The warm surf of tropical islands offers a pleasant escape from the snowy north. Not so before the dawn of travel agents. In tomorrow's issue of Nature, U.S. and South African geologists report that 2.2 billion years ago, glaciers turned parts of the balmy tropics into a frozen wasteland--extreme conditions that puzzle climate researchers.
During major ice ages over the last few million years, massive sheets of ice advanced from the poles and bulldozed their way down Europe and North America. But never did they push farther south than London or the Ohio Valley. Ice ages seem to have behaved quite differently in the Proterozoic era, which lasted from about 2.5 billion years ago to 570 million years ago. About 10 years ago, geologists realized that 750-million-year-old glacial deposits in Australia had originally been laid down within 5 degrees of the equator, and they began looking for other examples.
They have now found one from 2.2 billion years ago, close to the beginning of the Proterozoic. Dave Evans, a graduate student at the California Institute of Technology, and colleagues at Caltech and the Rand Afrikaans University in Johannesburg hit pay dirt when they looked at some lava flows in South Africa that had retained their original magnetic signal. When lava cools, tiny mineral grains align themselves to Earth's magnetic field like compasses. By measuring the inclination of the grains, geologists can determine the rocks' original distance from the equator. The glacial cobbles and pebbles mixed with the lava, Evans deduced, must have been deposited within 11 degrees of the equator.
The findings leave climate researchers wondering how glaciers could have formed in the tropics. Some speculate that carbon dioxide was somehow pulled from the atmosphere, removing the greenhouse blanket and forming a "Snowball Earth" so cold that glaciers covered entire continents. Others think that's too extreme. Jim Kasting, a paleoclimatologist at Pennsylvania State University, favors the notion that polar ice accumulation may have skewed the angle of Earth's axis such that for a time, the equator received less sunlight than the poles. Geologists find the new data impressive, if puzzling. Says Kasting, "It doesn't clear things up."