Things have been a little quiet here on the blog over the summer, but that is because we have been hard at work undertaking one of the largest things on our project ‘to do’ list: data collection. Although we are going to be submitting new samples for radiocarbon dating from a number of sites we are re-examining as part of the project (see our sites page), the majority of the radiocarbon measurements we will be using are what are known as ‘legacy data’. These are all of the radiocarbon measurements that have been generated by archaeological excavation and research in the eight decades since radiocarbon dating was developed and widely used. We are aiming to collate as many radiocarbon measurements that calibrate within the period 3500 to 1500 BC from sites in Ireland and Britain as possible. In particular, we are collecting a number of details about radiocarbon measurements-let’s take this measurement from Wyke Down 1 as an example:
|Lab Code||Measurement (BP)||Error||δC13||Sample|
|BM-2396||4140||±80||-22.9‰||Oak Charcoal, Heartwood|
Here we can see there are a number of pieces of information we are interested in for each radiocarbon measurement:
- The laboratory code for the radiocarbon determination (this allows us to refer to the specific preparation and measurement methods used by the laboratories)-for this example, ‘BM’ tells us this sample was submitted to the British Museum.
- The uncalibrated radiocarbon measurement (in this case, 4140 BP, where BP stands for “Before Present” and where present is 1950 AD)
- The error range of the measurement (in this case ± 80). We use the uncalibrated measurement and the error range to produce a ‘calibrated’ date – you can read more about calibration here on Oxford Radiocarbon Accelerator Unit’s website .
- Quality control measurements , most notably the δ13C measurement (and we also collect the percentage collagen yield and C:N ratio for bone samples). These are important for assessing the accuracy and precision of radiocarbon measurement – you can read more about quality control measurements (also referred to as corrections for isotopic fractionation) on Beta Analytic’s website
- Details about the sample submitted, including the material (such as charcoal, animal bone, human bone, carbonised residue), and more specific details (species, specific parts of trees, skeletal elements). In this case, we have the material (charcoal), the species (oak), and where from the tree the sample is from (heartwood). In this case, the fact that this sample is from oak heartwood is really important, because the measurement obtained from this sample identifies the year that the specific tree ring grew (as opposed to the moment the tree was cut down). Given that Oak can live for hundreds of years, this measurement may, therefore, overestimate the age of this deposit-hence why it is so important we collect these details!
One final, but vital piece of information we need to collect is the archaeological context – where did the sample come from, how did it get there (ie, what type of deposit), and how do radiocarbon measurements from a site relate to each other. This information allows us to think about how multiple measurements from a site relate to each other, and can inform on the sequence of events at a site. The combination of these measurements and our ‘prior beliefs’ – the archaeological relationships between the contexts from which the measurements come from – is a core component of Bayesian statistical modelling, which we will be using to model our dates once our data collection has finished (more on Bayesian modelling in future posts!).
So, “where have we been getting all of this data from?” I hear you ask. Well, most of this work is desk-based and in the next few blog posts, we will be taking you on a tour of some of the most important sources of radiocarbon data we have been looking at, many of which are freely available to all of us! So join us next time for a radiocarbon data adventure, and in the meantime, if you are part of an archaeological project, please get in touch on twitter ( @projecttimearch ) to let us know if you have data that you think we might have missed.