The transition of the Earth to the permanent hosting of an oxygenated atmosphere was a stopping process that lasted 100 million years longer than previously thought, 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, accompanied by massive climate change, including several glaciations that could have covered the entire globe in ice.
The chemical signatures trapped in the rocks that formed in this era had suggested that, 2.32 billion years ago, oxygen was a permanent feature of the planet’s atmosphere.
But a new in-depth study after 2.32 billion years ago finds that oxygen levels continued to go back and forth until 2.22 billion years ago, when the planet finally reached a permanent tipping point.
This new research, published in the journal The nature on March 29, it 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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“It’s only now that we’re beginning to see the complexity of this event,” said study co-author Andrey Bekker, a geologist at the University of California, Riverside.
Determining oxygen
The oxygen created in 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 early cyanobacteria eventually produced enough oxygen to restore 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 a variable number of neutrons in their nuclei.) When oxygen peaks, these sulfur isotopes disappear because the chemical reactions that create it do not take place in the presence of oxygen.
Bekker and colleagues have long studied the appearance and disappearance of these sulfur isotope signals. They and other researchers observed that the rise and fall of oxygen in the atmosphere appeared to follow with three global glaciations that occurred between 2.5 billion and 2.2 billion years ago. But, strangely, the fourth and last glaciation of that period had not been related to the oscillations of oxygen levels in the atmosphere.
The researchers were puzzled, Bekker told Live Science. “Why do we have four glacial events and three of them can be linked and explained by variations in atmospheric oxygen, but the fourth is independent?”
To find out, the researchers studied younger rocks in South Africa. These sea rocks cover the later part of the Great Oxidation Event, from the aftermath of the third glaciation to about 2.2 billion years ago.
They found that after the third glacial event the atmosphere was deprived of oxygen at first, then the oxygen increased and decreased again. Oxygen rose again by 2.32 billion years ago – the point where scientists previously believed the growth was permanent. But in the younger rocks, Bekker and his colleagues again detected a drop in oxygen levels. This decrease coincided with the final glaciation, which had not been previously related to atmospheric changes.
“Atmospheric oxygen in this early period was very unstable and rose to relatively high levels and dropped to very low levels,” Bekker said. “This is something we only expected in the last 4 or 5 years [of research]. “
Cyanobacteria vs. volcano
Researchers still find out what caused all these fluctuations, but they have some ideas. A key factor 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, as methane reacts with oxygen and disappears from the atmosphere within about a decade, while carbon dioxide remains for about 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.
So the sequence of oxygenation and climate change happened like this: Cyanobacteria began to produce oxygen, which reacted with the methane in the atmosphere at that time, leaving 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 have expanded, and the planet’s surface has become frozen and cold.
Saving the planet from permanent freezing, however, was the subglacial volcanoes. Volcanic activity eventually raised carbon dioxide levels high enough to warm the planet again. And while oxygen production remained 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 volcanic carbon dioxide levels had another major effect. When carbon dioxide reacts with rainwater, it forms carbonic acid, which dissolves rocks faster than pH-neutral rainwater. This faster degradation of rocks brings more nutrients, such as phosphorus into the oceans.
More than 2 billion years ago, such an influx of nutrients would have led to oxygen-producing marine cyanobacteria in a productive frenzy, again raising atmospheric oxygen levels, reducing methane and starting the whole cycle again.
Finally, another geological change broke this oxygenation-glaciation cycle. The model appears to have ended about 2.2 billion years ago, when rock recordings indicate an increase in buried organic carbon, suggesting that photosynthetic organisms had a period of glory.
No one knows exactly what triggered this tipping point, although Bekker and colleagues hypothesize that volcanic activity during this period provided a new influx of nutrients into the oceans, eventually giving cyanobacteria everything they need to thrive.
At this 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 dominant greenhouse gas to keep the planet warm.
There are many other rock sequences from this era around the world, Bekker said, including in West Africa, North America, Brazil, Russia and Ukraine. These ancient rocks need more study to reveal how early oxygenation cycles worked, he said, especially to understand how ups and downs have affected the life of the planet.
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This article was originally published by Live Science. Read the original article here.