Neil J. Loader
State of current knowledge
Whilst the measurement of stable isotope ratios in tree rings for paleoclimatology and plant physiology is well-established (McCarroll and Loader 2004), stable isotope dendrochronology is a relatively recent development in science-based tree-ring dating but one that has already demonstrated significant potential for dating timbers and wooden artefacts that would not normally be dateable using ring width dendrochronology alone. The technique is based upon the chemical analysis of a tree ring series and its statistical comparison against a reference chronology. Unlike ring-width dendrochronology which requires trees to be physiologically stressed to record a dating signal, the stable isotopic signal used for dating is recorded somewhat more passively, meaning that a reliable dating signal can be obtained from wide-ringed, fast grown trees. Such trees are common in Scottish vernacular timber buildings and native Scottish oaks but are often difficult to date. Stable isotope dendrochronology can therefore provide a complementary method for dating wood when conventional ring-width dendrochronology is unsuccessful (Loader et al 2019, Yamada et al 2018).
Stable isotope dendrochronology shares many elements with ring-width dendrochronology (Haneca et al. 2009). For oak and other ring porous species (eg chestnut, ash, elm) the latewood of each annual ring is dissected and from this the alpha cellulose is extracted for stable isotope analysis. The cellulose samples are pyrolised at high temperature using an elemental analyser and the resulting carbon monoxide gas measured for its oxygen and carbon isotope composition using an isotope ratio mass spectrometer. The resulting isotopic data are then converted to indices using a simple 9-year rectangular filter and the resulting index series compared against a precisely-dated reference chronology. Unlike ringwidth dendrochronology that reports dates using a variant of the Student’s t value, often termed the Baillie-Pilcher t value (Baillie and Pilcher 1973; Section 2. Principles of Dendrochronology), stable oxygen isotopes have different statistical properties that enable them to be reported using a framework of statistical thresholds. Only where these objective conditions are met, may a date then proceed to be considered (Loader et al 2019).
Key to the application of the method is the development of robust, well-replicated reference chronologies. The first reference chronology specifically developed for isotope dendrochronology published by Loader et al (2019) covered 800 years and was developed from more than 100 individual dendro-dated trees, all provenanced from across south central England. These data were combined to yield a chronology with a minimum sample replication of ten trees per year. It rapidly became clear that even with this relatively low level of replication there was a high degree of coherence in stable oxygen isotopes from building timbers sampled across a wide geographic range. Further applications including projects conducted in partnership with Historic England and the Royal Commission on Ancient and Historic Monuments of Wales were able to show that the Loader et al (2019) chronology could be used to date timbers from East Anglia and Kent, to Cornwall, Wales and SE Ireland. The ability of this chronology to securely date individual timbers is reduced as one moves northwards into Scotland highlighting the importance for the development of a network of regional isotopic reference chronologies. This finding was not unexpected as tree-ring oxygen isotopes reflect the amount and isotopic composition of the rainwater local to where the trees grow (Loader et al 2020a).
In light of this observation, one should expect that trees of different species, growing in the same region and sampling the same rainwater will record a broadly similar signal. Where this signal is strongly expressed it may be used for dating. Loader et al (2021) demonstrated this for both ring porous and diffuse porous (non-coniferous) species, meaning that there is also potential for dating non-oak species in Scotland using a stable isotope approach without the need for a genus specific reference chronology. Such reference chronologies (for both oak and for pine) must first be developed for Scotland, but these could prove invaluable given the ubiquity of non-oak species found in historical buildings and on prehistoric sites in Scotland for which there are no reference ring-width chronologies (See Section 3 Dendrochronology on Archaeological Sites).
Stable isotope dendrochronology is still a developing field, but it has already advanced understanding of historic buildings across large parts of the UK, providing chronology and interpretive details where this was not previously possible. It is likely that similar benefits and opportunities will stem from the development of stable isotope dendrochronology in Scotland.