Great Oxygenation Event
Photosynthesis was producing oxygen both before and after the GOE. The difference was that before the GOE, organic matter and dissolved iron chemically captured any free oxygen. Dissolved iron (of which there was a great deal) became iron oxide and huge deposits of this are found as banded iron rock from the Archaean and Proterozoic eras. The GOE was the point when these minerals became saturated and could not capture any more oxygen. The excess free oxygen started to accumulate in the atmosphere.
Oxygen was toxic to most of the Earth's anaerobic inhabitants at the time. As cyanobacteria produced oxygen, and built their stromatolites, they changed the environment for other protists. Since the other protists had no way to deal with oxygen, most would have become extinct. Another consequence was the effect of free oxygen on atmospheric methane, a greenhouse gas. The reaction removed the methane and caused the Huronian glaciation, possibly the longest snowball Earth episode ever. Free oxygen has been an important part of the atmosphere ever since.
The most widely accepted chronology of the Great Oxygenation Event suggests that oxygen was first produced by photosynthetic organisms (prokaryotic, then eukaryotic) that emitted oxygen as a waste product. These organisms lived long before the GOE, perhaps as early as 3500 million years ago (mya). The oxygen they produced would have quickly been removed from the atmosphere by the 'mass rusting' which led to the deposition of banded-iron formations. Oxygen only began to persist in the atmosphere in small quantities shortly (~50 million years) before the start of the GOE. Without a draw-down, oxygen could accumulate very rapidly. At today's rates of photosynthesis (which are much greater than those in the land-plant-free Precambrian), modern atmospheric O2 levels could be produced in around 2,000 years.
- 3,500 mya Archaean eon: production of oxygen by cyanobacteria in stromatolites.
- Oxygen causes deposit of iron as iron oxides in banded iron formations.
- c. 2,400 mya Palaeoproterozoic era: free oxygen escapes into atmosphere, mostly absorbed on the land.
- c. 850 mya Neoproterozoic era: oxygen starts to accumulate in the atmosphere. Continues to increase during the Palaeozoic era to present levels.
- Holland, Heinrich D. 2006.The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society: Biological Sciences. 361, 903–915.
- also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation
- Frei R. et al 2009. Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes. Nature 461 (7261): 250–253. 
- Lyons, Timothy W. & Reinhard, Christopher T. 2009. Early Earth: Oxygen for heavy-metal fans. Nature 461, 179–181. 
- Dutkiewicz A. et al 2006. Biomarkers from Huronian oil-bearing fluid inclusions: an uncontaminated record of life before the Great Oxidation Event. Geology 34 (6): 437. 
- Anbar A. et al 2007. A whiff of oxygen before the great oxidation event?. Science 317 (5846): 1903–1906. 
- Dole M. 1965. The natural history of oxygen. The Journal of General Physiology 49 (1): 5.