The Breathing Goddess: What's Happening to Oxygen in our Atmosphere?

The Goddess herself has created the oxygen she breathes (image by Dezgo.com) 

There is a hugely interesting paper that appeared this year on "Annual Reviews of Earth and Planetary Sciences." It deals with the evolution of oxygen during the Phanerozoic.  The story of the ecosphere is all in there. Unbelievably, some 300 million years ago, the Permian fauna and flora lived in a concentration of oxygen of about 35%, nearly double the current one (about 20%). Even the dinosaurs of the Cretaceous breathed air with about 25%-30% oxygen in it. Ever wondered why the diplodocus was so big? That's one possible reason. 

In comparison, we are oxygen-starved. And we are further reducing the oxygen concentration by burning fossil fuels (it can be observed experimentally). All this may have to do with the different breathing architecture of mammals and birds (aka dinosaurs).

But why these oscillations? They may be the result of geophysical factors. The oxygen concentration in the atmosphere is related to the interplay of carbon sedimentation and sedimented carbon oxidation. The former is mainly due to plate tectonics, the latter to large igneous provinces and similar volcanic events. The winding down of oxygen concentration might be explained by a slowdown of plate tectonics and hence of sedimentation at continental edges, but that's far from being certain. 

I tend to think that it is an indirect result of the sun becoming brighter over the Eons. This increase in irradiation forces the system to reduce the CO2 concentration to keep temperatures within the range needed for life. But that will also indirectly reduce the oxygen concentration because oxygen comes from a photochemical reaction that involves CO2 molecules. Less CO2, less oxygen. And that means we are winding down with the ecosystem complexity: it means, basically, that the biosphere doesn't produce as much oxygen as it used to in earlier times. 

If the oxygen concentration goes below certain limits, metazoa will not be able to survive. In a few tens of millions of years, Earth may have reverted to protozoa only (single-celled creatures), as it had been for more than 2 billion years.

Unless someone starts burning a fraction of the sedimented carbon to increase CO2 concentration AND simultaneously manages to reduce the solar irradiation. Maybe good old Gaia has exactly that in mind!!

https://www.annualreviews.org/doi/full/10.1146/annurev-earth-032320-095425