Scientists working under a grant from the National Science Foundation (NSF) have unveiled a new theory that winds in the tropics caused vast iceberg armadas to surge across the North Atlantic.
In the December 13 issue of Science, Andrew McIntyre and Barbara Molfino, paleoclimatologists at the Lamont-Doherty Earth Observatory in Palisades, New York, report evidence that strong westward-blowing trade winds died down along the equator at about the same time that iceberg flotillas sailed across the North Atlantic during the last ice age -- in approximately the same 7,000- to 10,000-year cycles.
"Ever since these cyclic iceberg pulses were first discovered in the early 1990s, scientists have tried to understand what caused them and have wondered whether Earth's climate system could shift so dramatically again in modern times," says Connie Sancetta, program director in NSF's marine geology and geophysics program, which funded the research.
The Lamont-Doherty scientists theorize that the equatorial winds relaxed periodically, allowing a large reservoir of warm tropical waters -- which had been pushed into the Caribbean Sea and the Gulf of Mexico by the winds -- to flow back eastward. The waters were whisked by the Gulf Stream to the north, where they warmed the North Atlantic region and triggered melting along the edges of the massive ice sheets that covered the Northern Hemisphere.
According to McIntyre and Molfino's theory, the strength of Atlantic equatorial trade winds over long intervals of time is governed by Earth's orbital cycle, which alters the seasonal intensity of solar radiation reaching the planet. The researchers found that over the past 45,000 years, a suborbital rhythm of 8,400 years has produced variations in the strength of the tropical winds.
McIntyre will present his new theory on December 16 at the American Geophysical Union's fall meeting in San Francisco. McIntyre and Molfino analyzed three ocean sediment cores taken near the equator just below the bulge of Africa. They looked for the fossilized plates of a marine alga called Florisphaera profunda. Unlike other algae, F. profunda lives further below the surface where sunlight levels are lower. When westward trade winds are strong, deeper nutrient-filled waters come nearly to the surface to replace the waters blown westward, and all algae thrive. But when the winds diminish, surface waters don't receive as many nutrients, surface algae don't fare well and the ratio of F. profunda to its cousins goes up.
The scientists found relatively high levels of F. profunda every 8,400 years, in a regular cycle stretching back 45,000 years. The high points of F. profunda along the equator correlated with times when other scientists have found high levels of pulverized rock in North Atlantic ocean sediment cores. This rock, which had been frozen into the bases of glaciers and carried out to sea in icebergs, is the signature of great iceberg launchings. Scientists have yet to determine whether these iceberg pulses are a response to climate changes, or a result of them -- or both. Explains McIntyre, "Our hypothesis is based on the premise that the equatorial bulge -- which receives the most solar radiation of anywhere on Earth -- is the central boiler that drives heat and energy around the planet. Small variations in received energy per unit area in the tropics translate into major changes in the total energy of the Earth's system."