The transition of the Earth to a permanent oxygen host in the atmosphere was a stopping process that lasted 100 million years longer than previously believed, according to a new study.
When the Earth first formed 4.5 billion years ago, the atmosphere contained almost no oxygen. But 2.43 billion years ago, something happened: oxygen levels began to rise and then fall, followed by massive climate change, including a few icicles that may have covered the entire world with ice.
Chemical signatures locked in the rocks that formed during this era suggested that 2.32 billion years ago, oxygen was a permanent feature of the planet.
But a new study dealing with the period from 2.32 billion years ago reveals that oxygen levels were still in motion until 222 billion years ago, when the planet finally reached a permanent turning point.
This new research, published in the journal Nature March 29 extends the duration of what scientists call the Great Oxidation Event by 100 million years. It can also confirm the link between oxygenation and major climate change.
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“It’s only now that we’re starting to see the complexity of this event,” said study co-author Andrey Bekker, a geologist at the University of California, Riverside.
The oxygen created in the great oxidation event was created by marine cyanobacteria, a type of bacteria that produce energy by photosynthesis. The main byproduct of photosynthesis is oxygen, and the early cyanobacteria eventually expelled enough oxygen to transform the face of the planet forever.
The signature of this change is visible in marine sedimentary rocks. In an oxygen-free atmosphere, these rocks contain certain types of sulfur isotopes. (Isotopes are elements with different numbers of neutrons in their nuclei.) When oxygen jumps, these sulfur isotopes disappear, because the chemical reactions that create it do not occur in the presence of oxygen.
Becker and his colleagues have long studied the appearance and disappearance of these sulfur isotope signals. They and other researchers have noticed that the rise and fall of oxygen in the atmosphere seems to be accompanied by three global glaciations that occurred between 2.5 and 2.2 billion years ago. But surprisingly, the fourth and last icing in that period was not associated with changes in atmospheric oxygen levels.
The researchers were confused, Becker told Live Science. “Why do we have four glacial events, and three of them can be connected and explained by variations in atmospheric oxygen, but the fourth of them stands independently?”
To find out, the researchers studied younger stones from South Africa. These sea rocks cover the later part of the Great Oxidation Event, from the effects of the third glaciation to about 2.2 billion years ago.
They discovered that after the third event of glaciation, the atmosphere was initially oxygen-free, and then oxygen rose and fell again. Oxygen rose again 2.32 billion years ago – a point at which scientists previously thought the rise was permanent. But in the younger rocks, Becker and his colleagues rediscovered a drop in oxygen levels. This decline coincided with the final icing, one that was not previously associated with atmospheric changes.
“Atmospheric oxygen at this early age was very unstable and climbed to a relatively high level and fell to a very low level,” Becker said. “It’s something we haven’t expected maybe in the last 4 or 5 years [of research]. “
Cyanobacteria against volcanoes
Researchers are still working out what caused all these fluctuations, but they have some ideas. One of the key factors is methane, a greenhouse gas that is more efficient at capturing heat than carbon dioxide.
Today, methane plays a small role in global warming compared to carbon dioxide because methane reacts with oxygen and disappears from the atmosphere within about a decade, while carbon dioxide is retained for hundreds of years. But when there was little or no oxygen in the atmosphere, methane lasted much longer and acted as a more important greenhouse gas.
Thus, the sequence of oxygenation and climate change could have gone something like this: cyanobacteria began to produce oxygen, which at that time reacted with methane in the atmosphere, leaving behind only carbon dioxide.
This carbon dioxide was not abundant enough to compensate for the warming effect of the lost methane, so the planet began to cool. The glaciers expanded, and the surface of the planet became icy and cold.
They saved the planet from permanent deep freezing, subglacial volcanoes. Volcanic activity eventually increased the level of carbon dioxide high enough to reheat the planet. And while oxygen production was lagging behind in the ice-covered oceans due to cyanobacteria receiving less sunlight, methane from volcanoes and microorganisms began to accumulate in the atmosphere again, further heating things up.
But the level of volcanic carbon dioxide had another important effect. When carbon dioxide reacts with rainwater, it produces carbonic acid that dissolves stones faster than rainwater with a neutral pH. This faster weathering of the rocks brings more nutrients like phosphorus into the oceans.
More than 2 billion years ago, such an influx of oxygen-producing marine cyanobacteria would drive a productive frenzy, again increasing atmospheric oxygen levels, suppressing methane, and starting an entire cycle.
Eventually another geological change interrupted this oxygenation-glaciation cycle. The pattern appears to have been completed about 2.2 billion years ago when a stone record indicates an increase in organic carbon being buried, suggesting that photosynthetic organisms were flourishing.
No one knows exactly what triggered this turning point, although Becker and his colleagues speculate that volcanic activity during this period provided a new influx of nutrients into the oceans, finally giving cyanobacteria everything they needed to thrive.
At this point, Becker said, oxygen levels were high enough to permanently suppress the excessive impact of methane on the climate, and carbon dioxide from volcanic activity and other sources has become the dominant greenhouse gas to keep the planet warm.
There are many other stone sequences from this era around the world, Becker said, including West Africa, North America, Brazil, Russia and Ukraine. These ancient rocks need more study to discover how the early cycles of oxygenation worked, especially to understand how the ups and downs affected the life of the planet.
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This article was originally published by Live Science. Read the original article here.