by Carolyn Gramling Monday, October 26, 2015
Like sentinels standing guard over a millennia-old secret, the 8-meter-tall stones of Stonehenge rise above the rolling green hills of England’s Salisbury Plain. The origin, date and purpose of the arrangement of the giant standing stones, located about 145 kilometers west of London, have puzzled people for thousands of years. But in 1952, physical chemist Willard Libby, a professor at the University of Chicago in Illinois, finally provided a concrete answer to one of the site’s most enduring questions: when it was built. To do this, Libby used a brand-new geochemical technique that he had been developing based on the radioactive isotope carbon-14. Only a few years later, his work on this groundbreaking technique earned him a Nobel Prize for Chemistry.
By the end of World War II, Libby was already a high-profile figure. He had played a key role in the Manhattan Project, creating a technology to enrich uranium-235 that became a critical part of the development of the atomic bomb that was later dropped on Hiroshima. When the war ended in 1945, Libby headed to the University of Chicago, and within a few years began investigating a way to use the radioactive decay of carbon-14 isotopes in organic material to determine the material’s age.
Carbon-14 was already a hot topic: Just before the war, scientists had realized that carbon-14, or radiocarbon, was produced high in the atmosphere as a byproduct of the collisions of cosmic rays with atmospheric nitrogen. These collisions had been happening for a very long time relative to the half-life of carbon-14 (the length of time it takes for half of the atoms of the isotope in a material to radioactively decay, about 5,730 years). As a result, the reaction that forms carbon-14 in the atmosphere had long since reached equilibrium, so that a predictable amount of carbon-14 was consistently forming and was also consistently being exchanged between Earth’s atmosphere, oceans and biosphere. Scientists could therefore roughly calculate how much cosmically produced carbon-14 there was throughout the planet. The question, Libby later noted in his Nobel Prize acceptance speech, was how scientists could make use of this knowledge.
One aspect of carbon-14’s usefulness soon became clear. While still living, all organisms on Earth, from humans to trees, are in equilibrium with the planet’s atmospheric carbon-14 concentration. But when an organism dies, its radiocarbon “clock” starts ticking as it slowly falls out of equilibrium: It stops taking in new carbon-14 through food, while the radiocarbon it already contains continues to decay. Libby realized it would be possible to go back and calculate the age of any carbon-containing artifact using this known information: the half-life of carbon-14, the remaining carbon-14 in the artifact and the carbon-14 content in the atmosphere when the organism was last “alive.”
Developing such a tool required first figuring out how to detect radioactivity, and next, how to measure it. Libby and his colleagues came up with a technique using a Geiger counter to inexpensively and accurately count each decay of an atom. By 1947, they were ready to test the method on materials of known ages spanning thousands of years of human history: the 3,000-year-old heartwood of an ancient redwood tree cut down in 1874; the 2,000-year-old linen wrapping of one of the Dead Sea Scrolls; carbonized bread from Pompeii, charred by the eruption of Vesuvius in A.D. 79; and timber samples from ancient Egyptian tombs or from the deck of an Egyptian king’s funeral barge. Libby plotted his radiocarbon-determined ages for these artifacts of already known age on a chart, calling it the Curve of Knowns.
The tests matched well to the known ages for these artifacts and showed that the carbon-14 method was so reliable that it could be used on objects as old as 30,000 years (later extended to 70,000 years). So Libby and his team began to use their dating method on prehistoric artifacts of unknown age. In 1952, they turned to Stonehenge, uncovering the charcoal remains of a campfire at the site. Those remains were dated to 1848 B.C., plus or minus 275 years.
In 1960, Libby received a Nobel Prize for developing what is now an essential dating tool for a broad range of scientists, from geologists to archaeologists to oceanographers. One of the scientists who nominated Libby for the prize noted, “Seldom has a single discovery in chemistry had such an impact on the thinking of so many fields of human endeavour. Seldom has a single discovery generated such wide public interest.” In his own Nobel Prize acceptance speech, Libby noted that radiocarbon dating “may indeed help roll back the pages of history and reveal to mankind something more about his ancestors, and in this way, perhaps about his future.”
Investigations of Stonehenge’s history, however, did not end with Libby’s date. Over the ensuing decades, scientists have found evidence of multiple phases of activity at the site, some going back nearly 8,000 years. The current arrangement of the megaliths of Stonehenge — a ring of sarsen sandstones surrounding an arrangement of bluestones — represents only the most recent of these phases of construction, many of which occurred essentially on top of one another. In 2008, scientists returned to Stonehenge to once again date the site. British archaeologists Tim Darvill and Geoff Wainwright led the first new dig at Stonehenge since 1964, intending to determine both new dates and gain a better understanding of Stonehenge’s ancient function (they contend it was a religious center and place of healing).
Focusing on a small patch of earth between two of the giant sarsen stones, Darvill and Wainwright uncovered about 100 pieces of organic material from the earliest “sockets” — the holes in which earlier stones had been placed — that had been buried beneath the current arrangement of stones. They determined a date of about 2300 B.C. for the first erection of stones at Stonehenge.
Those earlier stones, it was determined in the 1990s, were bluestones transported from the Preseli Mountains of south Wales, more than 200 kilometers away. How they were brought to the Salisbury Plain has provoked an ongoing ideological debate between archaeologists and geologists: The former suggest the stones were transported by people over land; the latter suggest that glaciers carried the stones to their current site. Reporting in EARTH in January 2009, geologists Brian S. John of the University of Durham in England and Lionel E. Jackson Jr. of the Geological Survey of Canada in Vancouver furthered the geologists' case for glacial transport of the Stonehenge stones by using a Canadian analogue: They described how glaciers in the Canadian Rockies acted as a conveyor belt for similarly sized stones during the last glacial maximum, transporting them nearly 600 kilometers through modern Alberta to the U.S.-Canada border.
That dispute is only one of the many questions still surrounding the mysterious site, not least of which is its actual purpose. But however the stones arrived, Libby’s initial date for Stonehenge was the first key that began unlocking the mystery.
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