Brent Cornell

    
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

Oceans

• 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

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

Link:  The Rise of Atmospheric Oxygen