Why Earth's Atmosphere Became Oxygenated

Earth scientists are offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere. Based on a new model that draws from research in diverse fields including petrology, geodynamics, vocanology and geochemistry, suggest that the rise of oxygen in Earth's atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics.

The new model suggests how atmospheric oxygen was added to Earth's atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago. 

 

Today, some 20 percent of Earth's atmosphere is free molecular oxygen. Free oxygen is not bound to another element, as are the oxygen atoms in other atmospheric gases like carbon dioxide and sulfur dioxide. For much of Earth's 4.5 billion-year history, free oxygen was all but nonexistent in the atmosphere. 

Researchers showed that around 2.5 billion years ago, the composition of Earth's continental crust changed fundamentally. Lee said the period, which coincided with the first rise in atmospheric oxygen, was also marked by the appearance of abundant mineral grains known as zircons.

The second rise in atmospheric oxygen was related to a change in production - analogous to turning up the flow from the faucet. The model of the research showed that Earth's carbon cycle has never been at a steady state because carbon slowly leaks out as carbon dioxide from Earth's deep interior to the surface through volcanic activity. Carbon dioxide is one of the key ingredients for photosynthesis.  The model also suggests that volcanic activity and other geologic inputs of carbon into the atmosphere may have increased with time and because oxygen production is tied to carbon production, oxygen production also must increase. The model showed that the second rise in atmospheric oxygen had to occur late in Earth's history.

Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, the team believes it may have been related to the onset of plate tectonics where Earth's surface for the first time became mobile enough to sink back down into Earth's deep interior.

Though the new model is not without controversy. For example, the model predicts that production of carbon dioxide must increase with time, a finding that goes against the conventional wisdom that carbon fluxes and atmospheric carbon dioxide levels have steadily decreased over the last 4 billion years.