By William J. Cromie
Seven hundred million years ago, Earth's oceans were completely frozen over. No rivers flowed, no rain or snow fell. Life, limited to simple plants and bacteria at the time, became severely depleted.
But inside Earth, the activity that leads to surface volcanism continued. Volcanoes belched carbon dioxide and other gases into the air. Carbon dioxide accumulated for millions of years, preventing heat from escaping into space (the greenhouse effect), and producing a global warming that eventually melted the ice.
Between 750 million and 570 million years ago, this icehouse to greenhouse cycle occurred several times. Glaciers turned Earth into a "snowball" that stayed unmelted for millions of years until volcanic gases finally freed it.
At least that's what the rocks in southwest Africa tell Harvard geologists. "It's staggering to think that such events are not only possible in theory, but actually occurred at a critical turning point in Earth's history," says Paul Hoffman, Sturgis Hooper Professor of Geology. "The first diverse fossils of large animals appear soon after the last snowball glaciation. There are reasons to believe that this is no mere coincidence."
For three billion years preceding the snowball glaciations, life was confined to algae, bacteria, and other simple organisms. Only the most adaptable of these creatures survived the global freezeovers. Soon after the last ice ages ended, about 565 million years ago, large animals with cells formed into tissues and organs suddenly began to appear. They included the ancestors of many groups of animals still alive.
Hoffman spent six summers examining rock formations in Namibia, Africa, where the rock record of ice ages and global warmings is clearly preserved. "Things didn't start falling into place until last December," he recalls. "What excites me about the snowball idea is that it provides a common explanation for many notable geological features in that period of Earth history, any one of which is puzzling when approached in isolation."
What do other scientists think about Hoffman's reading of the rocks? "They either love it, or they hate it," he replies. "We've had both support and stiff challenges, both of which have led to a stronger theory. At this point, the remaining questions appear minor compared to all the things that the idea explains."
A detailed report of the theory and evidence that supports it appears in Friday's issue of the journal Science. It was written by Hoffman; along with Alan Kaufman, a former Harvard post-doctoral fellow now at the University of Maryland; Galen Halverson, a graduate student working with Hoffman; and Harvard geochemist Daniel Schrag.
Do Worms Keep Us Warm?
Earth started to freeze over, Hoffman says, due to a lack of carbon dioxide in the air combined with a dimmer sun. Once ice and snow covered much of the land and ocean, a process called the albedo effect -- wherein ice and snow reflect the sun's energy back into space -- kicked in. The more ice and snow, the higher the albedo and the colder it gets.
"Albedo tends to drive global change to one extreme [all ice] or the other [no ice], causing catastrophic change in either direction," Hoffman explains. But for albedo-driven glaciation to start, it must first get cold enough for polar sea ice to expand to the latitude of Boston. During the most recent ice age, 18,000 years ago, land ice reached Cape Cod, but during summers the Atlantic stayed open as far north as Iceland.
The sun also radiated less energy 750 million years ago than it does now. Our star works like a nuclear reactor, converting hydrogen to helium and releasing energy. As the proportion of helium grows, the sun produces more heat. Some 750 million years ago, the solar furnace was an estimated 6-7 percent cooler than at present.
The sun, however, did not operate alone. "Over most of geological time, varying amounts of carbon dioxide in the air have regulated Earth's climate through the greenhouse effect," Hoffman notes. The gas acts like glass in a greenhouse; it lets light in but prevents heat from escaping into space. Carbon dioxide along with other gases coming from smokestacks, vehicle exhausts, and burning of tropical forests make a major contribution to the present global warming.
During the icehouse part of the greenhouse-icehouse transitions, a severe shortage of carbon dioxide in the air came from a loss of carbon, which entered the ocean and got buried with muddy sediments on the ocean floor. These sediments, which were later heated, compressed and uplifted by the shifting of continents and ocean floors, are beautifully preserved as rock layers in Namibia.
Chemical testing of these rocks reveals evidence for rapid removal of carbon from the atmosphere before the ice ages, then virtually no removal during the warmings. "At no time since the last snowball event do we observe carbon shifts of such magnitude," Hoffman comments.
How come there have been no snowball glaciations since that time? "We think we have worms and snails to thank," laughs Hoffman. They and many other animals that live on the sea bottom constantly churn muddy sediments searching for bits of food. This contributes to the breakdown of organic forms of carbon and its release into the water, then into the air.
"With the advent of bottom-dwelling animals, burial of organic carbon became seriously impeded by their feeding activities," Hoffman points out. Before the advent of animals, the rocks in Namibia show thin layers of undisturbed sediment on the sea bottom. After animals appeared, the fossil sediments were disrupted by feeding trails, burrows, and other signs of grazing activity.
But what caused the sudden appearance of such animals? Hoffman notes that a leading theory for the creation of new species involves mass mortality of organisms, disorganization of genetic material among the survivors, then renewed population growth in a different environment. "This is just what happened in the icehouse-greenhouse transitions," Hoffman says. "A succession of global glaciations, each terminated by intense warming conditions, may be just what the biologists ordered" for a sudden evolution of new forms of life.
If he is right, that explains one of the greatest mysteries of life on Earth: what caused the first appearance of animal life.
Snowball in Hell
"Whenever we see the top of deposits laid down by glaciers in Namibia, they are capped with layers of pure carbonate rock," says Hoffman, pointing to a chunk of such rock on his desk. These limestone caps consist of carbon and calcium that precipitated out of sea water. They appear all over the world and have always been a mystery to geologists.
Daniel Schrag, the Harvard geochemist, maintains that this unique combination of glacial deposits, sharply capped by carbonates laid down in warm water, can be neatly accounted for by the snowball theory. The high concentrations of carbon dioxide would break the ice's long grip. As the ice receded, rapid precipitation of carbonate from the water would occur.
"It was the most extreme and rapid change on record," says Schrag. "Organisms surviving the deep freeze would immediately have to face the heat."
But how did the snowballs get rolling in the first place? "For 300 million years before the cycles began," Hoffman explains, "all land was gathered together in a single supercontinent called 'Rodinia.' The name comes from the Russian work rodit, which means 'to beget.' When Rodinia began to breakup about 750 million years ago, it begot smaller continents which created many new continental margins. These margins are where most of organic matter, including carbon, settles to the ocean bottom and gets buried. The burials speeded up withdrawal of carbon dioxide from the air and begot the whole previously unimagined chain of events."
Copyright 1998 President and Fellows of Harvard College