|Roman mosaic from the 2nd century AD of a ship displaying similar hull shape to the Madrague de Giens wreck. |
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A confrontation among ancient and modern studies is pitting particle physicists seeking concrete evidence of dark matter against marine archaeologists intent on preserving material in centuries-old shipwrecks.
The source of the issue: samples of lead used for anchors and ballast in Roman ships that were sunk up to 2,000 years ago and remain underwater since then.
The ancient lead's purity makes it invaluable today for shielding underground experiments designed to detect evidence of dark matter, the mysterious invisible stuff that, according to physicists, accounts for 85 percent of all the matter in the universe. But some marine archaeologists assert that, as a part of the world's cultural heritage, the lead should stay in place for detailed historical study.
"The use of these objects as stock for experimentation had never been an issue before," wrote Elena Perez-Alvaro, a doctoral candidate in underwater cultural heritage maritime law at England's University of Birmingham, in the university's journal Rosetta
. "But now it is beginning to be deemed ethically questionable."
Both sides of the affair cite strong scientific justification for their use of the lead. "Underwater archaeologists and cultural heritage protection policymakers need to evaluate the value of this underwater lead for future generations," Perez-Alvaro explained. But the lead "is an essential element of state-of-the-art dark-matter searches," added Cambridge University physicist Fernando Gonzalez Zalba, who collaborates with Perez-Alvaro on studying the issue. "These experiments could shed light some of the most fundamental properties of the universe."
There's no shortage of the material. "I personally have seen dozens of lead anchor stocks during our expeditions in the Mediterranean and Aegean," recalled Brendan Foley of the Woods Hole Oceanographic Institution's Deep Submergence Laboratory, in Massachusetts.
For archaeologists, studying those stocks has value far beyond understanding ancient metallurgical methods. The pieces of lead "are marked with indicators of where they came from," said James Delgado, director of maritime heritage at the National Oceanic and Atmospheric Administration in the United States. "That helps us to reconstruct ancient economies and global trade."
Physicists have inferred the existence of dark matter by observing its gravitational influence in distant galaxies. But they don't know what it consists of. Among the most popular candidates are entities called weakly interactive massive particles, or WIMPs.
Theorists believe that, although WIMPs are about the size of atomic nuclei, they scarcely interact at all with any other forms of matter. "Very occasionally one of them will bump into a nucleus and rattle it around a bit," explained Daniel Bauer, project manager of the Cryogenic Dark Matter Search, or CDMS. "Our detectors are set up to measure the recoil of the nucleus when that happens," he added.
It doesn't happen often. "Nobody has yet had a completely confirmed sighting," Bauer said. Their detectors are sensitive to a rate of one incident per year.
Because the bumps happen so infrequently, CDMS has designed its experimental setup to minimize false positives. To avoid cosmic rays, the team has buried its detectors half a mile deep in a mine in Minnesota. It also shields them with copper, plastics, water, and, most important, lead.
"Lead is the material of excellence as a shielding material in radiation-rich environments," said Gonzalez Zalba, who does not work directly on dark-matter experiments. "Its low intrinsic radioactivity, good mechanical properties, and reasonable cost make it an excellent shielding material."
However, recently mined lead has one disadvantage. "Uranium and thorium that coexist with lead will leave a fair amount of the radioactive isotope lead-210 in it," Bauer noted. "In our experiments, even tiny amounts of radioactivity can lead to false signals. We want the purest possible material to shield the experiment from radioactivity."
That means lead mined a long time ago and preserved under water. "There's no chance that uranium and thorium are nearby," Bauer continued. "And since its decay half life is about 23 years, its radioactivity has basically gone." The ancient lead has over 1,000 times less radioactivity than modern lead.
The CDMS team bought its ancient lead from French company Lemer Pax, which had salvaged it from a Roman ship sunk off the coast of France. Later, the company "got in trouble with French customs for selling archaeological material," Perez-Alvaro reported.
"We assumed that this company was reputable, and I would believe that to be true," Bauer said. "They're still selling lead. That's the best evidence that everything is in order."
Another underground experiment, the Cryogenic Underground Observatory for Rare Events in Italy, also uses Roman lead. A museum gave it 120 archaeological lead bricks from a ship built more than 2,000 years ago and recovered in the early 1990s off the coast of Sardinia.
Marine archaeologists don't want to deny physicists the use of the ancient lead. But they fear that such use could help to commercialize the salvage of ancient shipwrecks.
"It's another example of something from a shipwreck that has value and will encourage an approach to shipwrecks that won't be available for careful meticulous study. Science and archaeology go out of the window in the quest for profits," Delgado said. "The issue is the salvaging and selling of the lead; that's where archaeologists say 'Wait a minute.'"
The 2001 UNESCO convention for the protection of the underwater cultural heritage preserves the Roman lead and other ancient artifacts from any use that would damage them. "However," Perez-Alvaro explained, "there is no reference anywhere to the use of shipwrecks for the purpose of experimentation – new uses of underwater cultural heritage."
Nevertheless, archaeologists and physicists see opportunities for agreements that would protect the ancient lead's heritage while still benefiting dark-matter searches. "It's all right if it's been documented – like taking a bit of DNA and putting it in the DNA bank," Delgado suggested. "That's a respectable scientific process that benefits all branches of science."
Gonzalez Zalba agreed. "We follow the idea of 'salvage for knowledge and not for the marketplace,'" he said. "Dark-matter searches follow under the idea of research for knowledge. Therefore I believe the resources should be granted if required under the adequate regulation and archaeological supervision."
Perez-Alvaro calls for a formal route to regulation. "There is a need for dialogue between the two fields," she said. "Especially there is a need for a protocol [on the acquisition and use of ancient lead] set up by archaeologists."
"Archaeologists will always view as unethical the outright sale of artifacts recovered from cultural sites," Foley added. "But other creative solutions could be devised which would be win-win for physicists and archaeologists."
- Peter Gwynne, Inside Science News Service
A former science editor of Newsweek, Peter Gwynne is a freelance science writer based in Sandwich, Massachusetts.