# What caused a fall in atmospheric oxygen about 1.9 billion years ago?

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?

The answer might lie in aerobic respiration. This is of course only one of the many possible explanations but mitochondria (and therefore $\ce{O2}$-breathing eukaryotes) are thought to have evolved circa 2.3 to 1.8 Ga (see for instance Hedges et al., 2001; 2004; 2006). This loose age bracket however is based primarily on molecular data with very few fossil evidence (as I mentioned in an earlier answer of mine, the Proterozoic fossil record of eukaryotes is almost inexistant).
An earlier apparition of aerobic respiration is unlikely (e. g. Knoll & Holland, 1995) as a certain level of $p_{\ce{O2}}$ needed to be reached for this metabolism to be possible.