Our second feature celebrating 70 years of 14c dating
In our second feature celebrating 70 years of radiocarbon dating, Mike Pitts looks at how old trees came to the rescue of an unexpected problem
As I explained in my previous feature (Nov/Dec 2018/163), the origins of carbon dating lay in the Second World War us Manhattan Project and the development of atomic bombs. Willard Libby, a Nobel prize-winning chemist, had a vision that the carbon-14 isotope his lab had discovered could be used to determine the age of historic artefacts. In the early 1950s he successfully showed that measuring the amount of radiocarbon in objects of known antiquity, mostly wood from ancient Egypt, more or less accurately, if not very precisely, predicted their age.
As radiocarbon dates accumulated around the world, archaeologists grew more confident with the “mysterious boffinry”, to use Colin Renfrew’s phrase. There was growing concern, however, that something seemed to be wrong. On one occasion in 1959, Glyn Daniel noted that many of the results were, “to put it mildly… at variance with archaeological dating”. In northern Europe the dates were “shaking”, adding a millennium of age to a Neolithic era that conventionally began in 2000bc and lasted only a few centuries.
Nonetheless, by the mid 1960s archaeologists were beginning to get used to the idea of scientific dating and the effect it often had of stretching out their old chronologies, which had inevitably relied on an element of guesswork. And then a huge spanner fell into the works. One of Libby’s original assumptions turned out to be wrong. But scientists figured out an ingenious way of adapting to the new
understanding. Radiocarbon dating underwent its first revolution, and scored its first major hit in changing the way we think about human history.
Now archaeologists really did have something to worry about.
Counting tree rings
Early studies seemed to support Libby’s theory that the level of atmospheric radiocarbon had been constant for the past 5,000 years, a necessary assumption for the dating process. But there was a problem. The fit between predicted and actual carbon dates was rarely spot on (even allowing for standard deviation ranges of two or three centuries). And it was curious how carbon dates for apparently known-age Egyptian Old Kingdom artefacts seemed always to be a few centuries too young. There was a need for testing with greater precision.
The solution lay with tree rings. Trees add a new growth ring just under the bark every year. Museum visitors are familiar with slices of trunks engraved with historic events, showing the size of the tree at the time: the American Museum of Natural History in New York and the London Natural History Museum display sections of the same giant sequoia (known as the Mark Twain tree), cut down in 1891. If you knew a tree’s felling date, you could count back through the years, one ring at a time, and establish precisely when a narrow strip of wood was growing. You could then carbon date that strip, and compare the result with the known age. The Mark Twain tree was born in ad550. There are older trees, also growing in California, high in the dry, subalpine White Mountains. Here can be found gnarled, almost deadlooking things known as bristlecone pine. There are three species. A Great Basin bristlecone pine ( Pinus longaeva) is currently said to be the oldest confirmed living organism on Earth, at (in 2019) 4,852 years.
Even that is not the limit for counting rings, however. Trees are affected by vagaries of climate and weather, so that rings will be thinner or thicker depending on conditions during the growing season. Within a given region and for a particular species, this creates consistent ring patterns across forests. That means any number of pieces of wood can be matched up against each other. Old wood in a tree that began growing a thousand years ago, for example, might be aligned with young wood from a tree that died 900 years ago – but itself lived for a thousand years, so reaching back a further nine centuries before the first tree. In this way, if suitable fossil wood is available (a key source turned out to be Irish bog oak), a ring sequence can be extended back into the past far beyond the age of any one tree. The idea had been proposed as long ago as 1837, by the English mathematician and computer inventor Charles Babbage. “The application of these principles i l to ascertaining the age of submerged forests, or to that of peat mosses,” he wrote with characteristic foresight, “may possibly connect them ultimately with the chronology of man.”
If this worked with trees preserved in English costal peats or Irish bogs, it applied too in the Californian White Mountains, where dead bristlecone pine wood had been found of greater age than living. With cross-checks to avoid confusion from missing or duplicated rings, dendrochronology, as the technique is known, has now created thousands of sequences around the world, which together reach back nearly 12 millennia. Just as cosmic rays in the outer atmosphere had for eternity been creating carbon-14, messengers which one day people would use to help read their forgotten story, so on the ground, around the world, trees had been storing up a record of what those rays had achieved year by year. It just needed someone to open up the book.
Something wrong
Inspired by a brilliant Dutch physicist called Hessel de Vries, in 1958 scientists at Cambridge University took a large chunk of sequoia wood from its display at the entrance lobby to the Botany School, and drilled it with holes (this was done from behind, and the polished slice, back in place, now looks good as new). The samples were carefully
trimmed so that each one covered 50 years’ worth of rings, and then analysed by the British radiocarbon lab, and again by their colleagues in Heidelberg, Germany, and Copenhagen, Denmark.
They could consider only the 1,300 years of the tree’s life, but the results showed a discrepancy: for much of that time, radiocarbon underestimated the sequoia’s age as indicated by its rings. This could mean only one thing. A key assumption in Libby’s theory – that the amount of radiocarbon in the Earth’s atmosphere was always the same – was wrong. Scientists soon realised that historic variations in atmospheric radiocarbon were not just random or cyclic, wobbling around a fixed level. It could be seen that before 2,000 years ago, the basic level itself was higher, and became increasingly so with time. A higher level of radioactive carbon in the atmosphere compared to the present, meant that it would take longer for absorbed radiocarbon in an archaeological sample to fall to the predicted amount than had the level been constant. Laboratory dates would need to be adjusted. These adjustments, which increased the calendar age of computed dates, became known as calibration. Then, as Colin Renfrew has described it, “assumptions which [had] sustained prehistoric archaeology for nearly a century” collapsed.
Up to this point, carbon dating in Europe and the Near East before 1000bc – where there was most interest from archaeologists, and most argument – had two notable features. On the one hand, for most of the region, science made things older than they were thought to be, with the effect that the past became bigger by as much as 3,000 years; the various cultures and eras that archaeologists had defined had more time to grow. On the other hand, in
Egypt, where there was already direct evidence for chronology in the form of ancient king lists, carbon dates seemed to point in the opposite direction – they were too young.
The first calibrations are associated with Hans Suess, an Austrian nuclear physicist and geochemist who moved to the us after the Second World War. His graphs had a dramatic impact on this already unsettling picture. First, they solved the Egyptian problem. Suitably adjusted, ancient Egyptian radiocarbon dates were seen to fall nicely into place, matching the historical record. Further north and west, where no histories had ever existed that far back, archaeologists were (mostly) starting to get used to the longer chronologies that radiocarbon dating had brought. Now it happened again: things got even older. Only this time, as Renfrew was quick to recognise, the effect was not just a surprise. It was revolutionary.
Liberating the past
Before science entered the fray, archaeologists had relied on their creative judgments to assign dates to events in European prehistory. Nineteenth-century antiquarians defined successive ages of stone, bronze and iron. The first of these could loosely be related to ice age chronologies computed by geologists (though these too involved much guesswork). The tools and weapons that characterised the metal ages could be compared to artefacts excavated around the eastern Mediterranean, where histories of sorts reached further back. Bronze daggers of similar design in England and Greece, it was supposed, showed a connection that implied synchronicity – or if, as was commonly assumed, the English design was influenced by the superior Greek version, the hf former ermight might be a century or so younger than the latter.
It was a simple device. But worked across the complex ancient cultures of the European continent by a myriad of independent scholars, writing in a variety of languages, the result was anything but simple. The edifice of prehistoric Europe came steadily to resemble a playing-card castle decorated with turrets and flying bridges, one lost people’s story propping up another’s, often with little basis in sound archaeology.
Arriving at Cambridge University in the year of the sequoia drilling, Renfrew first studied natural sciences (believing that “physics was the way to understand the world”), and then archaeology – both subjects he’d become interested in at school. His research took him to the Mediterranean, where he soon came to doubt some of the prehistoric connections archaeologists had been making across Europe. And then along came radiocarbon calibration. The effects were spectacular. Suess’s graphs turned a lab date of 2400bc into a calendar age of 3000bc – and the gap widened as you went further into the past.
Renfrew realised that this pushed northern Europe so far back, that the chronological links to the east favoured by traditional archaeologists finally snapped. Stonehenge, for example,