The transition from Earth to permanently housing an oxygenated atmosphere was a shutdown process that took 100 million years longer than previously thought, according to a new study.
When the Earth formed 4.5 billion years ago, the atmosphere contained almost no oxygen. But something 2,440 million years ago something happened: oxygen levels began to rise and fall, accompanied by massive changes in the climate, including several glaciations that could have covered the entire globe with ice.
The chemical signatures trapped in the rocks that formed during this time had suggested that 2,322 million years ago oxygen was a permanent feature of the planet’s atmosphere.
But a new study that delves into the period after 2,322 million years ago finds that oxygen levels were still coming and going until 2222 million 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 massive climate change.
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“Only now are we beginning to see the complexity of this event,” said study co-author Andrey Bekker, a geologist at the University of California, Riverside.
Establish oxygen
The oxygen created during the great oxidation event was produced by marine cyanobacteria, a type of bacterium that produces energy through photosynthesis. The main byproduct of photosynthesis is oxygen, and the first cyanobacteria ended up producing enough oxygen to redo the face of the planet forever.
The signature of this change is visible on marine sedimentary rocks. In an oxygen-free atmosphere, these rocks contain certain types of sulfur isotopes. (Isotopes are elements with a variable number of neutrons in their nuclei.) As oxygen increases, these sulfur isotopes disappear because the chemical reactions that create it do not occur in the presence of oxygen.
Bekker and colleagues have long studied the appearance and disappearance of these sulfur isotope signals. They and other researchers had realized that the rise and fall of oxygen into the atmosphere appeared to be followed by three global glaciations that occurred between 2.5 and 2.2 billion years ago. But, curiously, the fourth and final glaciation of this period had not been related to oscillations in atmospheric oxygen levels.
The researchers were baffled, Bekker told Live Science. “Why do we have four glacial events and three of them can be related and explained by variations in atmospheric oxygen, but a quarter of them are independent?”
To find out, the researchers studied younger rocks in South Africa. These sea rocks cover the back of the great oxidation event, from the aftermath of the third glaciation until about 2.2 billion years ago.
They found that after the third glacial event the atmosphere had no oxygen at first, then the oxygen increased and decreased again. Oxygen increased again to 2.320 million years ago, when scientists thought the increase was permanent. But in the younger rocks, Bekker and his colleagues again detected a drop in oxygen levels. This fall coincided with the final glaciation, which had not previously been related to atmospheric changes.
“Atmospheric oxygen during this first time was very unstable and went up to relatively high levels and went down to very low levels,” Bekker said. “This is something we didn’t expect until maybe the last 4 or 5 years [of research]. “
Cyanobacteris Vs. volcanos
Researchers are still figuring out what caused all these fluctuations, but they have some ideas. A key factor is methane, a greenhouse gas that is more efficient at trapping 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 in about a decade, while carbon dioxide remains 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 oxygenation sequence and climate change were similar to this: cyanobacteria began to produce oxygen, which at that time reacted with the methane in the atmosphere and left behind only carbon dioxide.
This carbon dioxide was not abundant enough to compensate for the heating effect of the lost methane, so the planet began to cool. Glaciers expanded and the planet’s surface became icy and cold.
However, saving the planet from permanent freezing were subglacial volcanoes. Volcanic activity eventually increased carbon dioxide levels high enough to reheat the planet. And while oxygen production was delayed in ice-covered oceans because cyanobacteria received less sunlight, methane from volcanoes and microorganisms began to accumulate in the atmosphere again, heating things up even more.
But volcanic carbon dioxide levels had another major effect. When carbon dioxide reacts with rainwater, it forms carbonic acid, which dissolves rocks faster than pH-free rainwater. This faster weathering of rocks brings more nutrients like phosphorus to the oceans.
More than 2 billion years ago, this influx of nutrients would have led oxygen-producing marine cyanobacteria to a productive frenzy, again raising atmospheric oxygen levels, lowering methane and starting the whole cycle again.
Finally, another geological change broke this cycle of oxygenation-glaciation. The pattern appears to have ended about 2.2 billion years ago when the record of the rocks indicates an increase in buried organic carbon, suggesting that photosynthetic organisms had a high point.
No one knows exactly what triggered this turning point, although Bekker and colleagues hypothesize that volcanic activity in this period provided a new influx of nutrients into the oceans, eventually giving cyanobacteria all that they needed. they needed to thrive.
At that time, Bekker said, oxygen levels were high enough to permanently suppress the oversized influence of methane on the climate, and carbon dioxide from volcanic activity and other sources became the effect gas. dominant greenhouse to keep the planet warm.
There are many other rock sequences from that era around the world, Bekker said, including West Africa, North America, Brazil, Russia and Ukraine. These ancient rocks need further study to reveal how the first cycles of oxygenation worked, he said, especially to understand how the ups and downs affected life on the planet.
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This article was originally published by Live Science. Read the original article here.