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February 2000 

Freeze-fry from the snowball Earth

Excitement is building over further developments of the snowball Earth theory. First proposed as a model to explain low-latitude glaciation during the Neoproterozoic, 750 to 580 million years ago, substantial observations are escalating the theory’s importance. The theory suggests that Earth experienced extreme glaciations that covered the entire planet, including the tropics, with ice.

“Every biogeochemical cycle needs to be rethought as a result of this theory,” says Daniel Schrag of Harvard University. On Dec. 14, Schrag presented recent observations in favor of the snowball Earth at the American Geophysical Union’s meeting in San Francisco, which lasted from Dec. 12–17. “This was an extreme perturbation in the Neoproterozoic. We can’t think that systems operated as they do today,” he says.

But some scientists still remain skeptical. During a presentation at the AGU meeting, Gregory Jenkins of Pennsylvania State University supported what is called the high obliquity hypothesis over the snowball Earth hypothesis.

Jenkins argued that climatic conditions during the Neoproterozoic resulted from a severe tilt of Earth on its axis, possibly caused by the impact of the planetoid that formed the moon. Although a tilted Earth may have cooled the equator and warmed the poles, the theory is still hotly debated as an explanation because it seems to require low-latitude cooling prior to Neoproterozoic glacial events.

Schrag and others defend the snowball Earth theory, saying it brings context to mysteries that have plagued the geological community for decades. It explains how glaciers survived in the tropics, how iron-rich rock emerged in an oxygen-enriched world and how warm-water carbonate rocks found themselves perched atop glacial deposits. It may even explain the explosion of life in the Cambrian.

Beginning in the 1960s, when plate tectonics was still emerging as a new science, the curiosity of Cambridge geologist Brian Harland was piqued over glacial rocks having ancient magnetic field lines that ran nearly horizontal—an indication of equatorial origin. Climate modelers at that time debated the idea of a frozen Earth, arguing that the high albedo from global ice cover would not permit sunlight to warm the planet. The freezing effect would be permanent, they said. And that was the end of it.

That is until paleomagnetism specialist Joe Kirschvink of Caltech linked the effect a frozen Earth would have on the hydrological cycle to a buildup of volcanic carbon dioxide. If ice covered the oceans to the equator, it would block off evaporation, dry up the clouds and deny water the chance to erode the land.

Carbon dioxide would build up in the atmosphere instead of being washed out by rain and carried back to the oceans as carbonates from land. With enough carbon dioxide, the “snowball Earth” would then become a hothouse, Kirschvink said in a 1992 paper published in the book The Proterozoic Biosphere.

Kirschvink also explained the Neoproterozoic iron formations that reappear in the stratographic record after a billion-year hiatus. He reasoned that the blanket of ice not only shut down the hydrologic cycle, but also allowed for a tremendous precipitation of iron rust when it finally thawed.

In oxygen-rich environments, ferrous iron immediately oxidizes and turns to rust (Fe3+). Under the extensive sea ice of the snowball Earth, the ocean turned anoxic and became rich in dissolved ferrous iron (Fe2+). The last time ferrous iron could build up was a billion years earlier, before oxygen-producing algae first evolved.

But without further support by geologists, the snowball theory stalled until Paul Hoffman of Harvard University got it rolling in 1998 with colleague Daniel Schrag. Hoffman had been digging in Namibia’s Skeleton Coast, investigating carbonate isotopes, when he found evidence that biological productivity in the surface oceans had collapsed for millions of years at least twice between 750 and 580 million years ago.

Inorganic carbon-13, like steak on a vegetarian’s dinner plate, is not the meal of choice for phytoplankton and settles to the bottom of the sea as leftovers in the carbonate rocks. Hoffman found that as the glaciers developed during the time of the snowball Earth, however, the amount of carbon-13 plummeted close to matching the ratio of carbon-12 to carbon-13 emitted from volcanic eruptions—the same ratios expected if biological productivity were to grind to a halt.

Schrag and Hoffman then looked at the cap carbonates, pure carbonate hundreds of meters thick that sit on top of the Neoproterozoic glacial deposits and point to a rapid change in temperature. “Postglacial cap carbonates are predictable consequences of the recovery from a snowball Earth,” wrote Hoffman and colleagues in the Aug. 28, 1998, issue of Science.

Jeff Serveringhaus and Dan Schrag stand next to bouquets of former aragonite crystal fans in cap carbonate rhythmites in Namibia. Courtesy of Paul Hoffman, Harvard University.
In 1992, Kenneth Caldeira of Lawrence Livermore National Laboratory and James F. Kasting of Pennsylvania State University calculated that the amount of carbon dioxide needed to reverse the snowball effect would be 350 times present-day levels. With a global average of 50°C below zero, Earth would bake under extreme global warming to 40–50°C in only a few thousand years.

This freeze-fry event would restore the hydrological cycle, scrub carbon dioxide from the atmosphere and bleach the land with acid rain dumping landlocked carbonate back into the oceans.

--Christina Reed