Ancient mineral provides clue to early climate

June 15th, 2008 - 4:05 pm ICT by IANS  

New York, June 15 (IANS) A new analysis of ancient minerals called zircons suggests that earth’s earliest continents were probably destroyed by an extremely harsh climate. Zircons, the oldest known materials on earth, offer a window in time back as far as 4.4 billion years ago, when the planet was a mere 150 million years old.

As these crystals are exceptionally resistant to chemical changes, they have become the gold standard for determining the age of ancient rocks, ScienceDaily reported.

University of Wisconsin-Madison geologist John Valley has used these tiny mineral grains - smaller than a speck of sand - to show that rocky continents and water formed on the earth much earlier than previously thought, about 4.2 billion years ago.

In a paper published in the journal Earth and Planetary Science Letters, Valley and co-authors show that rocky continents and liquid water existed at least 4.3 billion years ago and were subjected to heavy weathering by an acrid climate.

This could provide an answer to a long-standing question in geology: why no rock samples have ever been found dating back to the first 500 million years after the earth formed.

Previous explanations for the missing rocks have included destruction by barrages of meteorites and the possibility that the early earth was a red-hot sea of magma in which rocks could not form.

The current analysis suggests a different scenario. Valley and his team found that the zircons’ lithium signatures also hold signs of rock exposure on the earth’s surface and breakdown by weather and water, identified by low levels of a heavy lithium isotope.

“Extensive weathering earlier than four billion years ago actually makes a lot of sense,” said Valley. “People have suspected this, but there’s never been any direct evidence.”

Carbon dioxide in the atmosphere can combine with water to form carbonic acid, which falls as acid rain. The early earth’s atmosphere is believed to have contained extremely high levels of carbon dioxide - maybe 10,000 times as much as today.

“At those levels, you would have had vicious acid rain and intense greenhouse effects. That is a condition that will dissolve rocks,” Valley said.

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