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"Alien" Atmosphere Helped Unfreeze Early Earth

Anne Minard
for National Geographic News
January 13, 2009
 
Oxygen trapped in 635-million-year-old rocks from the Arctic has revealed that ancient Earth once had an otherworldly atmosphere that might have helped melt millions of years' worth of deep freeze.

Analysis of the chemical composition of rocks from the Norwegian island chain of Svalbard shows a surprisingly low amount of a particular type, or isotope, of oxygen.

Reduced levels of this isotope are linked to high levels of carbon dioxide in the atmosphere, and the new data suggest ancient Earth might have had 300 to 1,000 times more CO2 than current levels.

An atmosphere so rich in CO2 would still be breathable by modern standards, but it would place limits on the growth of life as we know it.

"The numbers indicate a very, very different world" than scientists had previously assumed, said study leader Huiming Bao of Louisiana State University.

Bao and colleagues think their finding supports the "snowball Earth" theory, which says that snow and ice accumulation worldwide once reached a threshold that caused the entire planet to stay frozen for millions of years.

Most scientists believe Earth could only emerge from such a state after ten million years or more, after atmospheric CO2 built up enough to finally trigger melting and lead to a rapid thaw.

(Related: "Ancient 'Snowball Earth' Melted Fast Due to Methane" [May 28, 2008].)

But Bao notes that there is no direct evidence for high CO2 levels, and it's possible his team's findings are due to a completely unfamiliar interaction between the atmosphere and the biosphere.

Weird Air

Bao and colleagues analyzed a compound called sulfate from rocks dated to the Neoproterozoic era, which lasted from a billion to 542 million years ago.

Sulfate forms when a charged sulfur atom binds with four oxygen atoms. The complete molecule is highly stable, which makes sulfate a useful tool for studying what types of oxygen atoms were present when the sulfate formed.

In the Svalbard study, which appeared last week in the journal Science, Bao and colleagues looked at the sulfate-containing mineral carbonate and found very low levels of an oxygen isotope called O-17.

Previously Bao and a different set of co-authors had examined sulfate in another mineral called barite in samples from south China and West Africa dated to roughly the same time period. Those rocks had also shown a low amount of O-17.

Given that the abnormal isotope ratios occurred in several locales, University of California, San Diego, chemist Mark Thiemens thinks the results are associated with a widespread phenomenon.

The atmospheric effects were, "if not global, at least equatorial," said Thiemens, who was not involved in the new study.

And geophysicist Raymond Pierrehumbert, of the University of Chicago, said the result "underscores what I call 'Neoproterozoic weirdness,' namely that there are a lot of strange geochemical signatures that indicate that the climate is doing something radically different from what it was doing before or since."

Fragile Ecosystem

Scientists don't know what would have caused snowball Earth scenarios in the past, but they do have ideas for what might trigger a repeat scenario in the future.

Either nuclear weapons or a catastrophic asteroid impact could conceivably shield Earth from the sun's rays long enough to turn it into a frozen planet, study leader Bao said.

Such a state wouldn't be favorable to life, just as it might not have been prior to 635 million years ago.

"Increasingly, lines of fossil evidence suggest that after this global glaciation event, [the first] animals started to appear in the geological record," Bao said. "Most believe this is not just a coincidence."

UCSD's Thiemens believes the new study presents a warning about the instability of Earth's atmosphere and climate, in light of increases in human-contributed greenhouse gases.

"I think the really important part is … it shows that the whole atmosphere in the Earth system is actually reasonably fragile," he said.
 

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