An ancient sea salt estimated to be about 2 billion years old sheds more light on how Earth's atmosphere transformed into an oxygenated environment that can support life.
Great Oxidation Event
Researchers believe that the emergence of oxygen in the Earth's atmosphere around 2.4 to 2.3 billion years ago was caused by cyanobacteria capable of photosynthesis, so it could take in carbon dioxide and produce and emit oxygen.
Scientists, however, used to be uncertain if the escalation of oxygen levels was a gradual process that took millions of years, or if it was a rapid event. The findings of the new research provided evidence that the oxygenation of the Earth involved a lot of oxygen production.
In a study published in the journal Science on March 22, Princeton University geochemist Clara Blättler and colleagues found that the Great Oxidation Event, which occurred about 2.3 billion years ago when free oxygen appeared in the Earth's atmosphere, was rapid. The crystallized salt rocks suggest that oxygen did not appear in a slow trickle but rather burst like a firehose.
Salt Crystals
The crystallized salt rocks, which were extracted from a 1.2 million deep hole in Russia, were left behind when primeval seawater evaporated, providing scientists hints to the composition of the Earth's atmosphere and oceans more than 2 billion years ago.
The large amounts of sulfate present in the mineral deposits provided clues of the increase in oxygen production. Sulfate is produced when sulfur reacts with oxygen.
The primeval seawater from which the minerals precipitated contained high concentrations of sulfate that were at least 30 percent of oceanic sulfate today. Researchers said that this is far higher than previously thought.
"The evaporite minerals provide a robust constraint that marine sulfate concentrations were at least 10 mmol/kg, representing an oxidant reservoir equivalent to over 20% of the modern ocean-atmosphere oxidizing capacity," the researchers wrote in their study.
"These results show that substantial amounts of surface oxidant accumulated during this critical transition in Earth's oxygenation."
What Caused Earth's Rapid Oxygenation?
It isn't yet clear what is behind the rapid oxygenation of the planet. Blättler said that rapid influx of oxygen could have been due to important changes in the feedback cycles in the oceans or in the land. It is also possible that this was due to the large increase in the amount of oxygen that microbes produced.
"Either way it was much more dramatic than we had an understanding of before," Blättler said.
