Earth’s ancient atmosphere was half as thick as it is today
Air pressure is crucial for life. Not only does it help the atmosphere retain water vapor and trap heat from the sun, but it also affects the very chemical reactions on which life depends. Yet very little is known about how thick Earth’s ancient atmosphere once was. Now, a new study suggests that Earth’s atmosphere 2.7 billion years ago was between a quarter to half as thick as it is today. The finding could force scientists to re-examine nearly everything they know about Earth’s early atmosphere, from nitrogen-fixing cycles to how the young planet trapped enough heat to give rise life as we know it.
Scientists have long assumed that Earth’s ancient atmosphere had a stronger greenhouse than today’s. That’s because the sun put out 20% less heat than it does today, and even with elevated levels of greenhouse gases, Earth would have struggled to keep global temperatures above freezing. A thicker atmosphere would have helped to compensate. But in recent research, this expected thickness hasn’t been found: A 2012 study of fossilized raindrops, for example, found that Earth’s early atmosphere was as little as half as thick as it is today.
Now, the same team of scientists has come up with a more precise way of measuring early pressure: gas bubbles trapped in ancient lava. In what is now the Australian outback, basalt lavas poured out over thousands of square kilometers billions of years ago, hardening when they hit seawater. The lavas contained dissolved gases that fizzed as they emerged onto Earth’s surface, “like when you take the top off a cola bottle,” says Roger Buick, a geologist from the University of Washington (UW), Seattle, and an author on the new study, which is published today in Nature Geoscience. Unlike the drink though, the hot lava quickly cools and freezes, trapping the bubbles. By measuring the size of the bubbles at the top—which were pushing against the weight of the atmosphere—and comparing them with the smaller bubbles at the bottom—which were pushing against both the atmosphere and the weight of the rock itself—the team came up with a proxy for ancient air pressure.
“It’s really bold, really ambitious, and fraught with difficulties,” says Dork Sahagian, a geologist at Lehigh University in Bethlehem, Pennsylvania, who invented the lava bubbling technique in the 1980s. “But you’ve got to try it. It’s as good a proxy of the pressure as you can hope to find.”
The difficulty was finding lava that was both old enough and pristine enough to have retained a true record of the deep past. The team needed to find samples that hadn’t been eroded away at the top, for example, or that hadn’t had fresh lava injected into their middles.
Buick and colleagues started their scouting 10 years ago, on 2.7-billion-year-old flood basalts from the Pilbara region of Western Australia. Importantly, some of the lavas there had clearly reacted with ancient seawater, proving they had erupted at sea level. After years of hunting, only three slabs from all this extent were adequate for the task. When they first eyeballed the bubbles, sitting around a campfire, the implications were immediately obvious, and disturbing.
They quickly saw from the size of bubbles exposed at the surface that the ancient atmosphere must have been much lower than previously assumed. “[At first] we thought the results would be really boring, that atmospheric pressure has always been much the same, just as everyone expects,” says David Catling, an atmospheric scientist at UW. “So the question was, why was the pressure so low?”
The key was nitrogen. Today nitrogen dominates the atmosphere, making up four-fifths of our atmosphere; oxygen makes up the other fifth. At the time of the basalt eruptions, there was no free oxygen in the atmosphere—that didn’t happen until cyanobacteria flooded the atmosphere with the gas hundreds of millions of years later. So that absence would explain some of the difference between now and then, but not the 50% or more the bubbles reveal. The explanation for that has to involve a lack of nitrogen as well.
Although nitrogen appears to be extraordinarily inert, life today works hard to catch this biologically essential element using fixation. Bacteria and phytoplankton draw tens of millions of tons from the atmosphere every year. And each year, an equal amount is returned to the skies by a mixture of other biological and geological processes. Because the cycle is balanced, the net nitrogen in the atmosphere remains unchanged. Catling argues that the balance of these cycles must have been quite different in the past.
A high rate of removal, implying substantial biological fixation, without the balancing return would do. Last year, Buick published evidence that biological nitrogen fixation was already well established half a billion years before these Pilbara basalts erupted. He and Catling say that in the absence of oxygen, the return mechanism was weakened, so that for hundreds of millions of years, life steadily transformed the atmosphere by removing nitrogen. That impact was reversed only when life’s better known transformation, the Great Oxygenation Event, arrived about 2.4 billion years ago.
The low nitrogen levels, however, close off a nitrogen-boosted greenhouse effect as one favored solution to the problem of keeping Earth warm when the young sun was faint. Evidence has accumulated that levels of the known greenhouse gases, like carbon dioxide and methane, were inadequate to compensate for the low warmth of the early sun. Yet evidence of ancient liquid water makes it clear the planet had not chilled below freezing. In 2009, Colin Goldblatt, an atmospheric chemist at the University of Victoria in Canada, proposed a way out. His calculations showed that in an atmosphere with double the nitrogen pressure, the effect of greenhouse gases could be amplified, producing enough warming to stop the planet freezing over. Goldblatt concedes that the new findings, if confirmed, kill that solution. “With such little pressure, it’s going to be very hard to account for the warm temperatures,” he says. But he still wants more evidence: A series of measures from other basalts of different ages would be ideal.
Buick and his team are already getting started. Right now, they are eyeing lavas even older than the Pilbara samples, in South Africa. But for now, they are keeping quiet about their exact location.