(Thanks to NoCamels) Weizmann Institute scientists have discovered how sulphur regulates oxygen in our atmosphere. Specifically, when the iron-sulphur mineral known as pyrite (or fool’s gold) dissolves in seawater, microbes use it to digest organic matter. This releases oxygen in the pyrite.
http://nocamels.com/2012/08/research-finds-fools-gold-regulates-world-oxygen-levels/