Isotopes in tree rings
Our research team specialises in using tree rings not just for dating, but also to reconstruct the climate of the past. We do not only use ring widths, but also study the chemistry of the wood, specifically the stable (non-radioactive) isotopes of carbon and oxygen. The carbon isotopes vary mainly in response to the amount of summer sunshine and the oxygen isotopes to summer rainfall. This allows us to analyse the chemistry of old tree rings to reconstruct summer climate through time. Trees do not need to be environmentally stressed to record a reliable isotopic signal in their rings and so this approach works particularly well in regions where tree growth is not strongly limited by climate such as in the UK and across north western Europe.
The isotopic match between trees is much stronger than for ring widths. This means that it is possible to use the annual pattern of isotopic variability to date wooden artefacts with far fewer rings than are currently required. Also, because ring widths and the two isotopes are only weakly related to each other, where all three indicators identify the same date then confidence in the match is significantly enhanced.
Using special increment corers we remove samples of wood from living trees and old buildings. Sampling does not cause untoward harm to the trees or damage the buildings. Often, we collect samples from structures under archaeological investigation to establish their age and construction history. For very old timbers, many hundreds or thousands of years old, we can sample logs preserved in peat bogs and sediments.
Once samples have been returned to the lab the process of measuring the annual growth rings and the extraction of cellulose can begin.