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Snowball Earth

Science grapples with a puzzling new theory: the extreme ice age

By David L. Chandler, Globe Staff, 6/12/2001

Even the scientists who first thought up the idea didn't think it was possible: That the entire Earth could have frozen over and been encased in a solid cover of snow and ice, perhaps for millions of years.

When the idea first showed up in a computerized climate model, it seemed out of the question. If Earth was frozen solid, how could any life have survived? And how could an ice-covered planet ever have thawed again?

But a rising tide of evidence over the last decade has turned what at first seemed like a crazy concept into a mainstream theory. In fact, it may be the only way to explain a whole series of seeming paradoxes and puzzles about this planet's geology and climate.

At a recent meeting in Boston of the American Geophysical Union, a long session on the so-called ''snowball Earth'' theory was concerned not so much with whether it happened - the evidence now indicates it happened at least twice, and perhaps as many as five times - but with the details of how, why and when the planet was suddenly plunged into the deep freeze, and how it just as suddenly thawed out like a Thanksgiving turkey.

While the ''snowball'' episodes may have lasted 10 million years or more, the actual freezing and thawing probably happened in just a matter of decades - perhaps providing an important clue into just how unstable and vulnerable the planet's climate system is. And, some scientists now suggest, the cycle of freezing and thawing may have been crucial in making all higher forms of life possible, including the humans who are now piecing together this ancient puzzle.

''Something about the snowball causes life to diversify and change,'' said Joseph Kirschvink, a geobiologist at the California Institute of Technology who first coined the term ''snowball Earth'' in a 1992 paper in Science.

The concept of snowball Earth first struck Kirschvink during a geological expedition to Australia in the late 1980s. There, he and a group of other geologists and biologists specializing in reconstructing the ancient environment were struck by abundant evidence of extensive glaciers, embedded in the layers of ancient rock. But there was a big problem: Other evidence clearly showed that, at the time of these deposits some 700 million years ago, Australia lay squarely on the equator.

How could sea-level glaciers possibly have existed under the tropical sun? The implication was clear. If glaciers reached all the way to the sea even in the tropics, the whole planet must have been frozen at the time.

And, it turned out, the theoretical explanation had already been proposed, back in the 1960s, from a computer climate model. In a kind of reverse-greenhouse effect, a sudden depletion of methane in the atmosphere could cause a dramatic drop in temperatures, leading to an advance of glaciers similar to the familiar ice ages of more recent times. But there comes a crucial turning point: If the ice reaches as far south as 30 degrees latitude - about the level of northern Florida - then it triggers an unstoppable ''runaway'' freezing cycle.

That's because ice and snow reflect most of the sunlight that falls on them, radiating away the heat. Darker open water or soil, by contrast, absorbs the sun's heat. So, as darker surface gives way to white snow and ice, less and less sunlight can be trapped to warm the planet, and the Earth cools faster and faster, producing more ice, which cools it even more.

The result: Within a span of a few decades, the Earth could freeze over. Any remaining simple, single-celled life would be limited to surviving in deep-ocean thermal vents, ponds melted on the land by volcanic heat, or patches of ocean where the ice was thin enough for sunlight to filter through and support photosynthesis.

The big puzzle, at first, was how such a snowball could ever reverse itself and thaw out again. That was Kirschvink's big breakthrough. He figured out that even on a frozen-solid Earth, volcanos would continue to erupt, belching the greenhouse gas carbon dioxide into the air. Over time - about 10 million years - at today's rate of volcanism, enough would build up to create a ''super-greenhouse'' effect, strong enough to melt the whole planet - again, in a span of just a few decades, once the process started.

''I didn't think it was possible'' when Kirschvink first proposed the idea, said James Kasting of Pennsylvania State University, who is now one of the concept's strongest advocates. ''I thought it would have extinguished all life. Now, I see there are ways out.''

And, Kasting said, it is now clear that the theory can explain many geological features that had been major enigmas for scientists. One of the most significant ones concerns deep deposits of carbonate rock that appear in the sedimentary layers immediately following those that show signs of tropical glaciers.

These ''carbonate capstones'' can be dozens of feet - in some places, hundreds of feet - thick, and are found all over the globe from the era immediately following the hypothesized snowball episodes. Geologists had been at a loss to explain them.

But, in 1998, Paul Hoffman and Daniel Schrag, two geologists at Harvard University, came up with the explanation: A sudden, dramatic weathering of exposed rock immediately after the planet thawed out again, flushing enormous quantities of carbonate into the oceans.

''They're predicted'' by the snowball Earth theory, Kasting said of the carbonate capstones. ''They are the smoking gun'' that points directly to the truth of the theory.

And it is largely because of that smoking gun, he said, that the whole theory has gained acceptance: ''When it was first proposed, there was a lot of resistance. Now, it's become mainstream.''

Hoffman himself is more guarded: ''The community is really polarized,'' he said last week. ''There are a lot of people sitting on the fence, because it's really such a radical idea.'' But, he said, while Kirschvink's original theory ''was kind of ignored,'' it is really since he and Schrag published their paper in 1998 ''that the real attention and intense interest has developed'' in the idea.

Now, the focus has shifted to understanding exactly how the process worked, and how it affected the rise and diversification of life as we know it.

Kirschvink, who presented his latest analysis at the recent geophysics meeting, said he thinks he now has the answer to what the ''something'' is that caused life to burgeon just after the frozen episodes: A huge increase in oxygen immediately following the snowball's big thaw.

''Snowballs produce massive oxygen spikes,'' he said, ''perhaps stimulating major evolutionary innovations.'' And that may include, he suggested, one of the most crucial innovations of all: The development of multicellular life, after more than 3 billion years when Earth was populated only by single-celled organisms.

In Kirschvink's view, the question is not so much why complex animals surged into existence when they did - about 550 million years ago, right after the last snowball episode - but why that didn't happen much sooner. ''There's nothing in the history of this planet that would have prevented animals from radiating a billion years ago, or 2 billion,'' he said in an interview last week.

''Something was holding them back,'' he said. ''I think it was oxygen.''

The reason for the sudden pulse of oxygen involves the action of billions of tiny single-celled organisms called cyanobacteria, also known as blue-green algae.

It turns out that the big thaw would have produced giant storms and intense acid rain, producing a surge of intense weathering of the freshly-exposed rocks and washing vast amounts of carbon, phosphorous, iron and other essential nutrients into the oceans. ''Everything you need for cyanobacteria to go haywire is there,'' Kirschvink said - very similar to the growth media used to grow colonies of bacteria in the lab. ''I think they'd be very happy.''

The result, he suggested, would have been ''a massive cyanobacteria bloom - I call it green Earth. If you have a whole planet of growth medium, and nothing to stop it, the expected result is a massive oxygen spike.''

And there is abundant geological evidence that exactly such a spike of oxygen did indeed take place, he said. For example, all this oxygen would be expected to mix chemically with manganese and iron, forming minerals that would then drop to the sea floor. And indeed, in formations that arose just after the snowball episode, massive layers of manganese and iron ores are found in many places.

In fact, he said, the world's largest commercial deposit of manganese, source of 80 percent of global manganese production, occurs in just such a formation in South Africa - called Hotazel, in reference to the super-greenhouse heating of the planet at that time.

But perhaps the biggest lesson from the whole snowball Earth concept is just how vulnerable the Earth's climate system is to dramatic, rapid changes and unanticipated feedback effects.

Snowball Earth ''is an extreme example of an instability of the climate system,'' Hoffman said. ''As a geologist, the remarkable thing about the last 10,000 years is how remarkably stable the climate has been.''

-- Anonymous, June 12, 2001