There is no simple answer to questions about the cause of either rises or falls in the amount of oxygen in the atmosphere, much as we would like there to be. For more technical details on the various geochemical and geological issues underlying the discussion that follows, Robert Berner’s book, The Phanerozoic Carbon Cycle, is highly recommended.
The major determinants of the changes in atmospheric oxygen levels are a series of chemical reactions involving many of the elements abundant on and in Earth’s crust, including carbon, sulfur, and iron. The chemical reactions involve both oxidation and reduction, processes that involve chemical reactions where certain elements either add or lose electrons. In the case of oxidation reactions, free oxygen (oxygen) combines with molecules containing carbon, sulfur, or iron to form new chemical compounds and in so doing oxygen is removed from the atmosphere and stored in the newly formed compounds. Oxygen is liberated back into the atmosphere by other reactions involving reduction of compounds. This is what happens during photosynthesis as plants liberate free oxygen as a by-product of the break-up of carbon dioxide through a complex series of intermediate reactions. Two important cycles ultimately dictate oxygen levels: the carbon cycle and the sulfur cycle. There may be other elements that are important as well, but currently they are deemed far less instrumental in affecting oxygen levels than are carbon and sulfur.
Let’s look first at the sulfur cycle. Sulfur is found in a wide variety of compounds, but the most important for understanding the rise and fall of oxygen over time is pyrite. This gold-colored cubic mineral is familiar to us as “fool’s gold,” and while of little value compared to gold in monetary terms, it is hugely important in dictating the amount of oxygen in the atmosphere and hence the state of the biosphere. Sulfur is added to the oceans from rivers as it weathers out of pyrite-bearing rocks on the continents, or it comes from sulfur-rich sedimentary rocks, such as gypsum and anhydrate. These latter are already in chemical states that do not react with oxygen. Such is not the case with pyrite, however. There are huge quantities of pyrite locked in a variety of rocks, most importantly dark shale that originated in the oceans, which are uplifted onto continents via plate tectonic mechanisms and then weathered under the onslaught of rain, wind, cold, and heat. There are