Refining radiocarbon dating

Professor of Archaeological Science and Deputy Director of the Oxford Radiocarbon Accelerator Unit Tom Higham explains the science behind radiocarbon dating and how he has refined this dating technique for archaeological research on ancient bones.

Nature of science

In this video, Tom says, “Radiocarbon has a half-life of 5,568 years”, yet in other places on the Science Learning Hub, we refer to radiocarbon as having a half-life of 5,730 years (this is known as the 'Cambridge half-life'). Both are in essence correct. Basically, calculating radiocarbon ages requires the value of the half-life for carbon-14. Nearly a decade after Willard Libby’s initial work to develop this method, the half-life was revised from 5,568 to 5,730 years. This meant that many calculated dates in papers published prior to this were incorrect. For consistency with these early papers and to avoid the risk of a double correction for the incorrect half-life, radiocarbon ages are still calculated using the incorrect half-life value of 5,568 years. A correction for the half-life is now incorporated into calibration curves, so even though radiocarbon ages are calculated using a half-life value that is known to be incorrect, the final reported calibrated date, in calendar years, is accurate.

Learn about developments in radiocarbon dating in our Athol Rafter heritage scientist timeline.

Transcript

PROF TOM HIGHAM

My expertise is in dating and archaeological dating using radiocarbon. Radiocarbon dating is a fantastic technique, but when you get down to 30 to 50,000 years ago, you’re dealing with much, much smaller amounts of radiocarbon than you are in the present day. So for those of you that know a little bit about it, radiocarbon has a half-life of 5,568 years. That means that, every 5,568 years, half the radioactive carbon in the bone, or the charcoal that we find in an archaeological site, has decayed back to its parent isotope, nitrogen-14.

So basically 10 half-lives is the limit of any radioisotopic dating method. So when you get down to 30,000 years ago, you’ve got about 3% of the amount of radiocarbon that you’ve got in the present day, just 3%. When you get down to 50,000 years ago, you’ve got 0.1% – that’s a tiny amount. As a consequence of this, small amounts of contaminating carbon can really affect the reliability of the dates unless they’re removed. So in the case of a bone, for example, that is 50,000 years of age, if you have 1% contamination with modern-day radiocarbon, you’ll produce a date that is over 7,000 years too young. And so this experience of ours in knowing about the contamination issues led us to be somewhat sceptical about some of these radiocarbon dates and these estimates here, because we were worried that some of them could well be underestimates. So what we decided to do was to try and address these issues by a new programme of dating.

This is the radiocarbon accelerator in Oxford, where I work, and it’s a £2.5 million piece of kit that enables you to date really small pieces of bone, really small pieces of charcoal. We’re talking here about dating about a milligram of carbon – that’s really a tiny amount. And as a result of this extremely small size requirement, we’re able to date things using much better pre-treatment chemistry designed to remove the contamination that otherwise would affect the reliability of our dates. And so over the last few years, we’ve been working quite hard on improving this pre-treatment chemistry. So for example, what we do is we take a bone like this, we drill the sample with a dental drill, get about half a teaspoon-sized aliquot of bone, and then we extract the collagen from the bone, and this is – this little stuff here that you can see on the end of the set of tweezers – it looks a bit like cotton wool – this is the protein fraction of your bone. About 20% of your bone is protein, and the rest of it is a mineral cement. And the protein is the best material that we can get for dating.

So what we do in Oxford is we use something called an ultrafilter. We put the proteins into this filter – they’re in a liquid form – and then we zoom them around in a centrifuge, and the centrifuge and the filter together act to remove small molecular weight contaminants that come out of the collagen. We trap most of the collagen above the filter, and this is what we end up radiocarbon dating. And we started using this in 2001, and we noticed immediately some big differences between dates previous to ultrafiltration and post-ultrafiltration. With the ultrafilter, we started to get dates that were a lot older when we redated samples that we thought were a little bit too young.

The Science Learning Hub would like to acknowledge: Professor Tom Higham, University of Oxford The Allan Wilson Centre for Molecular Ecology and Evolution Images of Tom with skeleton, skulls in lab and protein fraction of bone courtesy of Professor Tom Higham, University of Oxford Colour map of distribution of Neanderthals, Eric Delson and Katerina Harvati, Palaeoanthropology: Return of the last Neanderthal, Nature, 443, 762-763 (19 October 2006) | doi:10.1038/nature05207, published online 13 September 2006 Images of radiocarbon dating laboratory at Oxford University, James King-Holmes/SciTech Images Footage of Tom Higham drilling bones, courtesy of National Geographic Creative Ultrafilter tube going into centrifuge, New on the Market, Nature Biotechnology, 20, 1277-1279 (2002), doi:10.1038/nbt1202-1277