Widespread volcanic activity, cyanobacteria and global glaciation may sound like the plot of a new, blockbuster disaster movie, but in reality, they are all events in the mystery surrounding the development of our oxygen-rich atmosphere, according to a Penn State geoscientist.
The most extreme fluctuation in the Earth's carbon cycle occurred about 2.2 billion years ago, according to Dr. Lee R. Kump, professor of geosciences and member of the Penn State Astrobiology Research Center, and the conventional explanation is that it marks the debut of our oxygen atmosphere. Recently, however, better geological dating and a better proxy measure of when oxygen occurred in the atmosphere suggest that the oxygen atmosphere appeared long before this supposedly seminal event.
"The new dating and proxy clearly show that the rise of oxygen preceded its apparent cause by at least 100 million years," Kump told attendees at the Geological Society of America conference Nov. 8 in Denver.
The proxy measure of when significant oxygen appeared in the atmosphere is sulfur. In an oxygen atmosphere, which is very oxidizing, all sulfur eventually becomes sulfate, but in a reducing atmosphere – one without significant oxygen – sulfur deposits as sulfate, sulfite or even pure sulfur and retains an unusual isotopic signature of upper atmospheric processes. Better dating of these strange isotopes in rocks found them to be 2.3 billion years old or older and does suggest that oxygen appeared earlier than the carbon cycle perturbation.
If the large carbon marine sequestration episode does not coincide with the increase of oxygen, what does? Cyanobacteria -- marine organisms that are photosynthetic – produced oxygen during these early pre-oxygen atmosphere years. However, because the atmosphere was heavily reducing, the oxygen was quickly removed from the atmosphere.
The shift from a reducing atmosphere to an oxidizing one occurred when volcanic activity gradually switched from volcanoes that outgas hydrogen and carbon monoxide to those that produce water vapor and carbon dioxide, according to Kump. These mantle plume volcanoes bring molten rock up from deep within the earth.
With larger amounts of water and carbon dioxide in the atmosphere, hydrogen and carbon monoxide were no longer using up all the oxygen produced by the Cyanobacteria and the amounts of oxygen increased. In a few million years, the oxygen levels reach those of Earth's atmosphere today.
"The increases and mantle plume changes came in pulses, not all at once," says Kump. "There was a step wise increase first at 2.7 billion years ago and then at 2.4 billion years ago."
As the oxygen built up, iron rich layers called red beds, because the iron is oxidized to rust, were deposited around the world. Redbeds are considered a sign of atmospheric oxygen.
But also, as the oxygen increased, the levels of methane decreased.
"You can have a methane-rich atmosphere with a little oxygen or an oxygen-rich atmosphere with a little methane, but both cannot be high," says Kump. "Methane was the most important greenhouse gas left in the atmosphere."
Without greenhouse gases and in the presence of a faint young Sun that produced less heat than the Sun does today, the Earth cooled, according to a theory published earlier by Penn State colleagues James Kasting and Alexander Pavlov. Glaciation on a global scale followed. However, the volcanoes continued to produce carbon dioxide – a greenhouse gas – until the atmosphere warmed enough to melt the glaciers.
"We have found really oxidized basalt, which is anomalous, in Northern Russia near Scandinavia. This basalt would be a very poor sponge for soaking oxygen out of the atmosphere," says Kump. "We need to look for more evidence of these oxygen producing mantle rocks."
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