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Dead Sea Dried Up in Past
7 December 2011 1:32 pm
SAN FRANCISCO, CALIFORNIA—The Dead Sea region has been a center of human activity for hundreds of thousands of years—and the layers of sediments buried beneath the lake, scientists think, hold clues to the changing environment in which those cultures existed. Now, an analysis of sediments drawn from the center of the lake basin reported here this week at the American Geophysical Union's annual meeting reveals that, contrary to what scientists previously believed, the lake once completely dried up.
The study is part of the Dead Sea Deep Drilling Project, a $2.6 million effort begun 10 years ago to recover the longest, most continuous, and best-preserved archive of environmental and seismic information in the Middle East. Following two drilling efforts, one lasting from November 2010 to January 2011, and one in March 2011, project researchers have now extracted a 1-kilometer-long core of sediment from the center of the basin, representing roughly 200,000 years of climate and seismic data for the region. The biggest surprise so far? About 120,000 years ago, the Dead Sea essentially dried up.
Scientists didn't think that would happen. At 425 meters below sea level, the Dead Sea is at the lowest continental elevation on Earth, and it is about 34% saline. That extreme saltiness, scientists thought, would ultimately prevent the lake water from completely evaporating. They were convinced that the lowest the water level in the Dead Sea could sink was about 150 meters below its current level.
But a layer of pebbles discovered in the new core calls that assumption into question, team member Steven Goldstein, a geochemist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, said at a press conference at the meeting. Most of the core is a series of black and white layers of sediment, representing seasonal variations. Dark sediments containing mud and silt from winter floods alternate with summertime sediment rich with white calcium carbonate precipitated from a seasonally shrinking lake. "It's an absolutely phenomenal record," Goldstein said.
Overprinted on those finely detailed, seasonal layers in the core is a similar but larger-scale variation between wetter ice ages and drier interglacial warm periods. "Salt represents the Dead Sea declining and precipitating out the salt, which wasn't happening during the ice ages," Goldstein said. And in the core, about 235 meters down, the layer of pebbles appears. It is likely a beach deposit where the center of the lake used to be. The pebbles sit atop a 40-meter-thick layer of salt. Together, the scientists reported, they reveal an extreme drying event.
It's an important finding—and, significantly, these results are also the product of an international collaboration that includes both Israeli and Jordanian scientists, Ulrich Harms, the executive secretary of the International Continental Scientific Drilling Program, said at the press conference. That collaboration is particularly key in a region where political and societal needs, rather than climatic change, are now diminishing the amount of water flowing into the basin, said Harms, who was not involved with the study. Over the past 80 years or so, countries in the region have increasingly siphoned off fresh water for irrigation from the Jordan River, the primary feeder river into the basin. As a result, the water level in the Dead Sea has been dropping dramatically. In 1997, the lake level was 411 meters below sea level. By 2008, the level had dropped another 10 meters, and by 2010, it had dropped 3 meters farther. However, given the evaporation constraints, scientists thought it would never completely disappear. Now, they say, the new data suggest otherwise.