for National Geographic News
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.
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.
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