[MUSIC] What if the human remains that have been found are really old? What technique can we use for dating them? The answer is a technique called carbon-14 dating. Now, the element carbon is ubiquitous in living organisms, it's the basis of all life on Earth. Now, carbon comes in three forms. By far, the most common is carbon-12, this is the isotope carbon-12. So in carbon-12, the nucleus of the atom is made up of 6 protons and 6 neutrons, and it's a quite stable nucleus. A very small amount of carbon is carbon-13, and in carbon-13, the nucleus of the atom is made up of six protons and seven neutrons, and again, this is a perfectly stable atom. But there's a small amount of carbon-14, and in carbon-14, the nucleus is made up of six protons and eight neutrons and this is an unstable nucleus. That means carbon-14 is radioactive. And if you have a sample of carbon which contains 12, 13, and 14, gradually, slowly over time, that carbon-14 will decay and disappear. Now, radioactive decay occurs at a very fixed and regular rate, and for carbon-14, we know what the rate is and we measure that the rate by the so-called half-life. A half-life is the time it takes for half the atoms in your sample to undergo radioactive decay. For carbon-14, that is just over 5000 years, so in 5000 years, half your carbon-14 will disappear. In another 5000 years you'll be down to a quarter, and another 5000 years down to an eighth, and so on and so on and so on. Now, how can we use this for dating? The key assumption is, the amount of carbon dioxide which is carbon-14 in the atmosphere is approximately constant. The carbon-14 is produced by the bombardment of the atmosphere by cosmic rays. As long as the bombardment by cosmic rays is constant, then the formation of carbon-14 is going to be constant. Now where does the carbon-14 go? It's present in the atmosphere in carbon dioxide. So whenever a plant absorbs some carbon dioxide from the air, it absorbs that amount of carbon-14, and that carbon-14 will then make its way through the food chain and end up in herbivorous animals, carnivorous animals, us, anything made of wood, et cetera. So we continually intake small amounts of carbon-14 from the diet, and of course, we give out carbon-14 when we breathe, which means it reaches a steady state. So in a living organism, the amount of carbon-14 is at a steady state during life, but of course when you die, all this stops. But the decay of carbon-14 through radioactive processes doesn't stop. So after death, the amount of carbon-14 drops with radioactive decay at a very nice and fixed mathematical rate. So Willard Libby, who won the Nobel Prize in 1960, developed this as a dating method and he was able to calibrate it against historical objects, so we can have a great deal of confidence in this method. The limitation on carbon-14 is that the object you're studying must be more than a few centuries old, more than, say, 400 years old. And the reason for that is very simple, because if something is only 100 or 200 years old, the amount of carbon-14 that is decayed is too small for us to measure accurately. So if something is 500 years old, then we can get a good measurement; if something is 10,000 years old, we can also get a good measurement. Future archaeologists are going to have a problem, and that is because the assumption of the carbon-14 in the atmosphere being constant has now broken down, and this is due to testing of atomic bombs in the atmosphere back in the 50s and the 60s. Those atomic bomb explosions produced all sorts of isotopes including carbon-14. And you can see on the graph here that prior to atom bomb testing, the carbon-14 level was nice and steady, and then it jumps up to a big spike because of atom bomb testing, and is slowly dropping away, but it's going to take some time for it to get back to normal. What's this got to do with forensic science? We're talking about things here that have been dead for centuries and centuries. Well, let's look at the story of Peter Reyn-Bardt. It was suspected that Peter Reyn-Bardt murdered his wife Malika back in 1960, but there was no evidence. More to the point, there was no body, her body was never found. It was suspected that Reyn-Bardt had buried her body in this place called Lindow Moss, and this is a peat bog on the edge of Manchester. Now, peat is formed when the vegetation of the bog is compressed. And traditionally in Northern Europe, peat has been used for centuries, particularly for fuel. If the peat is dug out, it's cut up and dug out and dried, it can then be burned as fuel. So, in 1983, a man was doing just that. He was peat cutting when he found a skull fragment. An examination of the skull fragment showed it still had tissue attached, even brain tissue, which of course indicates it must be quite recent. So this was handed to the police, the police went to Reyn-Bardt, confronted him with the fact that they found this fragment, Reyn-Bardt confessed and he was convicted of murder and sent to prison. Then someone had the idea of sending that fragment to Oxford University for carbon-14 dating, and when the results came back, it must have been quite a surprise to Peter Reyn-Bardt because it turned out to be nearly 2,000 years old. It wasn't from his wife after all. But what is surprising is that it's 2,000 years old, but it still contains tissue. How can tissue survive that period of time? And the answer comes in the very unusual chemistry of the peat bog. So on the left there is Lindow Moss. It's a peat bog, and as I said, peat is formed by the compression of this vegetable matter in the swamp, and the vegetable matter, as it breaks down, releases chemicals into the water. Most particularly, it releases tannins, and this is why the water on peat bogs is often a brown colour. It's the same chemicals that make tea brown, and in fact they taste about the same too. So the peat bogs are acidic, and also these tannins have antibacterial properties, and that means that the microorganisms which are responsible for the tissue breakdown can't grow. Their growth is inhibited by these natural antibiotics, the tannins. Now, it's not true for all peat bogs. On the left there you have Lindow Moss where that skull fragment was found, on the right is Dartmoor. Dartmoor should be well known to all of us because quite a few of the Sherlock Holmes stories are set on Dartmoor, most famously "The Hound of the Baskervilles". The Dartmoor chemistry is quite different. Dartmoor has 20,000 archaeological sites, and that's one in the picture. Many of those archaeological sites are graves. That one is a grave; the depression in the centre of that stone circle is where the body was buried. And yet, there's been almost no human remains found on Dartmoor. The chemistry there is a little bit different, the human remains dissolved away. Now, peat and peat bogs are not restricted to England, they're spread all across Northern Europe, Northern Germany and especially Denmark. And as people have been cutting the peat for fuel and for other reasons, they've been finding bodies in the bog. And these bodies, typically, are extremely well preserved and they date back to about 2,000 years ago. And they are from the Celts, the Celts were the people living in this part of Europe at that time, and they, it is believed, practiced human sacrifice. And what they would do is they would ritually kill the person involved and then drop their body in the bog. What they didn't know is that body would be preserved for thousands of years. So, here's one of them. Here's Grauballe Man, he was found in Denmark. And you can see remarkable preservation; that may be his original hair colour, it's not his original skin colour, that is a result of the tannins. And typically, what used to happen when bodies such as Grauballe Man were found by the peat cutters, they looked so well preserved that the peat cutters would assume this was a murder case, and they would call the local police. The local police would come and they would take a look, and then they would call the University and a professor specializing in this would come over. And in fact, in Denmark, they have a museum dedicated to these people. One bog person has been found in Lindow Moss, where Reyn-Bart is believed to have buried the remains of his wife. This is Lindow Man, and if you happen to be in London, you can go to the British museum and take a look at him. [BLANK_AUDIO]
