New limits on prehistoric oxygen levels

The evolution of life on Earth is closely intertwined with the evolution of Earth's physical environment, particularly the concentration of atmospheric O₂. The oxygen increase arising from the development of photosynthesis in the Precambrian is thought to have created an "oxygen catastrophe" that challenged the dominant anaerobic organisms but spawned increased biodiversity and set off the Cambrian explosion in the fossil record. And low oxygen availability has been associated with two large-scale extinctions in the past 550 million years. One record of atmospheric oxygen levels is geological evidence—such as charcoal or polycyclic aromatic hydrocarbons—for prehistoric wildfires. Such wildfires would have required a minimum O₂ concentration for combustion, and new work by Claire Belcher and Jennifer McElwain at Ireland's University College Dublin has refined what that minimum concentration is. Unlike prior investigations of combustion limits, the researchers simulated realistic atmospheric conditions and used natural samples of pine and moss. The team found that the threshold O₂ concentration for combustion is 15%. Several models infer O₂ levels from paleogeochemical records for carbonates, sulfates, and other materials. But some of those models may now need to be reevaluated, since they predict prolonged periods with oxygen concentrations below the 15% threshold at times in the Mesozoic Era for which there is fossil evidence for wildfires. (C. M. Belcher, J. C. McElwain, Science 321, 1197, 2008.) — Richard J. Fitzgerald