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."
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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