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Written in stone

点击量:   时间:2019-02-26 10:17:01

By Adrian Barnett NEW light is being shed on the history of Greek limestone monuments thanks to a technique normally used to date fossil soils. The usual way to assess the age of ancient buildings relies on artefacts found in or near them. Carbon dating these items can pin down when a building was being used. But the artefacts may have been left behind after building work was completed, so the true age of a monument can remain uncertain. Now, using a phenomenon called thermoluminescence, researchers are getting much more precise dates. In 1663, Robert Boyle noticed that transparent crystals such as feldspar, calcite and quartz emit bursts of light of particular colours when they are heated. This phenomenon was unexplained until the 1950s, when it was found to be caused by electrons that have been excited by alpha, beta and gamma radiation and then trapped in impurities in the crystal. Provided background radiation levels stay constant, the number of captured electrons increases over time. Heating frees the electrons in a burst of visible light of an intensity proportional to the number of electrons. Robert Galloway from Edinburgh University, and Ioannis Liritzis and Pericles Theocaris from the Academy of Athens are now using the technique—previously only used to date fossil soils and marine sediments—to find out the age of building stones made of sedimentary rock. Sunlight also evicts the electrons, so the material on the outer surfaces of the blocks will not thermoluminesce. But the researchers realised that the blocks would have had no light on their internal surfaces since the ancient workers built them into a wall, so crystals extracted from these surfaces would reveal how much time had passed since the day the buildings were erected. Archaeologists have effectively been given a “crystal clock”. The team developed its ideas while studying two mysterious Greek limestone pyramids, one at Hellinikon and the other at Ligourio in the Argolid district of the Peloponnese. The trio decided to try thermoluminescence after testing the technique on pottery shards. Previously, based on the style of pottery found at the site, the pyramids were thought to have been built in the Classical period around 400 BC. But thermoluminescence dated the Hellinikon pyramid from around 2730 BC, while the one at Ligourio was built around 2260 BC. “These pyramids were being constructed at the same time as those in Ancient Egypt,” says Galloway. “They were always something of a stylistic oddity. Now we know why.” Galloway says the new date dovetails neatly with the writings of the Classical scholar Pausanias. In the 2nd century AD he recorded that the Hellinikon pyramid was constructed as a cenotaph for the dead of a fratricidal battle between Proetus and Acrisius, grandsons of Danaüs, King of Argos, and pretenders to the kingdoms of Tiryns and Mycenae. “The period given by Pausanias fits nicely with that derived by our new method,