Although it’s hard to imagine, Earth didn’t always have the oxygen-rich atmosphere we breathe today. In fact, until about two and a half billion years ago, the air contained almost no oxygen gas at all. You can learn how geologists made this discovery in our module, the History of Earth’s Atmosphere II: The Rise of Atmospheric Oxygen.
But major questions remain, and researchers continue to investigate what the early atmosphere was made of, and when it changed. This has proved challenging because researchers can’t sample ancient air. Instead, they have to look at rocks, which can preserve indirect information about the nature of the early atmosphere. In the past week, three new studies on this subject have come out, and each one uses a different approach to fill in a missing piece of the puzzle.
Above the clouds
Most of the evidence for low oxygen concentrations in the early atmosphere comes from minerals found at the Earth’s surface that could only form in the absence of oxygen. But what was going on higher up in the atmosphere? A new study in the journal Nature suggests that above 50 or 75 kilometers, the atmosphere might have had about the same amount of oxygen gas as it holds today.
Scientists drew this conclusion after studying micrometeorites — tiny fragments of extraterrestrial material that constantly rain down on Earth — trapped in 2.7 billion-year old rocks from Australia. As they stream through the upper atmosphere, these particles melt and react with the air to form new minerals. In the ancient micrometeorites they studied, the researchers found iron oxides that most likely formed in the presence of oxygen. The results suggest that the gas was present in the upper atmosphere — potentially produced by the breakdown of carbon dioxide released by volcanoes — even if very little oxygen existed at the surface.
A thinner atmosphere
Scientists have also puzzled over how the early Earth managed to stay warm enough to maintain liquid water on the surface. The young sun burned cooler than it does today, and until about 2 billion years ago, the planet should have been frozen. Scientists have speculated that perhaps there were more greenhouse gases in the atmosphere at that time, or that the early atmosphere was thicker, which would have made greenhouse gases more efficient at trapping heat. However, a new study in the journal Nature Geoscience shows just the opposite: it suggests the early atmosphere was much thinner than it is today.
Researchers reconstructed the pressure of the ancient atmosphere by studying bubbles in lava rocks in Australia. In the 2.7 billion years since these rocks formed, the holes — known as vesicles — have filled in with other minerals, but the vesicles have retained their original shape and size. These depend in part on the weight of the atmosphere bearing down on them when the lava solidified. The results suggest atmospheric pressure was only half what it is today, and appear to rule out a thicker atmosphere as an explanation for the so-called Faint Young Sun Paradox.
A rapid rise
Although scientists think that photosynthesis — the primary source of modern oxygen gas — evolved very early in Earth’s history, oxygen didn’t build up in the atmosphere until much later. This jump is known at as the Great Oxidation Event (GOE), and is thought to have occurred sometimes in the 200 million-year-window between 2.4 and 2.2 billion years ago. Now, a new study in the journal Science Advances suggests that the atmosphere gained oxygen relatively quickly — within a period of 10 million years — starting 2.33 billion years ago.
The researchers determined this by studying the isotopes of sulfur preserved in South African rocks. Isotopes are atoms of the same element with different masses, and they give clues about the types chemical reactions that happened in the past, which tend to discriminate between isotopes in particular ways (learn more about this in our module on the Rise of Atmospheric Oxygen). In the new study, the researchers measured sulfur isotopes in sedimentary rocks that span the GOE and found evidence for an abrupt change in the types of chemical reactions that occurred — from those that indicate an absence of oxygen to those that indicate its presence — starting 2.33 billion years ago. Knowing the rate and timing of the GOE will help researchers understand the mechanisms that drove this dramatic change in Earth’s atmosphere.
Learn how photosynethesis produces free oxygen in our module on the process.
Learn more about the evolution of Earth’s atmosphere from the Smithsonian Institution.
Discover how the rise of atmospheric oxygen impacted Earth’s early inhabitants, from Slate.
Written by Julia Rosen
Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelors degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her Master's in backpacking around the world and skiing. Julia is a periodic contributor to Oregon States research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.