For the last 15 years, most climate researchers have looked to space for an explanation of the ice ages that have repeatedly gripped our planet in recent geologic times. The established theory, called the Milankovitch hypothesis, holds that wiggles and wobbles in Earth's orbit serve as a pacemaker that determines when the planet plunges into a glacial period and when it thaws out of one. But new evidence from a deep crack in the Nevada desert threatens to over-turn the Milankovitch theory and replace it with a more down-to-Earth solution.
Geoscientists report this week that the ice ages did not follow any pattern consistent with orbital variations. Rather, chaotic elements in Earth's own climate dictated when the planet slipped into and out of a deep freeze, the researchers suggest.
"We feel that the Milankovitch theory is incapable of explaining the climate shifts," says Isaac J. Winograd of the U.S. Geological Survey in Reston, Va. He and his colleagues discuss their findings in two papers in the Oct. 9 SCIENCE.
The climate information collected by Winograd's group comes from Devils Hole, an open fault zone in southwestern Nevada. The fissure is filled with mineral-rich water that has coated the rock walls with layer upon layer of calcite over the last 500,000 years. Divers equipped with scuba gear entered the fault and used a drill to cut a 36-centimeter-long cylinder out of the calcite coating.
By analyzing the ratio of two isotopes--oxygen-18 and oxygen-16 -- at hundreds of spots along the calcite core, Winograd and his colleagues identified changes in the temperature of the atmosphere when rain fell in the Devils Hole region. They dated these climate shifts by using the radioactive decay of uranium within the calcite as a clock. Previously, they had drilled a core that recorded information going back 250,000 years (SN: 12/3/88, p.356). The new core doubles the length of that record.
Scientists who study the glacial cycle have traditionally relied on climate records constructed by measuring the oxygen isotopes in seafloor sediments. But the dates of the climate events in these marine records are less certain than those in the Devils Hole core.
The idea that ice ages result from alterations in Earth's orbit goes back to the 19th century. However, the theory did not enjoy widespread support until 1976, when oceanographers analyzed two marine records and found that the glacial cycle closely matched changes in the shape of Earth's circular orbit, the tilt of the planet, and the way its axis wobbles. …