'Fossil air' provides climate clues

Sulphur-rich rocks in the Namib Desert

By BBC News Online science editor Dr David Whitehouse

Scientists have extracted samples of Earth's atmosphere trapped in rocks millions of years old.

It is a technique that could provide new data about past climate upheavals and mass extinctions.

Until now, the oldest samples of "fossil air" have come from ice cores from the central Greenland ice cap. They provide evidence for the past 400,000 years.

To go back further, scientists have to analyse rocks that have acquired some recognisable chemical signature through interaction with the ancient atmosphere during their formation.

Huiming Bao, of the University of California San Diego, La Jolla, US, and colleagues, have shown that just such a signature can be found in some sulphate rocks.

These retain the distinctive pattern of the different types, or isotopes, of oxygen that existed much earlier in Earth's history

The pattern, or ratio, of isotopes was locked into the minerals when the oxygen reacted with molten rock on the planet's surface.

Mars lander

This isotopic pattern is affected by the amount of ozone present in the ancient atmosphere and hence may serve as a monitor of atmospheric conditions in the past.

The rocks they studied came from the United States and Africa and were formed in the Miocene geological epoch.

This period extended from 23.7 to 5.3 million years ago. It was a time when the great mountain ranges of Europe, Asia and America were forming.

The team examined sulphur-rich rocks from the Namib desert in Africa and volcanic-ash-beds from the western United States.

The researchers say that their new technique is a promising way to explore changes in the Earth's atmosphere in the past.

They add that it may also be a new way to analyse rocks from Mars, as the Pathfinder Mars Lander showed that some rocks contain high quantities of Sulphur.

The research is published in the journal Nature.