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Web Extra Friday, April 18

Microfossil lineages support sloshy snowball Earth

Scientists studying the snowball Earth hypothesis have two very different models: one hardball and one slushball. The hypothesis suggests that large segments of Earth's surface were frozen over during the Neoproterozoic (about 900 to 540 million years ago), and that the glacial periods were followed by rapid and intense greenhouse warming, acid precipitation and deposition of extensive carbonate deposits, or "cap carbonates." This idea has attracted significant research attention since it was initially proposed in 1998.

It remains controversial, but many earth scientists believe that Earth experienced several significant glaciations between 700 and 540 million years ago. Whether Earth's surface was completely frozen over during the glaciations (a hardball) or experienced open water near the equator (a slushball) is up for debate. A recent paper in the Proceedings of the National Academy of Sciences (PNAS) now suggests that slushball conditions were more likely.

The two scenarios would have differed in the severity of their effects on the evolution of life. Complete coverage of ice and snow around the globe would have decimated many ancient lineages of life, says Alan Jay Kaufman, a geologist at the University of Maryland and one of the co-authors of the 1998 Science paper suggesting a snowball Earth. "Only the most extremophilic bacteria would have been able to survive the snowball Earth and repopulate the planet afterwards," Kaufman says.

Even with some open water, however, slushball-like conditions would not have been easy for life. The chances, though, for a more diverse array of multicellular life forms to evolve would have been greater, according to the recent research.

In the PNAS study, Frank Corsetti of the University of Southern California, and Stanley Awramik and David Pierce of the University of California, Santa Barbara, examined microfossil lineages from the Neoproterozoic Kingston Peak Formation in Death Valley, California. The strata span what is known as the Sturtian glaciation (750 to 700 million years ago). The researchers believe the deposits formed within 10 degrees of the equator, but acknowledged some uncertainty on that point.

Locality information for the study area, Kingston Range, eastern California. View larger version here.

"The effect of low latitude glaciation on life has been predicted to be extreme … but little or no fossil evidence from the interval in question has been brought to bear on the issue," Corsetti says. "We bring the fossil record to the debate. Our data would suggest that microbial diversity in prokaryotes and eukaryotes was nearly identical before the snowball and during the snowball. That is, the predicted crash in the biosphere is not supported by our data. This led us to conclude that the climate was not as severe as predicted by some of the snowball hypotheses or that life is much more resilient than we thought," he adds. In their paper, Corsetti and his colleagues concluded that slushball conditions were more probable.

Kaufman agrees. Hardball conditions, he explains, would have decimated many ancient lineages, particularly among eukaryotic organisms — those that gave rise to what we now know as animals, plants and fungi. "If the snowball Earth was such a dire event for life you would not see the same lines of eukaryotes before and after the event," he says.

Gabrielle Walker, author of a book on the snowball Earth hypothesis (Snowball Earth, Crown Books, 2003) says that the results are not that surprising. She explains that while many species would have gone extinct during the snowball episodes, some would have survived in safe havens, which scientists call refugia.

Corsetti, however, downplays the idea that the strata he and his colleagues studied were merely a refuge. "This is highly unlikely, given the difficulty in finding microbial fossils in the first place, let alone stumbling across a purported refugia," he says. Still, given the low level of detail that can be seen in microbial fossils, Corsetti acknowledged difficulty identifying individual species. This could be problematic for some, but he does not think identification problems pose critical problems for their analysis.

"Most of us would agree that Earth got cold enough for ice to form at the equator at least twice in Neoproterozoic time and that the direct aftermath of the cold times was highly unusual," Corsetti says. This was manifested by the isotopic anomalies and the unusual carbonates capping the glacial units, he adds. "From this point, no one agrees on anything. The debates are far from over, I suspect. But that's what makes it interesting."

Dave Lawrence
Geotimes contributing writer

Links:

PNAS abstract

Frank Corsetti's home page

Snowball Earth by Gabrielle Walker


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