Based on a new model that draws from research in diverse fields including petrology, geodynamics, volcanology and geochemistry, the findings of Earth scientists from Rice University, Yale University and the University of Tokyo offer a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere.
The study suggests 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.
“It’s really a very simple idea, but fully understanding it requires a good bit of background about how the Earth works,” said study lead author Cin-Ty Lee, professor of Earth science at Rice.
“The analogy I most often use is the leaky bathtub. The level of water in a bathtub is controlled by the rate of water flowing in through the faucet and the efficiency by which water leaks out through the drain. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the sink: reaction of oxygen with iron and sulfur in the Earth’s crust and by back-reaction with organic carbon,” he added.
The authors’ explanations are based on a new model that 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, or O2.
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.
Lee and colleagues 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.
Lee said the model 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.
Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, but Lee said the team believes it may have been related to the onset of plate tectonics, where the Earth’s surface, for the first time, became mobile enough to sink back down into Earth’s deep interior.
Lee added that the team’s 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.
The study was published in Nature Geoscience.