Recipe for an oxygenated atmosphere = tectonics + continents + life
How did Earth’s atmosphere get its oxygen? Scientists have been contemplating, hypothesizing, and testing answers to that question for years. Ideas currently range from cyanobacteria producing oxygen as a waste product to a reduction in underwater nickel erupting volcanoes that decreased the number of methane emitting organisms.
Now, using a new model, Earth scientists from Rice University, Yale University, and the University of Tokyo are offering an alternative answer to how our planet acquired its oxygenated atmosphere. They suggest that the rise of Earth’s atmospheric oxygen was a consequence of the formation of the continents in the presence of life and plate tectonics.
The study, published in the journal Nature Geoscience, suggests how atmospheric oxygen was added to Earth’s atmosphere at two key points: the first about 2 billion years ago and another around 600 million years ago.
Today, roughly 20 percent of Earth’s atmosphere is free molecule oxygen (O2) — meaning it is not bound to another element. However, for most of Earth’s 4.5-billion-year-history, free oxygen was nonexistent in the atmosphere.
"It was not missing because it is rare," said study lead author Cin-Ty Lee, professor of Earth science at Rice, in a press release. "Oxygen is actually one of the most abundant elements on rocky planets like Mars, Venus and Earth. However, it is one of the most chemically reactive elements."
This means it forms strong chemical bonds with other elements, and remains bound within rocks. According to Lee, "almost all of Earth's oxygen still remains locked away in its deep rocky interior."
However, 2.5 billion years ago, the composition of Earth’s continental crust changed, coinciding with the first rise in free oxygen and the appearance of mineral grains known as zircons.
"The presence of zircons is telling," said Lee. Their appearance signifies a change from silica-poor to silica-rich volcanism. This is an important difference because silica-rich rocks have far less iron and sulfur, which are both very reactive with oxygen, so more oxygen was freely available.
What caused the composition of the crust to change is still a mystery, but Lee and his colleagues believe it may have been related to the onset of plate tectonics — where Earth’s surface, for the first time, became mobile.
The second rise in atmospheric oxygen was related to a change in the production of oxygen, according to the study authors. Oxygen production is inherently linked to the global carbon cycle — the cycling of carbon between the biosphere, atmosphere and ocean — and carbon dioxide is one of the key ingredients for photosynthesis, where plants and bacteria harness energy from the sun and produce oxygen as a byproduct.
So if carbon emitted into the atmosphere increases, so does oxygen production.
According to the study, the second rise in atmospheric oxygen must have occurred late in Earth’s history. "Exactly when is model-dependent, but what is clear is that the formation of continental crust naturally leads to two rises in atmospheric oxygen, just as we see in the fossil record," Lee said.
However, the model Lee and his team used to test their hypothesis is far from perfect. For example, the model predicts that the production of carbon dioxide must increase with time, a finding that goes against the accepted view of how carbon fluxes, and atmospheric carbon dioxide levels have steadily decreased over the last 4 billion years.
"The change in flux described by our model happens over extremely long time periods," explained Lee. "However, our work does suggest that Earth scientists and astrobiologists may need to revisit what we think we know about Earth's early history."
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