According to a new study, the transition to the permanent host of an oxygen-like atmosphere was a halting process that took 100 million years longer than previously believed.
When the earth first formed 4.5 billion years ago, the atmosphere contains almost no oxygen. But 2.43 billion years ago, something happened: oxygen levels began to rise and then dropped, coupled with massive climate change, including several glaciers that covered the entire globe in ice.
Chemical signatures trapped in rocks formed during this era suggested that oxygen was a permanent feature of the planet’s atmosphere by 2.32 billion years ago.
But a new study that went into the period after 2.32 billion years ago found that oxygen levels still yo-yo back and forth until 2.22 billion years ago, when the planet finally reached a permanent tilt point. This new research, published in the journal Earth on March 29, the duration of what scientists call the Great Oxidation Event extends by 100 million years. It could also confirm the link between oxygen and massive climate change.
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“We are only now beginning to see the complexity of this event,” said Andrey Bekker, co-author of the study, a geologist at the University of California, Riverside.
Establish oxygen
The oxygen created during the Great Oxidation event was made by marine cyanobacteria, a type of bacteria that produces energy via photosynthesis. The most important byproduct of photosynthesis is oxygen, and early cyanobacteria eventually eliminated enough oxygen to recreate 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 varying numbers of neutrons in their nuclei.) When oxygen rises, these sulfur isotopes disappear because the chemical reactions they cause do not occur in the presence of oxygen.
Bekker and his colleagues have long studied the occurrence and disappearance of these sulfur isotope signals. They and other researchers noted that the rise and fall of oxygen in the atmosphere appears to be with three global glaciers that occurred between 2.5 billion and 2.2 billion years ago. But oddly enough, the fourth and final icing in that period was not linked to fluctuations in atmospheric oxygen levels.
The researchers were surprised, Bekker told WordsSideKick. “Why do we have four glacier events, and three of them can be linked and explained by variations of atmospheric oxygen, but the fourth of which is independent?”
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 aftermath of the third icing to about 2.2 billion years ago.
They found that the atmosphere was oxygen-free only after the third icing event, when oxygen rose and fell again. Oxygen rose again 2.32 billion years ago – the point at which scientists previously thought the rise was permanent. But in the younger rocks, Bekker and his colleagues again observed a drop in oxygen levels. This decline coincided with the final icing, the one that had not previously been linked to atmospheric changes.
“Atmospheric oxygen during this early period was very unstable and rose to relatively high levels and fell to very low levels,” Bekker said. ‘This is something we did not expect until maybe 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 important 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 within a decade, while carbon dioxide is trapped for hundreds of years. But when there was little or no oxygen in the atmosphere, methane lasted much longer and acted as a major greenhouse gas.
The sequence of oxygenation and climate change may have gone something like this: Cyanobacteria began to produce oxygen, which at that time reacted with the methane in the atmosphere, leaving only carbon dioxide. This carbon dioxide was not abundant enough to replenish the heating effect of the lost methane, and so the planet began to cool down. The glaciers expanded and the surface of the planet became icy and cold.
However, the planet has escaped from a permanent freezing point, but below glacier volcanoes. Volcanic activity eventually raised the carbon dioxide levels high enough to warm the planet again. And while oxygen production in the ice-covered oceans lagged behind due to the cyanobacteria that received less sunlight, methane from volcanoes and microorganisms began to build up in the atmosphere again, heating things up further.
But volcanic carbon dioxide levels had another major effect. When carbon dioxide reacts with rainwater, it forms carbon dioxide that dissolves rocks faster than pH-neutral rainwater. This faster weathering of rocks brings more nutrients like phosphorus into the oceans. More than 2 billion years ago, such a supply of nutrients would have driven the oxygen-producing marine cyanobacteria into a productive frenzy, which in turn increased the atmosphere of oxygen in the air, expelled methane, and resumed the entire cycle.
Finally, another geological change breaks this cycle for oxygenation and icing. The pattern seems to have ended about 2.2 billion years ago when the rock record indicates an increase in buried organic carbon, indicating that photosynthetic organisms had a heyday. However, no one knows exactly what caused this tipping point Bekker and his colleagues suppose that volcanic activity during this period provided a new influx of nutrients to the oceans, which eventually gave cyanobacteria everything to thrive. At this point, Bekker said, oxygen levels were high enough to permanently suppress the major impact of methane on the climate, and carbon dioxide through volcanic activity and other sources became the dominant greenhouse gas to keep the planet warm.
There are many other rock series from this era around the world, Bekker said, including in West Africa, North America, Brazil, Russia and Ukraine. These ancient rocks need more research to explain how the early cycles of oxygenation worked, especially to understand how the ups and downs affected the planet’s life.
Originally published on Live Science.