Study Casts Doubt On 'Snowball Earth' Theory

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

Geologists agree that prehistoric Earth was locked in a deepfreeze 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 werecovered by thick ice, explain the survival of life by hypothesizing theexistence of small warm spots, or refugia.

On the other side are supporters of a "Slushball Earth" thatwould have included large areas of thin ice or open ocean, particularlyaround the equator.

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

The team, which included scientists from USC, Caltech, theUniversity of Maryland and a Brazilian mining company, identified "acomplex and productive microbial ecosystem," includingphotosynthesizing organisms that could not have existed under a thicklayer 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 the USC College, one of Olcott's advisers anda co-author on the paper, said: "What she has provided is the firstreal evidence that substantial photosynthesis occurred in the Earth'soceans during the extreme ice age 700 million years ago, which is achallenge for the snowball theory."

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

But, she said, "finding the one anomalous spot would be quiteunlikely," adding that the drill core came from an extensive formationof 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 dateto a glacial era, Olcott said. But her team found evidence of glacialactivity in the samples, such as dropstones (continental rocks droppedby melting glaciers into marine deposits) and glendonites (mineralsthat 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 ofmicrobes left in the rocks. This is the first direct look at theecosystem during this time period," said Olcott, who credited USC'sgeobiology program, one of a handful in the country, with influencingher 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 betweengeologists and biologists," he said, "is unusually good at USC ... itwas this way of thinking that provided the impetus for the project inthe first place."

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