments by heating various combustible materials on a hot plate in one of several 8.3-cubic-meter chambers permitting control of temperature, humidity and atmospheric chemistry. The researchers gathered data at oxygen levels between 9 percent and 21 percent for both a modern-day carbon dioxide concentration of 380 parts per million and a CO2 level of 1,500 ppm, which is representative of some ancient atmospheres, says Belcher. For some combinations of conditions, especially those simulating a low-oxygen atmosphere, the researchers wore space-suit–like gear. When oxygen levels drop below 13 percent, people typically have heart attacks and die, Belcher notes.