Late Paleozoic Hyperoxia
Robert Berner (Yale University, New Haven, Connecticut, USA)
Bownocker Lecturer, Dept. of Geological Sciences, Ohio State University, Columbus, Ohio, USA
26 April 2001
Presenting arguments for an interval of time during the Permo-Carboniferous of high atmospheric oxygen levels (hyperoxia).
Evidence for this is from:
1) abundance of organic carbon and pyrite sulfur in sedimentary rocks over time
2) mass balance equations based on carbon & sulfur isotope records
3) Late Paleozoic insect gigantism
4) plant fossil carbon isotopic fractionation
5) burning experiments not definitive
Organic matter burial & weathering
CO2 + H2O <--> CH2O + O2
This equation going to the right is photosynthesis. This equation going to the left is respiration (organic matter being oxidized). Need net photosynthesis in order for O2 accumulation with organic matter burial going on.
Sedimentary pyrite formation & weathering
2Fe2O3 + 16H+ + 8SO4- <--> 15O2+ 4FeS2 + 8H2O
Many intermediate reactions in this equation here. For example, oxygen does not bubble out with pyrite precipitation. Bacterial sulfate reduction is a necessary process here. The equation shows formation of pyrite and generation of O2. The reverse of this equation is pyrite weathering (easily grasped, compared with forward aspect of equation).
Fractionation goes up if land plants & plankton are grown in high O2 conditions.
Dragonflies grown at 35% O2 (rather than 21%) have higher metabolisms and fly better. There is a 25% increase in the size of Drosophila embryos if they are grown in high O2 conditions. Only the Permo-Carboniferous was a time of giant insects. 30-38% O2 estimates for the Permo-Carboniferous.
People object to this idea based on forest fires would go nuts if there was such high atmospheric O2. The experiments upon which this claim is based involve sparks on strips of paper (not a good representation of the biosphere!). New experiments of forest fire susceptibility in high O2 conditions show that some trees won’t burn at 35% O2. So, this is not an objection. We do find charcoal then - there were forest fires. But, O2 levels are not restricted to <25% O2.
Conclusion: the Permo-Carboniferous was a time of high O2 and low CO2.