Oldest rocks contain oxygen clues

Free oxygen allowed life to expand

By BBC News Online science editor Dr David Whitehouse

Scientists looking at some of the oldest-known rocks have discovered a way to read details about the evolution of oxygen and ozone in the Earth's early atmosphere.

They have identified an increase in our planet's oxygen levels over two billion years ago.

Chemists from the University of California-San Diego say that their analysis of the rocks reveals a "profound change" in chemical reactions involving sulphur and oxygen between 2.1 and 2.5 billion years ago.

This confirms oxygen levels increased sharply, a development that would have allowed life to colonise our world.

Major shift

"This is the first time that anyone has been able to see a record of oxygen from the ancient atmosphere," says Mark Thiemens, professor of chemistry at the University of California-San Diego.

He added: "We now know it's possible to track the evolution on Earth of oxygen and ozone, which both coincide with the evolution of life and the build-up of the conditions on the planet that led to a major shift in the atmosphere 2.2 billion years ago."

Co-researcher Dr James Farquhar added: "What we found is a geochemical indicator that originated in the atmosphere and it's clearly a global signature.

"It appears in samples that are older than 2 billion years, but is most pronounced in samples older than 2.5 billion years."

Solar radiation

Banded iron formations in 2.2-billion-year-old rocks show that significant quantities of oxygen were present to oxidise the iron in the rocks.

It is believed that photosynthetic bacteria, which were known to exist as far back as 3.5 billion years ago, generated some of that oxygen. Some oxygen could also have come from the chemical separation of water molecules into oxygen and hydrogen.

But until now, scientists had no way of knowing what proportion each process may have contributed to this sharp rise in oxygen and to the development of the Earth's ozone layer, which permitted the expansion of terrestrial life by shielding organisms from the most damaging effects of solar radiation.

"The banded iron formations tell you that the Earth had to have significant quantities of oxygen then," says Professor Thiemens.

"But you don't know how much or where it came from. The period from the earliest-known rocks, at 3.9 to 2.2 billion years ago is a black hole of knowledge about the atmosphere and about life. This method provides a way to track the record of oxygen in the atmosphere and, more importantly, of ozone in the earliest rocks."

Global warming

By analysing variations in the common isotopes, or forms, of sulphur that were incorporated into the rocks, the scientists were able to conclude that the atmosphere 2.45 billion years ago had limited free oxygen.

"It is a new discovery," says Robert Clayton, professor of chemistry at the University of Chicago. "No-one has seen anything like that before. It's another handle on ancient atmospheric chemistry. It is surely going to be important."

Besides improving knowledge about the ancient atmosphere, the discovery has implications for improving the understanding of long-term atmospheric events in the future, such as global warming.

The research is published in the journal Science.