July 29. Oxygen levels

On July 14 when we talked about the giant insects of the Pennsylvanian, I mentioned the significantly greater oxygen content of the Pennsylvanian atmosphere as a possible contributor to the large size of those bugs. The extra oxygen could have provided more energy for growth as well as better support for winged insects.  

Oxygen levels in the Pennsylvanian are estimated at as much as 31% to 35%, versus today’s 21% oxygen. The cause is probably complex, but it seems reasonable to expect that the abundance of plants must have had something to do with it, and they could have also helped reduce carbon dioxide in the atmosphere as well, as I mentioned a few days ago. Plants take in CO2 and give off oxygen, and especially if the carbon is sequestered in coal in the ground, this could have a profound impact on the atmosphere.

The high in atmospheric oxygen during the Pennsylvanian and early Permian was probably the highest it has ever been, at least during Phanerozoic time, the past 550 million years. And it was followed in the Permian and Triassic by one of the lowest lows – around 15% oxygen. That crash might have been related to the crash in the Pennsylvanian rainforest ecosystem – many fewer plants, much less oxygen, and that in turn might be connected in part to the growing glaciers in Gondwana, which we’ll talk more about next month, during the Permian. But whatever the causes, these dramatic shifts in atmospheric oxygen content must have had dramatic impacts on life, positive for some groups and negative for others.

How do we know the concentration of oxygen in ancient atmospheres? It’s estimated mostly using geochemical models that integrate fluctuations in the carbon cycle with measurements of oxygen isotope ratios in rocks and fossils, as well as erosion and sedimentation rates and other expectable chemical reactions, all of which give us indications of ancient concentrations of atmospheric gasses. The oldest known amber, fossil resin from plants, is from the Carboniferous, and while some younger amber specimens have trapped air bubbles that can be analyzed, I haven’t found any reports that that has been done for Carboniferous amber. So it’s pretty much a combination of observations such as abundant plants and giant insects plus detailed chemical modeling. The nature of Carboniferous plants – thick corky outer layers that could have served as protection from fire, as well as some modern analogs that suggest Carboniferous plants thrived in an ecosystem that had abundant fires, also points toward the likelihood of significantly more atmospheric oxygen then, to fuel such fires.

It seems to be pretty much accepted today that Carboniferous atmospheric oxygen was at least 31% or as much as 35% of the total, much greater than today’s 21% oxygen content.

—Richard I. Gibson

Links and References:
Historical CO2 levels

Atmospheric oxygen over Phanerozoic time (Berner, 1999) 

Geochemical modeling 

A new model for atmospheric oxygen over Phanerozoic time (Berner, 1989)

PHANEROZOIC ATMOSPHERIC OXYGEN (Berner et al., 2003): Ann. Rev. Earth Planet. Sci. 2003, 31:105.

Oxygen balance 

Insect growth in various oxygen concentrations