Great Oxygenation Event
The great oxygenation event (GOE) corresponds to the time when excess free oxygen started to accumulate in noticeable concentration in the atmosphere. This first rise of oxygen in the atmosphere took probably place at the Archean/Proterozoic boundary, around 2.45 Ga ago and was the result of photosynthetic activity of cyanobacteria. Oxygen partial pressure in the atmosphere before life was likely less than 10−13 bars. Oxygen could have been locally produced by photosynthesis well before the GOE, as early as 3.0 Ga. However, most of the produced oxygen was then consumed to oxidize the rocky surface of the Earth and the pO2 was maintained as low as 10−5 times the present atmospheric level (21% vol. of O2). At around 2.32 Ga there is strong evidence that O2 was accumulating in the atmosphere, and may have reached 0.2–2% by volume.