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An international team led by a scientist at Columbia University's Lamont-Doherty Earth Observatory has found that tiny soil particles trapped in a Greenland glacier during the last ice age traveled halfway around the globe from a single surprising source - the deserts of Mongolia and northern China.
The finding, reported today at the American Geophysical Union's spring meeting in Baltimore, Md., provides an important new clue to understand the workings of earth's climate system and the causes of abrupt climate changes in the past and future.
The study makes less probable several previously considered atmospheric circulation patterns for the earth at the height of the last ice age between 23,000 and 26,000 years ago. Earlier theories held that the soil particles were transported to Greenland from much closer sources, such as Alaska, the continental United States, or the Sahara Desert in Africa.
Instead, turbulent, high-speed Asian winds - similar to those that caused the dust bowls during 1930's in the United States - cast fine grains of soil and ground-up rock from the Gobi and Takla Makan deserts up into the atmosphere. A powerful jet stream carried this dust across the northern Pacific, around the great Laurentide ice sheet that covered Canada, all the way to Greenland, said Pierre Biscaye, a geochemist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y.
Throughout the 3,000-year span studied by the team - during which earth's climate shifted abruptly between cold, dusty periods and somewhat warmer and less-dusty periods - the dust continued to come solely from the same parts of Asia, Dr. Biscaye told the AGU. The new evidence indicates that fluctuating dust levels in the earth's atmosphere were not due to changes in atmospheric circulation patterns that picked up dust from alternative source areas. Nor did the size of the source area increase and decrease to change the amounts of available dust. Instead, East Asian winter monsoon winds, which whip across the vast Asian deserts, may have grown stronger and weaker, transporting more and less dust to Greenland.
These new observations will have to be incorporated into computer models that simulate earth's past climate conditions to predict future climate change more reliably, said Dr. Biscaye. In particular, scientists will have to explore relationships between shifts in the strength of East Asian monsoon winds; dramatic Greenland temperature fluctuations that occurred over decades or less (as seen in the ice cores), and sudden massive iceberg discharges into the North Atlantic (as documented in ocean sediment cores), he said.
Drilling of the two-mile-thick ice sheet at Summit, Greenland, was completed in 1993. The scientists extracted milligrams of dust that had been frozen in the cores at the height of the last ice age between about 26,000 and 23,000 years ago. In the first analysis of this kind, they examined minerals and geochemical isotopes in the Greenland dust and compared these characteristics with those found in some 100 samples of soils from 28 different locations all over the northern hemisphere.
Because soils from different areas have different mineralogical and isotopic characteristics, the scientists could eliminate some locations as possible sources for the Greenland dust and identify the best match for the Greenland samples. For example, the Greenland dust contained no smectite, a mineral common to soils deposited in the continental United States by advancing glaciers. The ratios of neodymium, strontium and lead isotopes closely matched those in Asian source areas but not those in other samples.
The team led by Dr. Biscaye included Francis Grousset of Lamont-Doherty and the UniversitŽ de Bordeaux in France, Marie Revel of the UniversitŽ de Bordeaux, Sjerry Van der Gaast and Aad Vaars of the Netherlands Institute for Sea Research, and Gregory Zielinski of the Institute for the Study of Earth, Oceans and Space at the University of New Hampshire.
For decades, scientists have long known that earth's atmosphere was 10 to 50 times dustier during ice ages than it is now, but they have not understood why. A first step is to figure out where the dust comes from, Dr. Biscaye said.
In 1992, Dr. Grousset, Dr. Biscaye and several of the same researchers analyzed dust in glacial ice cored in Antarctica. They found the dust came solely from Patagonia, an arid region at the southern tip of South America, and not from deserts in southern Africa, Australia and Antarctica itself. The study provided a linchpin for climate modelers reconstructing past atmospheric circulation patterns in the southern hemisphere.
Unlike Antarctica, Greenland is surrounded by many more land masses that were potential source areas for dust. Dr. Biscaye said the researchers were surprised to find that the dust came from a single source and one of the most distant ones, at that.
The scientists also found that the isotopic signature of the Greenland dust did not change over the 3,000-year span. That casts doubt on the theory that the periodic increases in dust in the Greenland and Antarctic ice cores were caused when the source areas of the dust grew larger, creating more dust available for transport. The Asian source area could not have enlarged without encompassing rocks and soils with different isotopic and mineralogic compositions - leaving stronger winds carrying more dust as the best explanation, Dr. Biscaye said.
The research was supported by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the French Centre Nationale de Recherche Scientifique.
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