Earth’s transition to permanent host to an oxygenated atmosphere was a stopping process that took 100 million years longer than previously thought, according to a new study.
When the Earth was first formed 4.5 billion years ago, the atmosphere contained almost none oxygen. But 2.43 billion years ago, something happened: Oxygen levels began to rise and then fall, accompanied by massive climate change, including several ice sheets that may have covered the entire globe in ice.
Chemical signatures locked in rocks formed in this era had suggested that oxygen 2.32 billion years ago was a permanent feature of the planet̵
But a new study that goes back to the period after 2.32 billion years ago finds that oxygen levels are still yo-yoing back and forth until 2.22 trillion 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 fluctuations.
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“We are only now beginning to see the complexity of this event,” said study author Andrey Bekker, a geologist at the University of California, Riverside.
Establishment of oxygen
The oxygen created during the great oxidation event was made by marine cyanobacteria, a type of bacteria that produce energy via photosynthesis. The most important byproduct of photosynthesis is oxygen, and early cyanobacteria eventually knock out enough oxygen to recreate the planet’s surface 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 varying numbers of neutrons in their nuclei.) When oxygen peaks, these sulfur isotopes disappear because the chemical reactions that create them do not occur in the presence of oxygen.
Streams and his colleagues have long studied the appearance and disappearance of these sulfur isotope signals. They and other scientists had noticed that the rise and fall of oxygen in the atmosphere seemed to be tracing with three global glaciers that occurred between 2.5 and 2.2 billion years ago. Strangely, the fourth and final ice age of this period had not been linked to fluctuations in atmospheric oxygen levels.
The researchers were confused, Bekker told WordsSideKick.com. “Why do we have four glacial events, and three of them can be connected and explained through variations of atmospheric oxygen, but the fourth of them stands independently?”
To find out, the researchers studied younger rocks from South Africa. These marine rocks cover the later part of the great oxidation event, from the wake of the third icing up to about 2.2 billion years ago.
They found that the atmosphere after the third ice age event was first oxygen-free, after which oxygen rose and fell again. Oxygen rose again 2.32 billion years ago – the time when scientists previously thought the rise was permanent. But in the younger cliffs, Bekker and his colleagues again discovered a drop in oxygen levels. This decline coincided with the final ice age, the one that had not previously been linked to atmospheric changes.
“Atmospheric oxygen during this early period was very unstable and it went up to relatively high levels and it dropped down to very low levels,” Bekker said. “It’s something we did not expect until maybe the last 4 or 5 years [of research]. “
Cyanobacteria vs. volcanoes
Researchers are still working on 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 because methane reacts with oxygen and disappears from the atmosphere within about a decade, while carbon dioxide is stuck 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.
So the oxygenation sequence and climate change possibly went something like this: Cyanobacteria began to produce oxygen that reacted with the methane in the atmosphere at that time, leaving only carbon dioxide behind. This carbon dioxide was not abundant enough to compensate for the heated effect of the lost methane, so the planet began to cool down. The glaciers expanded, and the planet’s surface became icy and cold.
However, rescuing the planet from permanent freezing were volcanoes underground. Volcanic activity eventually increased carbon dioxide levels high enough to warm the planet again. And while oxygen production was delayed in the ice-covered oceans due to the cyanobacteria, which received less sunlight, methane from volcanoes and microorganisms began to build up again in the atmosphere and further heat things up.
But volcanic carbon dioxide levels had another major effect. When carbon dioxide reacts with rainwater, it forms carbon dioxide, which dissolves rocks faster than pH-neutral rainwater. This faster weathering 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 the oxygen-producing marine cyanobacteria to a productive madness, which in turn would have increased atmospheric oxygen levels, run down methane and started the whole cycle again.
Eventually, another geological change broke this oxidation-glaciation cycle. The pattern appears to have been completed for approx. 2.2 billion years ago, when the mountain record indicates an increase in organic carbon being buried, suggesting that photosynthetic organisms had a heyday. No one knows exactly what triggered this turning point Streams and his colleagues assume that volcanic activity during this period provided a new influx of nutrients to the oceans and finally gave cyanobacteria everything they needed to thrive. At this point, Bekker said oxygen levels were high enough to permanently suppress methane’s oversized impact 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 the early oxidation cycles worked, he said, especially to understand how the ups and downs affected the planet’s life.
Originally published on WordsSideKick.com.