Radiocarbon Dating the Early Iron Age Crannogs within Loch Tay
Our ability to date the crannogs in Loch Tay stems from what happens in outer space. Particles from space continuously enter our atmosphere and interact with the nitrogen (approximately 80% of our atmosphere) to form a tiny amount of radioactive carbon called radiocarbon or 14C. These radiocarbon atoms very quickly bond with oxygen (approximately 20% of our atmosphere) to form radiocarbon dioxide (14CO2), which constitutes a very small proportion of the total atmospheric carbon dioxide. This very quickly mixes with all the other stable (non-radioactive) carbon dioxide in the atmosphere (12CO2 and 13CO2). The fact that radioactive carbon is produced might seem frightening but the quantity is very small: there is only around one radioactive carbon atom for every million million stable carbon atoms in our atmosphere.
A small proportion of the carbon dioxide in the atmosphere (including the 14CO2) is taken up by terrestrial plant life in the process called photosynthesis, whereby the carbon dioxide is transformed into plant material and at the same time, oxygen, which sustains life on Earth, is produced. Thus, the plants become labelled with radioactive carbon dioxide. Plants are eaten by animals and so they too become labelled. While the plants and animals remain alive, they continue to take up radiocarbon dioxide through photosynthesis and direct consumption of plant material, respectively. Therefore, they retain a fixed concentration of radiocarbon in their tissues, similar to that of their contemporary atmosphere. However, when the plants or animals die, they can no longer take up new carbon into their tissues.
Because the radiocarbon atoms are radioactive, they undergo what is termed radioactive decay, whereby they convert back to nitrogen. The important point here is that this radioactive decay occurs at a fixed rate, whereby the amount of radiocarbon in the plant or animal halves in a fixed period of time. This is termed the half-life and for radiocarbon, this is 5730 years.
We can use this to date how long ago something (or someone) died. Because a dead plant or animal cannot continue to take up radiocarbon, the amount within it will start to decline as soon as the tissue dies. So, if we know how much radiocarbon should have been in a sample in the first place, and we also know how much radiocarbon is in the sample now, we can use our knowledge of how fast radiocarbon decays (5730 years for the amount to half) to calculate how much time passed since the organism died. We know how much was in the plant or animal to start with because radiocarbon is formed at an almost constant rate, so by measuring the amount of radiocarbon in living tissue and comparing it to the amount in our sample, we can calculate the time that has elapsed since death.
One of the reasons why dating crannogs is difficult is because radiocarbon is not formed at an absolutely constant rate, there are small variations in production rate and at some periods in time, these variations increase. So, before we can date a sample properly, we need to understand how radiocarbon production varied through time. Trees from bogs in Ireland and Germany often have ring patterns that can be overlapped from the present day (living trees) over many millennia, so we know exactly when each individual ring grew. Tree-ring (dendrochronology) laboratories have managed to secure known age material that covers approximately the last 13,000 years. Now, unlike muscle and bone in animals, which live as long as the animal lives, tree rings are dead with respect to the carbon cycle as soon as they are formed, and as such, each known-age tree ring contains a radiocarbon signature of the year in which it grew. In a huge effort, radiocarbon laboratories from around the world measured the radiocarbon content of these known-age tree rings in 10-year blocks and combined them into a calibration curve in which the radiocarbon age is plotted against the true calendar age. Hence, whenever we find a certain amount of radiocarbon in a sample of unknown age, we can look at the calibration curve and determine at what period in time this amount lies and hence how old the sample is.
Here is where the challenge begins. For much of the earlier Iron Age, the expected amounts of radiocarbon in the atmosphere are the same – it does not matter if a sample lived in 750 BC or 350 years later, the amount of radiocarbon we’ll find in it will be roughly the same. As a result, if we tried using isolated radiocarbon dates alone, we wouldn’t be able to tell if the different crannogs we find in Loch Tay were built and lived in by the same people, or by people many generations apart! In the figure of the calibration curve, you can see where the grey horizontal lines cut the blue calibration curve in several places. This is the problem area of the curve, known as the Hallstatt Plateau. The grey represents the radiocarbon age plus the error on the age.
There is however a way to overcome this challenge. The calibration curve is full of small wiggles, even on this plateau – a record of times when there was a little more or a little less radiocarbon in the atmosphere. Because crannogs were built of timber, we can measure the radiocarbon content in their tree-rings and build a picture of wiggles over the time when that timber was growing. We can then look for a part of the calibration curve with similar wiggles and this will let us know, with much better precision, how old the tree is. Indeed, in some very special cases we can determine when the tree was cut to the exact year! This technique, called radiocarbon wiggle-match dating, is how we will learn when people built the earliest crannogs of Loch Tay, how long they lived in them and when they abandoned them. This is the primary subject of the Living on Water project.