The principles of radiocarbon dating are relatively straightforward – there is 14C in the atmosphere which is absorbed by living things. When living things die the uptake of 14C ends, and the 14C, being radioactive, begins to decay. The 14C decays at a known constant rate, so the remaining quantity of 14C in any particular sample of once living matter will reveal its age of death. However, in addition to issues with how to calibrate the radiocarbon age (which you can read more about over on the 14C page), there is also the issue of what material you are actually measuring. In our case, we are dealing with wood, more specifically individual rings of wood. So how do we make sure we measure only individual rings and not anything else? Our previous blog post covered how we count and cut out the rings we are interested in, but there are other sources of potential contamination.
Wood is a complex structural arrangement of cells. As trees grow they typically lay down annual rings, which become physiologically dead after each year. So in theory, each ring preserves 14C from the year it was growing only, even though the tree may continue to live happily for many years afterwards. It is this quality that we exploit in wiggle-match dating. But, other living things can get into these otherwise dead rings either during the life of the tree (like lignin) or after it has died (like fungus or bacteria). We need to remove these sources of carbon not associated with the date of our sample, so that our 14C measurement reflects only the 14C from the sample and not what might have entered that tree ring decades, centuries or millennia later.
To do this we follow a process that has been developed at SUERC. The samples are put in a bath of acid, followed by a bath of a base, followed by a final bath in acid (so Acid-Base-Acid). The make-up and strength of the acids are slightly different, as are the lengths of the baths, but the goal is to remove everything but cellulose. The result is that we can be confident that all that is left within this sample is cellulose associated with the formation of the wood – ie. precisely what we are trying to date, the year of formation of the tree-ring.
There are other pre-treatment processes for different types of samples such as bone or charcoal. A full description of all the specific methods has been published by SUERC staff <https://doi.org/10.1017/RDC.2015.2> After the acid-base-acid pre-treatment, our samples need to be prepared for measuring. This process will be covered in the next post!