June 26, 2006 — Smidges of primeval oil found inside grains of Canadian rocks are providing new evidence of an oxygen-rich Earth almost 2.5 billion years ago — suggesting oxygen infused Earth's lower atmosphere 500 million years earlier than previously thought.
The clue comes from hydrocarbons, known as sterols, discovered in tiny amounts of oily water hermetically trapped inside the mineral grains. The sterol could only have gotten there in one way: from the residue of ancient algae that required oxygen to make the compound, say researchers who published their discovery in the June issue of the journal Geology.
“The (rock-forming) environment had to have O2,” said geologist Jay Kaufman of the University of Maryland.
Kaufman is a specialist in what’s called the Great Oxidation Event, in which Earth’s oceans and atmosphere went from being oxygen-poor to oxygen rich. The shift probably had effects on the climate — possibly triggering frigid “snowball Earth” episodes — as well as making possible the evolution of animal life, Kaufman explains.
Earlier research had put the Great Oxidation Event at 2.1 billion to 2.2 billion years ago, said Kaufman. But sterol has also been found in rocks called shales that are up to 2.7 billion years old. That suggests that at least some places on Earth had high oxygen levels very early, he said.
But the reliability of those shales has been questioned because they are vulnerable to contamination, said Adriana Dutkiewicz of the University of Sydney, and the lead author on the Geology paper.
That’s why she and her colleagues have been looking inside the grains of rocks for old oils that have been safely locked away.
“It’s a very new area of research and it’s really taking off,” said Dutkiewicz. The latest chemical detection instruments make it possible to analyze incredibly small amounts of oil trapped inside mineral grains that are narrower than a human hair, she said.
To hunt down and study ancient oils, Dutkiewicz and her team first located rocks of the right age, in this case at Lake Elliot, Canada. They sliced the rocks very thin and looked at them through microscopes under ultraviolet light, which causes any trapped oils to glow.
The researchers carefully crushed the rocks found to contain oils, to free the individual mineral grains. The next step was to meticulously wash the grains to be sure no other oils were present when the researchers finally opened the grains to analyze the pristine ancient oil.
Dutkiewicz’s success at finding sterol in the 2.45 billion-year-old rocks is strong confirmation of other work that pushes the Great Oxidation Event back to 2.4-2.5 billion years ago, said Kaufman. It could also be telling another story: that there wasn’t a single “great event” at all, he said.
“It could have been an oscillation of oxygen over a long period of time,” said Kaufman. That might even explain why there were also a few severe global ice ages between 2.2 and 2.4 billion years ago.
Oxygen, it turns out, breaks down methane in the atmosphere, and methane is a potent greenhouse gas. So spikes in oxygen may have triggered the “snowball Earth” ice ages, he said.
