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. Last Updated: 07/27/2016

Rocks Hold Promise of Sampling Bygone Air

Scientists have developed a new technique to analyze the chemical composition of ancient rocks for hints about what the air was like billions of years ago. The information could help them decipher the evolution of Earth's earliest life forms.

Researchers at the University of California at San Diego reported in Friday's issue of the journal Science that 2.1 billion to 2.5 billion years ago, the mix of gases in the atmosphere shifted radically, probably signaling the rise of oxygen.

Most scientists agree the Earth's atmosphere contained very little oxygen when the planet formed 4.5 billion years ago.

But iron deposits on the ocean floor, which can exist only when oxygen in the water causes the metal to precipitate out, indicate that the air was oxygen-rich 2.3 billion years later.

The question is, what happened to create the change? And did it occur suddenly or over hundreds of millions of years?

"What you didn't know was how much was there and how did it change over time,'' said Mark Thiemens, dean of physical sciences at the university and professor of chemistry and the paper's senior author. "You didn't really see the change with time.''

The new clue lies in the relative amounts of different types, or isotopes, of sulfur in the rocks.

The prevalence of the different isotopes varies according to whether sulfur molecules have been broken apart by ultraviolet radiation from the sun.

That, in turn, varies according to the amount of oxygen, in the form of ozone, that is in the atmosphere.

The ratio of sulfur isotopes "is an oxygen barometer, if you will,'' said Thiemens.

Rocks about 2.1 billion years old contained sulfur ratios similar to those of rocks today, indicating large amounts of oxygen in the air. Rocks about 2.5 billion years old indicate little oxygen.

The change may have occurred much more quickly and researchers plan to refine their oxygen timetable by analyzing more rock samples.

They also plan to measure levels of oxygen isotopes in the rocks, which could offer additional clues to the atmosphere's composition.

With a detailed history of oxygen, biologists could then see if the life on Earth during this period, all one-cell organisms, evolved in tandem with the changing atmosphere.

"Now you've got a record that is quantitative,'' said Thiemens. "One can compare that with the empirical observations and fossils.''

James Kasting, a professor of geosciences at Pennsylvania State University, said the findings by the San Diego researchers were "consistent with the story that many of us already believe," that oxygen levels in the atmosphere jumped about 2.2 billion years ago.

Even so, Kasting said, the paper is still "very exciting.''

"It's a very creative application of chemistry and physics.''

The San Diego researchers had earlier performed the same analysis on several Martian meteorites, reported in March in the journal Nature.