Resetting the Carbon Clock

Most archaeologists have looked upon the peoples of prehistoric Europe as no more than primitive barbarians. True, certain prehistoric monuments, like Britain's Stonehenge--whose great slabs are now thought to have formed a sophisticated solar observatory--did indicate a high order of culture. But such structures were usually ascribed to the influence, if not the actual workmanship, of skilled migrants from the much more civilized areas of the Near East. Now, in a surprising about-face, archaeologists are sharply questioning their old assumptions about the cultural inferiority of early Europeans. What has prompted this major reassessment is a change in archaeology's key dating tool: the so-called carbon 14 "nuclear clock."

Introduced by the American chemist

Willard Libby two decades ago, the clock depends on the decay of carbon 14, a radioactive isotope of ordinary carbon 12, which is nonradioactive and stable. Both forms of carbon are found in all living things, and their proportion remains constant during the life of the organism. New carbon of both forms is continuously added through normal metabolic processes. But when the organism dies and the intake of fresh material stops, this ratio of carbon 14 to carbon 12 begins to change. The amount of carbon 12 stays the same, but the unstable carbon 14 begins to disintegrate. The radioactive decay occurs at a regular and predictable rate, like the flow of sand through an hourglass. Thus by measuring the ratio of the carbon 14 to carbon 12 in a rafter, say, or in a bone, or in seeds found in a clay pot, scientists can calculate the age of ancient objects.

Archaeologists were delighted with the new technique, which brought Libby a Nobel Prize. By using it to date artifacts of questionable vintage, archaeologists found that it lent fresh support to one of their pet theories--that there was a gradual diffusion of culture from the advanced Near East to barbarian Europe. There were a few puzzling exceptions: Stone Age tombs in Brittany, for example, were found to date back to at least 3000 B.C. Yet the oldest comparable tombs in the eastern Mediterranean--built by the Minoans on Crete --were known indirectly from actual historical records to date from only 2500 B.C. But except for a few iconoclastic prehistorians like Britain's Colin Renfrew of Sheffield University, most archaeologists remained thoroughly convinced "diffusionists." If a few prehistoric European monuments or artifacts happened to show unusual antiquity, they contended, it was the carbon 14 clocks that were in error, and not their well-entrenched ideas.

Now the debate is rapidly being resolved. When Libby first proposed carbon 14 dating in 1950, he assumed that the total world supply of the isotope was about the same as it had been in the past. Reason: carbon 14 is being produced continuously in the atmosphere as cosmic rays bombard the earth from deep space and leave a trail of atomic debris. But as the number of puzzling carbon 14 dates increased, scientists at the universities of Arizona, Pennsylvania and California began testing Libby's assumption by turning to some of the oldest living things on earth--California's bristlecone pine and sequoia trees, which have been growing for as long as 4,000 years. By carefully analyzing the carbon 14 content in the annual growth rings of the trees, they found that there have, in fact, been small but significant changes in the isotope's production over the centuries, apparently as a result of variations in the cosmic-ray bombardment.

Before the Pyramids. Working with hundreds of ancient wood samples, Geochemist Hans Suess of the University of California at San Diego recalibrated archaeological ages derived by carbon 14 dating in all parts of the world. His corrections did not affect the commonly accepted dates of Near East events and artifacts, which have been largely deduced from ancient calendars. But they did show that carbon-14-based European dates before 1500 B.C. must be adjusted by the addition of as many as 700 years.

Those adjustments have already stirred what Renfrew calls a major revolution in archaeological thinking. Brittany's tombs, for example, are now acknowledged to have been built at least a millennium before the first stone tombs in the eastern Mediterranean and 1,500 years before the first pyramids. The revised carbon 14 dates have also shown that skilled coppersmiths may have been at work in the Balkans and possibly prehistoric Spain even before the Greeks managed to master the metallurgical arts. "The central moral is inescapable," Renfrew recently wrote in Scientific American. "We have completely undervalued the originality and creativity of prehistoric Europe."

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