The Oxygenation of Earth’s Atmosphere, Plate Tectonics, and the Deep Carbon Cycle

Today, some 20 percent of Earth’s atmosphere is free molecular oxygen, or O2. For much of Earth’s 4.5-billion-year history, free oxygen was all but nonexistent in the atmosphere.

Earth scientists from Rice University, Yale University and the University of Tokyo 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, volcanology and geochemistry, the team published their findings this month in Nature Geoscience [1]. They 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.

 “It’s really a very simple idea, but fully understanding it requires a good bit of background about how Earth works,” said study lead author and DCO collaborator Cin-Ty Lee (Rice University, USA). “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 Earth’s crust and by back-reaction with organic carbon. For example, we breathe in oxygen and exhale carbon dioxide, essentially removing molecular oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources and sinks, but the 3-billion-year narrative of how this actually unfolded is more complex.”

Lee co-authored the study with Laurence Yeung and Adrian Lenardic (both of Rice University, USA), Ryan McKenzie (Yale University, USA), and Yusuke Yokoyama (University of Tokyo, Japan). The authors’ explanations are based on a new model that suggests oxygen was added to Earth’s atmosphere at two key times: one about 2 billion years ago and another about 600 million years ago.

The new model suggests Earth’s carbon cycle has never been at a steady state because carbon slowly leaks out from Earth’s deep interior to the surface through volcanic activity. The model also shows volcanic activity and other geologic inputs of carbon into the atmosphere increasing over time, and because oxygen flux is tied to carbon, oxygen production also must increase. The model showed that the second rise in atmospheric oxygen had to occur 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.

Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, but the team believes it may have been related to the onset of plate tectonics.

Article adapted from source.

Image: A view of Earth’s atmosphere taken from the International Space Station in 2003. Photo courtesy of ISS Expedition 7 Crew, EOLNASA

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