New York: In a fresh look at how Earth's atmosphere became oxygenated, a team of researchers has found that the rise of oxygen in the Earth's atmosphere was an inevitable consequence of the formation of continents in the presence of life and plate tectonics. Today, some 20 percent of the Earth's atmosphere is free oxygen or O2. For much of the Earth's 4.5-billion-year history, free oxygen was all but nonexistent in the atmosphere. “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 University. Plants and certain types of bacteria produce oxygen as a byproduct of photosynthesis. This oxygen production is balanced by the reaction of oxygen with iron and sulfur in the Earth's crust and by back-reaction with organic carbon. “For example, we breathe in oxygen and exhale carbon dioxide, essentially removing oxygen from the atmosphere. In short, the story of oxygen in our atmosphere comes down to understanding the sources but the three-billion-year narrative of how this actually unfolded is more complex,” Lee explained. Earth scientists from Rice University, Yale University and University of Tokyo prepared a new model that suggests how atmospheric oxygen was added to the Earth's atmosphere at two key times: One about two billion years ago and another about 600 million years ago. Oxygen is actually one of the most abundant elements on rocky planets like Mars, Venus and the Earth. However, it is one of the most chemically reactive elements. It forms strong chemical bonds with many other elements, and as a result, it tends to remain locked away in oxides that are forever entombed in the bowels of the planet -- in the form of rocks. “In this sense, Earth is no exception to the other planets; almost all of Earth's oxygen still remains locked away in its deep rocky interior,” Lee noted in a paper published in the journal Nature Geoscience. Lee and colleagues showed that around 2.5 billion years ago, the composition of Earth's continental crust changed fundamentally. The period, which coincided with the first rise in atmospheric oxygen, was also marked by the appearance of abundant mineral grains known as zircons. Zircons crystallise out of molten rocks with special compositions and their appearance signifies a profound change from silica-poor to silica-rich volcanism. The relevance to atmospheric composition is that silica-rich rocks have far less iron and sulfur than silica-poor rocks, and iron and sulfur react with oxygen and form a sink for oxygen. “Based on this, we believe the first rise in oxygen may have been due to a substantial reduction in the efficiency of the oxygen sink," Lee said. “In the bathtub analogy, this is equivalent to partially plugging the drain.” The second rise in atmospheric oxygen was related to a change in production, just like turning up the flow from the faucet. The model showed that the 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. According to the model, production of carbon dioxide must increase with time -- a finding that goes against the conventional wisdom that atmospheric carbon dioxide level has steadily decreased over the last four billion years. “This work does suggest that Earth scientists and astrobiologists may need to revisit what we think we know about Earth's early history,” Lee noted.