Researchers have retrieved samples of 1.4 billion-year-old air from ancient crystals and found something surprising about a supposedly “boring” time period.
The team studied the gases and fluids locked in halite crystals (rock salt) from Canada, shedding light on the composition of the atmosphere hundreds of millions of years before dinosaurs walked the Earth. It turns out the planet was sporting more oxygen than expected—at least, in that exact moment in time—as they explained in a study published last month in PNAS.
Direct samples of air
“The carbon dioxide measurements Justin obtained have never been done before,” Rensselaer Polytechnic Institute’s (RPI) Morgan Schaller, co-author of the study, said of the lead author, RPI graduate student Justin Park, in a university statement. “We’ve never been able to peer back into this era of the Earth’s history with this degree of accuracy. These are actual samples of ancient air!”
Over a billion years ago, a lake in modern-day Ontario evaporated. During the evaporation, some of the resulting brine was captured within halite crystals along with air bubbles—precise records of our early atmosphere’s makeup. Extracting accurate readings from these fluid inclusions, which have both air and brine (salt water), is difficult, however—some gases, including oxygen and carbon dioxide, act differently based on whether they exist in water or air. As such, it’s been challenging to precisely measure the gases in their original state.
The team solved the problem using a method previously developed by Park, uncovering that around 1.4 billion years ago, during the Mesoproterozoic era (1.6 to 1.0 billion years ago), Earth’s atmosphere contained ten times more carbon dioxide than today. This high CO2 level helped maintain a climate similar to the present, even though the Sun was significantly weaker at the time. These high levels, along with temperature estimates based on the salt, indicate that the Mesoproterozoic climate was more mild than researchers theorized.
The atmosphere also had 3.7% of today’s oxygen levels. While this might not seem like a lot, it’s still an unexpectedly high quantity. At the time, life was ruled by bacteria, and red algae was a newcomer, but these oxygen levels could have sustained complex animal life, such as animals and plants, that would not emerge until 800 million years later.
Why so slow, animals?
So why was animal life so slow to appear? “It may reflect a brief, transient oxygenation event in this long era that geologists jokingly call the ‘boring billion,’” Park explained. In other words, the readings reflect a brief moment in time. The “boring billion” is an epoch during which there were low oxygen levels, stable atmospheric and geologic conditions, and little evolutionary shifts.
Today, red algae produce a major amount of the planet’s oxygen. Since this life form emerged around this time period, the surprisingly large quantities of oxygen may have resulted from algae’s growing numbers and complexity. “It’s possible that what we captured is actually a very exciting moment smack in the middle of the boring billion,” Schaller said.
As for carbon dioxide, earlier approximations based on indirect information suggested lower CO2 levels at the time. This was incompatible with other data demonstrating that the Mesoproterozoic era did not have major glaciers.
“Despite its name, having direct observational data from this period is incredibly important because it helps us better understand how complex life arose on the planet, and how our atmosphere came to be what it is today,” Park said.
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