Cynthia to Steve Topletz: >> "A few hundred years ago, available oxygen was about 36%+." > A FEW HUNDRED YEARS?? Try 300 million. "
What an enjoyable topic. This site has some useful information (with the caveat that I'm nowhere near an authority on this topic and I'm only quoting one source): http://www.globalchange.umich.edu/globalchange1/current/lectures/samson/evolution_atm/index.html **************** Fluctuations in Oxygen The history of macroscopic life on Earth is divided into three great eras: the Paleozoic, Mesozoic and Cenozoic. Each era is then divided into periods. The latter half of the Paleozoic era, includes the Devonian period, which ended about 360 million years ago, the Carboniferous period, which ended about 280 million years ago, and the Permian period, which ended about 250 million years ago. According to recently developed geochemical models, oxygen levels are believed to have climbed to a maximum of 35 percent and then dropped to a low of 15 percent during a 120-million-year period that ended in a mass extinction at the end of the Permian. ****************** and prior to the above entry: ******************** Why does present-day oxygen sit at 20%? This is not a trivial question since significantly lower or higher levels would be damaging to life. If we had < 15% oxygen, fires would not burn, yet at > 25% oxygen, even wet organic matter would burn freely. ******************** Cheers...Steve _______________________________________________ the OrchidGuide Digest (OGD) orchids@orchidguide.com http://orchidguide.com/mailman/listinfo/orchids_orchidguide.com
