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University of Southern California
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Carl Marziali, 213-740-4751

09/29/05

Study casts doubt on 'Snowball Earth' theory

Remains of photosynthesizing microbes in prehistoric rocks suggest Earth
was not ice-bound, as long proposed by supporters of the ‘Snowball
Earth' theory.

By Carl Marziali

A study that applied innovative techniques to previously unexamined rock
formations has turned up strong evidence on the "Slushball Earth" side
of a decades-long scientific argument.

The study appears in the Sept. 29 Science Express. The lead author is
Alison Olcott, a Ph.D. student of earth sciences in the USC College of
Letters, Arts and Sciences.

Geologists agree that prehistoric Earth was locked in a deep freeze
during Precambrian times, about 750 to 600 million years ago. They
disagree over the severity of the glaciation.

"Snowball Earth" proponents, who say that Earth's oceans were covered by
thick ice, explain the survival of life by hypothesizing the existence
of small warm spots, or refugia.

On the other side are supporters of a "Slushball Earth" that would have
included large areas of thin ice or open ocean, particularly around the
equator.

The debate has tended to revolve around the same rock samples and
analytical techniques, Olcott said. So she and her team focused on a
drill core of little-known black shale deposits from southeastern Brazil
and applied lipid biomarker techniques to identify prehistoric organisms
based on the fatty remains of their cell membranes.

The team, which included scientists from USC, Caltech, the University of
Maryland and a Brazilian mining company, identified "a complex and
productive microbial ecosystem," including photosynthesizing organisms
that could not have existed under a thick layer of ice.

"If there was ice, it had to have been thin enough that organisms could
photosynthesize below it or within it," Olcott said.

Frank Corsetti of USC College, one of Olcott's advisers and a co-author
on the paper, said: "What she has provided is the first real evidence
that substantial photosynthesis occurred in the Earth's oceans during
the extreme ice age 700 million years ago, which is a challenge for the
snowball theory."

The evidence from the drill core does not prove that large parts of the
ocean remained free of sheet ice during the pre-Cambrian glaciation. It
is statistically unlikely but possible, Olcott said, that the drill core
found one of the tiny "refugia" for marine life whose existence is
allowed under the "Snowball Earth" hypothesis.

But, she said, "finding the one anomalous spot would be quite unlikely,"
adding that the drill core came from an extensive formation of rocks
with similar characteristics.

"At what point does an enormous refugium become open ocean?" she asked.

Skeptics also may argue that the rocks do not necessarily date to a
glacial era, Olcott said. But her team found evidence of glacial
activity in the samples, such as dropstones (continental rocks dropped
by melting glaciers into marine deposits) and glendonites (minerals that
only form in near-freezing water).

Objections aside, the paper's main contribution may be the application
of new techniques to an old chestnut.

"Geologists don't necessarily think of looking for traces of microbes
left in the rocks. This is the first direct look at the ecosystem during
this time period," said Olcott, who credited USC's geobiology program,
one of a handful in the country, with influencing her thinking.

"They really try to synthesize between geology and biology. It was a new
way to attack the problem."

Corsetti agrees. "The climate of collaboration between geologists and
biologists," he said, "is unusually good at USC … it was this way of
thinking that provided the impetus for the project in the first place."

Funding for Olcott's project came from the National Science Foundation
and NASA.