As the title says, I'd like to know what the O2 concentration was during the late Miocene period. I've tried to look this up but I'm having a hard time finding any conclusive data on it, or even any graphs of oxygen concentration on a scale smaller that 100 million years. From what I can tell it should be lower than today, although I can't tell by how much.
R. Tappert et al (2013) used carbon isotopes in ambers as proxies for atmospheric oxygen concentrations. Their work indicates an initial Miocene O2 concentration of 16% and rising to about 20% at the end:
It's pretty remarkable how different their results look from other models. Some think that some inferred upward trend in O2 levels since the Eocene gave rise to the mammals, or at least bigger brains in mammals.
Hope this helps!