Twenty-one percent of the air we breathe is made up of molecular oxygen, but it was not always in such ample, life-sustaining supply. A new study by EAPS scientists pinpoints the timing of oxygen’s emergence in Earth’s atmosphere.
Exactly when the Great Oxygenation Event (GOE) began, and how long it lasted, has long tantalized scientists hungry to understand the early history of life on Earth. Now Professors Roger Summons and Shuhei Ono, Postdoctoral Associate Genming Luo, and Graduate Student David Wang have been able to refine the date of this critical transition to an oxygen rich atmosphere to 2.33 billion years ago, while also demonstrating that atmospheric oxygen built up to near modern levels in a trifling (in geologic terms) 10 million years.
Whiffs in the Air
For the most part, scientists agree that oxygen, though lacking in the atmosphere, was likely brewing in the oceans as a byproduct of cyanobacterial photosynthesis as early as 3 billion years ago. However, as Schlumberger Professor of Geobiology Roger Summons puts it, “oxygen in the ancient ocean would have instantly been sucked up by hungry microbes, ferrous iron, and other sinks, keeping it from escaping into the atmosphere.”
“There may have been earlier and temporary ‘whiffs’ of oxygen in the atmosphere, but their abundances and durations are not currently measurable,” Summons says.
That changed with the GOE, a period which scientists believe marked the beginning of oxygen’s permanent presence in the atmosphere. Previous estimates have placed the start of the GOE at around 2.3 billion years ago, though with uncertainties of tens to hundreds of millions of years. ”The dating of this event has been rather imprecise until now,” says Summons.
A Transition, Pinned
To get a more precise timing for the GOE, Luo first analyzed rocks from around this period, looking for a particular sulfur isotope pattern. When volcanoes erupt, they emit sulfur gases, which, when exposed to the sun’s ultraviolet radiation, can fractionate chemically and isotopically. The pattern of isotopes generated in this process depends on whether or not oxygen was present above a certain threshold.
Luo looked to pinpoint a major transition in a particular sulfur isotope pattern called mass-independent fraction of sulfur isotopes (S-MIF), in order to determine when oxygen first appeared in the Earth’s atmosphere. To do this, he first looked through sediment cores collected by Ono on a previous expedition to South Africa.
“Genming is a very tenacious and thorough guy,” Summons says. ”He found that rocks from deep in the core had S-MIF, and rocks shallow in the core had no S-MIF, but he didn’t have anything in between. So he went back to South Africa.”
There, he was able to sample from the rest of the sediment core and two others nearby, and determined that the S-MIF transition—marking the permanent passing of the oxygen threshold—occurred 2.33 billion years ago, plus or minus 7 million years, a much smaller uncertainty compared with previous estimates.
Getting a “Decent Hold”
The team also discovered a large fractionation of the isotope sulfur-34, indicating a spike in marine sulfate levels around this same time. Such sulfate would have been produced by the reaction between atmospheric oxygen with sulfide minerals in rocks on land, and sulfur dioxide from volcanoes. This sulfate was then used by ocean-dwelling, sulfate-respiring bacteria to generate a pattern of sulfur-34 in subsequent sediment layers that were dated between one and 10 million years after the S-MIF transition.
The results suggest that the initial buildup of oxygen in the atmosphere was relatively rapid. Since its first appearance 2.33 billion years ago, oxygen accumulated in high enough concentrations to have a weathering effect on rocks just 10 million years later. This weathering process, however, would have leached more sulfate and certain metals into waterways and ultimately, the oceans. Summons points out that it would be quite some time before the Earth system would reach another stable state, by the burial of organic carbon, and exceed the higher oxygen thresholds needed to encourage further biological evolution.
“Complex life couldn’ t really get a decent hold on the planet until oxygen was prevalent in the deep ocean,” Summons says, “and that took a long, long time. But this is the first step in a cascade of processes.”
Now that the team has constrained the timing of the GOE, Summons hopes others will apply the new dates to determine a cause, or mechanism, for the event. One hypothesis the team hopes to explore is the connection between oxygen’s sudden and rapid appearance, and Snowball Earth, the period in which Earth’s continents and oceans were largely ice-covered. Now, thanks to improved precision in geochronology which Summons largely credits to EAPS Professor Samuel Bowring, scientists have the tools to start to really nail down the mechanisms behind major events in Earth’s history, with more precise dates.
Adapted from the full article in MIT News: http://bit.ly/timing-the-goe
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