Evolution of atmospheric O2 during the ages.

Early photosynthetic prokaryotes added oxygen to the atmosphere.

Two estimates of evolution of atmospheric O2.

The upper red and lower green lines represent the range of the estimates. Stage 1 (3.85–2.45 Ga) represents the primordial reducing atmosphere. Stage 2 (2.45–1.85 Ga) coincides with the emergence of oceanic cyanobacteria where O2 was being absorbed by the oceans and sediment. O2 escaped the oceans during Stage 3 (1.85–0.85 Ga). O2 sinks filled in Stage 4 (0.85–0.54 Ga) and Stage 5 (0.54 Ga–present) leading to atmospheric accumulation.

Banded iron formations in 2.1 billion year old rock.

The oxidation of dissolved oceanic iron that precipitated in response to accumulating O2 concentrations resulted in red bands in this 2.1 billion year old rock..

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