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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?

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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.
If aerobic eukaryotes expanded rapidly they might have constituted a strong enough sink of oxygen, which could be responsible for the drop observed by Frei et al., 2009.

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