Oxygenation of Earth


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•  Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis

Only one significant source of oxygen gas exists in the known universe – biological photosynthesis

  • Before the evolution of photosynthetic organisms, any free oxygen produced was chemically captured and stored

Approximately 2.3 billion years ago, photosynthetic organisms began to saturate the environment with oxygen

  • This led to changes in the Earth’s atmosphere, oceans, rock deposition and biological life


  • Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
  • When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
  • When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere


  • For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
  • The current concentration of oxygen gas within the atmosphere is approximately 20%

Rock Deposition

  • The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
  • These deposits are not commonly found in oceanic sedimentary rock younger than 1.8 billion years old
    • This likely reflects the time when oxygen levels caused the near complete consumption of dissolved iron levels
  • As BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels

Biological Life

  • Free oxygen is toxic to obligate anaerobes and an increase in O2 levels may have wiped out many of these species
  • Conversely, rising O2 levels was a critical determinant to the evolution of aerobically respiring organisms

Changes to Oxygen Levels on Earth

oxygenation of earth

Link:  The Rise of Atmospheric Oxygen