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Pottery shows Nile delta dropping fast.

Ancient pottery fragments discovered by chance near the mouth of the Nile reveal that parts of this agriculturally important delta are sinking substantially faster than scientists had thought, a finding that bodes ill for the future of Egypt's breadbasket.

The delta is dropping in part because the massive load of sediment laid at the mouth of the Nile causes Earth's crust to sag there. Called subsidence, this process occurs at most major river deltas. Natural floods normally rebuild the sinking land surface by depositing new sediment, but the Nile's Aswan dams have stopped the annual floods and shut off the source of replenishing silt.

To gauge the rate of subsidence across the Nile delta, Daniel Jean Stanley of the Smithsonian Institution in Washington, D.C., and Andrew G. Warne of the U.S. Army Corps of Engineers in Vicksburg, Miss., have drilled 100 holes in the region, collecting cores of the layered sediments deposited on the delta. Because the layers record where sea level was millennia ago, the researchers can determine the speed of subsidence by dating the sediments. In the April 30 SCIENCE, Stanley and Warne reported that carbon-14 dating technique suggested that various parts of the delta were sinking 1 to 5 centimeters per decade.

That now appears an underestimate, according to archaeological dating of potsherds preserved within one of the cores. The shards date to various periods of Egyptian history from 3,600 to 2,000 years ago. In all cases, the shards appear significantly younger than the carbon-14-dated sediment layers in which they were found, Warne and Stanley report in the August GEOLOGY.

For various reasons, researchers regard dates obtained through archaeological methods as more reliable than those determined by the carbon-14 method. In some of the Nile cores, the carbon-14 dates record the much greater age of organic matter preserved in the younger sedimentary layers.

The revision in dates suggests that the site where the shards were preserved is sinking more than 1-1/2 times faster than previously estimated. The authors suggest that calculated subsidence rates at other sites are also too low. The effects of subsidence combine with those of rising sea levels, which are currently climbing 1 centimeter per decade and could accelerate in the future.

The combined effects of subsidence and sea-level rise create erosion along the coast and threaten to inundate some delta areas in the next century, Stanley says. Even more important, he adds, the amount of arable land will shrink as saline groundwater seeps southward and salts accumulate in the surface soils.
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Title Annotation:sinking land and rising water
Author:Monastersky, Richard
Publication:Science News
Date:Aug 7, 1993
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