Measuring Oxygen Content of the Ancient Earth

[Drakkith]

I read that the level of oxygen in the atmosphere reached its highest concentration during the Carboniferous period, with over 32% of the atmosphere by volume as O2. I was wondering, how is this measured?

The reference on wiki only links to a graph with no other content.

Thanks.

 

[Borek]

Composition of some precipitates depends on the partial pressure of oxygen, so you can determine the partial pressure by examining deposits.

I also seem to remember in some cases we were able to find trapped bubbles of original atmosphere.