Using chromium isotopes geochemistry, Frei et al. 2009, describe in the article Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes that after the Great Oxidation Event (2.2-2.45 billion years ago), there was a precipitous decline in atmospheric oxygen at about 1.88 billion years ago, to have "all but disappeared" as described by Lyons and Reinhard 2009 in the article Early Earth: Oxygen for heavy-metal fans.
What evidence is there for a the drop in atmospheric oxygen approximately 1.9 billion years ago?