Earth’s atmosphere did not always contain oxygen. The prevailing thinking is that algae produced the first free oxygen when they evolved the capacity for photosynthesis. However, the timing is problematic since oxygen-producing algae evolved 3.4 billion years ago, but a permanently oxygenated atmosphere didn’t appear until over a billion years later. Recent research suggests that this delay was due to volcanic gases reacting with the available free oxygen, thus removing it from the atmosphere. An oxygen tug-of-war between biology and geology delayed the oxygenation of the atmosphere.
Cyanobacteria
Signs of life in the ancient oceans of the earth appeared as early as 3.4 million years ago as cyanobacteria (blue-green algae). Stromatolites, fossilized in old Archean-age rocks, are the remains of bioherms formed by these algae. Scientists determine the age of these fossilized algae bioherms by age-dating the rocks containing the stromatolites. Archean stromatolites exist on continents around the world, including Australia, South Africa, and Greenland.
Cyanobacteria are “oxygenic phototrophs,” meaning they use light for nourishment and give off oxygen as a by-product. These algae were the first organisms to develop photosynthesis, and thus, they ruled the earth during the early Archean. For a billion years, they basked in the sunlight and excreted free oxygen into the oceans and atmosphere.
The mystery
The mystery behind the success of cyanobacteria lies in determining what happened to all of that oxygen for over a billion years. MIT scientists studying sulfur isotopes in ancient rocks place the timing of the Great Oxygenation Event (GOE) at 2.33 billion years ago. Also, they believe that full oxygenation of the atmosphere took about 10 million years.
The geological record of the GOE lies in a transition detected in specific sulfur isotopes. These sulfur isotopes represent reactions between volcanic gases and the ancient atmosphere. But the isotope pattern changes when oxygen is present. Through studying these isotope patterns in rock cores from South Africa, Genming Luo and his associates determined when the transition took place from an anoxic atmosphere to an oxygenated one.
This work tells us when, but not why. A 2020 paper in Nature Communications (S. Kadoya et al.) proposes an idea of why the GOE was over a billion years after algae first started producing oxygen.
Sources and sinks, and oxygen tug-of-war
Concentrations of chemicals in the atmosphere, including oxygen, represent a balance between sources and sinks. An example of sources and sinks occurs when fossil fuels burn and become a source of carbon dioxide (CO2). However, trees use CO2 to grow through photosynthesis; thus, trees are a sink. Oxygen production by algae began 3.4 million years ago (or earlier), and the sulfur isotope work at MIT indicates that the final oxygenation of the atmosphere took only 10 million years to achieve. So, if oxygen did not initially accumulate in the atmosphere, it must have been taken up by a sink.
The work by Kadoya indicates that the sink for that oxygen was volcanic gases. But the story begins deep in the mantle of the earth. The mantle lies below the earth’s crust and is the source of magma for volcanic hotspots like in Iceland and Hawaii. The early Archean mantle’s composition was different from today, and that first mantle contained less oxygen. A less-oxidized mantle produces higher volumes of gases that react with oxygen. Gases, like hydrogen, naturally combine with free oxygen, thus removing it from the atmosphere.
The implication is that volcanic gases were the oxygen sink, and the mantle’s oxygenation took over a billion years. Once the mantle was oxygenated, the composition of volcanic gases changed, and these gases ceased to be an effective sink for all of the oxygen the algae produced. In this oxygen tug-of-war, geology prevailed for over a billion years, but biology had the final say.
ArcheanWeb:
The Icelandic Plume (Source: ArcheanWeb) – https://archeanweb.com/2020/03/25/the-icelandic-plume/ Also:
Yellowstone quakes and shakes, but will it blow? (Source: ArcheanWeb) – https://archeanweb.com/2020/06/12/yellowstone-quakes-and-shakes-but-will-it-blow/ Also:
Sources:
Study pinpoints timing of oxygen’s first appearance in Earth’s atmosphere (By Jennifer Chu; MIT News Office) – http://news.mit.edu/2016/oxygen-first-appearance-earth-atmosphere-0513 Also:
Mantle data imply a decline of oxidizable volcanic gases could have triggered the Great Oxidation (By Shintaro Kadoya, David C. Catling, Robert W. Nicklas, Igor S. Puchtel & Ariel D. Anbar; Nature Communications) – https://www.nature.com/articles/s41467-020-16493-1 Also:
Feature Image: Tuvurvur volcano (Modified) – By Taro Taylor edit by Richard Bartz – originally posted to Flickr as End Of Days, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=6113476
