Surprisingly Oxygen-Rich Layer Found In Earth's Ancient Atmosphere 2.7 Billion Years Ago

The upper atmosphere contained as much oxygen as it does today. Aphelleon/Shutterstock

The Great Oxygenation Event (GOE), thought to have occurred around 2.4 billion years ago, changed everything. Thanks to the persistence of photosynthesizing organisms, the planet was enriched with free oxygen, and life on Earth took a radically different evolutionary path.

However, it wasn’t just life that first put oxygen into Earth’s atmosphere. New research describes how extremely small meteorites, excavated from submarine sedimentary rocks in Australia’s Pilbara region, reveal that the upper atmosphere of Earth 2.7 billion years ago was surprisingly rich in oxygen.

Previously, it had been thought the whole ancient atmosphere around this time contained just 0.001 percent oxygen before the GOE. But the researchers think that an oxygen-rich upper layer, separated from the lower layer by a haze of methane, may have had as much oxygen as is present in our atmosphere today (where oxygen makes up about 21 percent of our entire atmosphere).

This new Nature study highlights the fact that this is the first time the chemistry of the upper atmosphere of the ancient Earth has been sampled. Its findings confirm that photochemical reactions between sunlight and atmospheric gases were successfully producing small quantities of oxygen long before the GOE was initiated. But the process through which this occurred is unknown.

“Our research has opened up new avenues to modelling the atmosphere,” lead author Dr. Andrew Tomkins, a geologist and meteorite hunter from Monash University, told IFLScience. “We’ve just sampled it at 2.7 billion years. What about 3.5 to 2 billion? There’s a huge period of Earth’s early history we could look into, including the Great Oxygenation Event itself.”

One of the excavated micrometeorites as viewed under a scanning electron microscope. Tomkins et al./Nature

Lifeforms able to produce oxygen have been around for about 3.5 billion years, but it took at least a billion more to reach a critical point wherein the Earth became an oxygen-rich environment. Throughout this time period, the energetic interaction between the Sun’s radiation and the early Earth atmosphere would have probably led to the production of oxygen, but there was no direct evidence of this until now.

In order to address this problem, Tomkins and his international team decided to look for “fossil micrometeorites” – incredibly tiny fragments left over from the formation of the Solar System that had been slowly buried by sedimentary rock offshore. By dissolving away the surrounding limestone, they removed a total of 60 of these little bits of space dust – all of which are about as small as the width of a human hair – and examined their chemical compositions.

Micrometeorites fall to Earth as much as 30 times as frequently as their larger meteorite companions. They impact the upper atmosphere at such speeds that the air in front of them becomes highly compressed. This causes them to rapidly heat up and react with the gases around them, before they fall to Earth and rapidly cool, leaving them inert.

Thanks to this mechanism, Tomkins realized that micrometeorites would provide a chemical record of the gases they reacted with in the upper atmosphere. After dating these micrometeorites as being 2.7 billion years old, chemical analysis revealed that their iron segments had turned into iron oxide through a powerful oxidation reaction.

 

Andrew Tomkins, meteorite hunter. Monash University Faculty of Science via YouTube

The degree of oxidation indicated that the upper atmosphere of Earth back then contained the same concentrations of oxygen as it does today. Not only that, but a methane haze layer, one that separated the upper atmosphere from the oxygen-starved lower atmosphere, was also recorded by these micrometeorites. It would be another 300 million years or so before the GOE converted much of the lower atmosphere into an oxygen-rich one.

Importantly, this study shows that micrometeorites can reveal a remarkable amount of information about ancient atmospheres, and the hunt is now on for those that can be dated even further back in time.

“There are also micrometeorites on the surface of Mars,” Tomkins added. “If the Curiosity rover found them and analyzed them, perhaps we could get a clue as to what Mars’ early atmosphere was like.”

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