Dec. 29, 2008 -- In a dripping, stalactite-filled cave outside of Jerusalem, geologists have unearthed clues about what happened to a bustling civilization more than a millennium ago.
The researchers used a new technique to figure out exactly how much rain fell in the Eastern Mediterranean between about 1,000 and 2,000 years ago. In some stretches, the scientists were able to pinpoint what the region's weather was like from one week to the next -- by far the most detailed climate history ever produced.
The results showed a gradual drying between about 100 and 700 A.D., with sharp drops in rainfall at 100 A.D. and 400 A.D. Overall, annual rainfall fell 50 percent during those centuries, dropping from an average of more than 3 feet per year to 1.6 feet.
During the same period, Roman rule declined in the area. This is the first study to link the two events.
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"Such a large change in rainfall may have played an important part in the historical events that took place in that region at that time," said Ian Orland, a Ph.D. candidate in geology at the University of Wisconsin, who co-led the study.
As their crops suffered, the Romans probably began to struggle until finally succumbing to the growing Islamic empire at the Battle of Yarmouk in 636 A.D.
The new insights were especially surprising because the scientists weren't expecting to find anything interesting. Their study began simply as an attempt to test the capabilities of a tool called an ion microprobe.
The device uses a laser to excite the organic particles within an object. Like an astronomer's mass spectrometer, the resulting signature reveals details about the chemical composition of the substance.
With the tool's precision, the researchers were able to look at single layers of stalactite that were just 1/100th of a millimeter thick--100 times thinner than what scientists can analyze with standard techniques.
Like a tree's growth rings, stalactites grow in layers from the top of a cave downward. In each layer, a preserved chemical signature called the oxygen isotope ratio reveals whether a particular period was especially wet or dry.
Orland and University of Wisconsin geologist John Valley used a new generation ion microprobe to analyze a stalactite sample form Israel's Soreq Cave, one of the best-studied caves in the world.
Compared to standard methods, the new technique revealed four times as much variability in rainfall during the period covered by the sample -- from 2,200 to 900 years ago. The drastic change in climate would have had a profound effect on the people living in the region, the researchers speculate.
"It's the first time they've put together this kind of high-resolution record," said Larry Edwards, a geochemist at the University of Minnesota, Twin Cities. "There have been other measurements of this sort done, but they haven't been as accurate."
Future work will attempt to confirm these results with other samples and look for similar data in other regions. Eventually, Orland said, building a detailed database of past climate should help scientists refine their models of future climate change.
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