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. …