The rise of oxygen in the Earth’s air was a consequence of the formation of continents in the presence of life and plate tectonics, suggest scientists, offering a new answer to the long-standing question of how our planet acquired its oxygenated atmosphere. The 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 per cent of Earth’s atmosphere is free molecular oxygen (O2). Free oxygen is not bound to another element, as are the oxygen atoms in other atmospheric gases like carbon dioxide and sulphur dioxide.
For much of Earth’s 4.5-billion-year history, free oxygen was all but nonexistent in the atmosphere. “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,” said Cin-Ty Lee, professor at Rice University in US.
“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,” he said.
Researchers showed that abound 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,” Lee said.
“The relevance to atmospheric composition is that silica-rich rocks have far less iron and sulphur than silica-poor rocks, and iron and sulphur react with oxygen and form a sink for oxygen,” he said.
“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. The study suggests that the second rise in atmospheric oxygen was related to a change in production.
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.
It showed 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.
Exactly what caused the composition of the crust to change during the first oxygenation event remains a mystery, but Lee said 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. The study was published in the journal Nature Geoscience.