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Empires in the dust
Discover, March, 1998 by Karen Wright
DeMenocal told Weiss that if a large-scale drought had in fact occurred, it would have left a mark in the sediments of nearby ocean floors--the floor of the Gulf of Oman, for example. Lying approximately 700 miles southeast of ancient Mesopotamia, the gulf would have caught any windblown dust that swept down from the Tigris and Euphrates valleys. (The Persian Gulf is closer, but because it's so shallow, its sediments get churned up, thereby confusing their chronology.) And deMenocal just happened to know some German scientists who had a sediment core from the Gulf of Oman.
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Analysis of the gulf core is ongoing, but deMenocal has already extracted enough information to confirm Weiss's suspicions. To track dry spells in the sediments, he and his colleague Heidi Cullen looked for dolomite, a mineral found in the mountains of Iraq and Turkey and on the Mesopotamian floodplains that could have been transported to the gulf only by wind. Most of the Holocene section of the core consists of calcium carbonate sediments typical of ocean bottoms.
"And then all of a sudden, at exactly 4,200 calendar years, there's this big spike of dolomite," says deMenocal--a fivefold increase that slowly decays over about three centuries. The chemistry of the dolomite dust matches that of the dolomite in the Mesopotamian mountains and plains, verifying the mineral's source. And not only did deMenocal and his colleagues figure out what happened, they may have figured out how. Studies by Gerard Bond at Lamont-Doherty have shown that the timing of the drought coincided with a cooling period in the North Atlantic. According to a survey by Cullen of current meteorological records, such cooling would have dried out the Middle East and western Asia by creating a pressure gradient that drew moisture to the north and away from the Mediterranean.
"The whole disruption, collapse bit, well, I just have to take Harvey at his word," says deMenocal. "What I tried to do is bring some good hard climate data to the problem." Why hasn't anybody seen this signature of calamity before? Simple, says deMenocal. "No one looked for it."
Weiss's first hints of climate-associated calamity came from a survey of his principal excavation site, a buried city in northeastern Syria called Tell Leilan. Tell Leilan (rhymes with "Ceylon") was one of three major cities on the Habur Mains to be taken over by the Akkadian Empire around 2300 B.C. The city covered more than 200 acres topped by a haughty acropolis, and was sustained by a tightly regulated system of rain-fed agriculture that was co-opted and intensified by the imperialists from the south. Weiss had asked Marie-Agnes Courty of the National Center for Scientific Research in France to examine. the ancient soils of Tell Leilan to help him understand the agricultural development of the region. She reported that a section dating from 2200 to 1900 B.C. showed evidence of severe drought, including an eight-inch-thick layer of windblown sand and a marked absence of earthworm tunnels.
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