For nearly a decade, a cloud of suspicion has hung over the idea that fluctuations in Earth's orbit triggered the ice ages. But now scientists have developed a way to double-check their dates for the corals and other deposits that hold clues to ancient climate change. The results, described in today's issue of Science,* could put the doubts about the orbital theory to rest.
Twenty years ago, oceanographers had gathered enough evidence to conclude that the march of ice ages over the last million years was paced by the cyclical stretching and squeezing of Earth's orbit around the sun, which would have altered how sunlight fell on the planet's surface. But in 1988, researchers scuba diving in Nevada's Devils Hole came up with a climate record--captured in carbonate deposits in the crack--that was out of step with the orbital theory. Most glaringly, these carbonates indicated a profound warming trend, which appeared to signal the end of the penultimate ice age, thousands of years before orbital variations could have begun to melt the ice. If the Devils Hole chronology was a true record of the world's ice ages, researchers would have to dump the astronomical mechanism and look for something new.
Geochronologist Lawrence Edwards of the University of Minnesota and his colleagues applied a new clock, based on the radioactive decay of uranium-235 to protactinium-231 in marine sediments, to check the dates of both the Devils Hole record and records of sea-level change in Barbados coral. To get the most precise reading from their new clock, the researchers directly counted individual protactinium atoms with a new mass-spectrometric technique, instead of trying to estimate the element's abundance by measuring its own slow decay. "It's a technological tour de force," says Kenneth Ludwig of the Berkeley Geochronology Center in California.
The preliminary verdict: The marine record is right, and the astronomical theory is on solid ground. The new clock puts the last interglacial as recorded in the coral somewhere around 129,000 to 120,000 years ago--about where oceanographers always had it. But the new findings haven't settled the issue cleanly. Puzzlingly, the Devils Hole record seems to be correct as well. To most oceanographers, this bolsters their contention that the Devils Hole and marine records "are two fundamentally different beasts," says Steven Clemens of Brown University. He and others suggest that while the marine records trace the ebb and flow of the ice ages, Devils Hole may chronicle only the climate of a region as small as southwestern North America.