New Insight Into the Early Atmosphere of Earth

The rise of oxygen in a thin Earth atmosphere and the deepening of a paradox.

shark bay stromatolites.jpg
Stromatolites in Shark Bay, Australia, give us an idea of how life looked two and a half billion years ago on Earth.

Two studies published this week add new detail to our knowledge of environmental conditions on the early Earth. The first study, led by Genming Luo from M.I.T. and appearing in Science Advances, pinpoints the rise of oxygen in Earth’s atmosphere rather precisely, at 2.33 billion years ago. Based on sulfur isotopic ratios, the authors determined the time when Earth became oxygen-rich, meaning that it crossed the threshold of one ten-thousandth of today’s oxygen concentrations.

That doesn’t sound like much, but keep in mind that oxygen was toxic to all organisms living on Earth at the time. In fact, the cyanobacteria that caused the rise in oxygen—what is known as the Great Oxidation—triggered the first global environmental catastrophe. Most life today, including humans and all higher life forms—is adapted to oxygen-rich conditions, but it took a long time for organisms to become accustomed to the destructive properties of oxygen and start taking advantage of its positive effects. The Great Oxygenation Event is believed to have caused a Snowball Earth event, because the increased oxygen removed warming greenhouse gases from the atmosphere, resulting in the glaciation of all or most of the planet.

Surprisingly, the study by Luo’s group indicates that the rise in oxygen concentrations was limited to a sediment layer only five meters thick, which represents a geological time span of one million to ten million years. That’s a rather short time, considering that all of Earth’s atmosphere became irreversibly aerobic (oxygen-rich) during the Great Oxygenation Event.

In another study published in Nature Geosciences, Sanjoy Som from NASA’s Ames Research Center and colleagues analyzed lava bubbles that released gas from a lava flow 2.7 billion years ago. Their surprising finding was that Earth’s atmosphere at the time was at most half as dense as it is today. They base their estimate on the size of gas bubbles that formed when the lava cooled, while constrained by air pressure. A thinner atmosphere might have helped oxygen levels in the atmosphere rise faster than expected, since less of it would be needed for the atmosphere to become aerobic.

However, this makes another early Earth puzzle, called the Faint Sun Paradox, more difficult to solve. About 2.7 billion years ago the Sun was about 20 percent less luminous than it is today, yet all indications point to warm conditions during that time period. If Som et al.’s findings are correct, it will be even harder to explain the Faint Sun Paradox, because a thinner atmosphere is generally associated with less greenhouse gas and colder conditions. In fact, to compensate for the fainter Sun and thinner atmosphere, greenhouse gas concentrations would have to be incredibly, almost impossibly high. The solution to the Faint Sun paradox is now more challenging than ever.

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