Dendrochronology (dating timbers by analysing tree-rings) is a vital weapon in the archaeological arsenal, and one that is often mentioned in CA. This month’s ‘Science Notes’ features a new approach, using stable isotopes, which could help date samples that cannot normally be analysed using traditional methods. We will be looking at how this method was able to shed light on the history of construction at the Tower of London.
Generally, dendrochronology determines a tree’s age through the annual growth pattern and width of its rings, assigning each ring to a single calendar year by matching it with samples of known age (many regional and local ring-width chronologies have been developed for dating purposes). This technique is most effective when trees have experienced an environmental (climatic) stress, which affects the width of the annual growth ring, creating a clear dating signal. In the UK and regions with similarly mild climates, this signal can be weakly expressed; in such cases, long, continuous sequences of at least 80 rings are usually required to date a sample securely. This figure is less than is found in many timber structures and artefacts, however, so ring-width dating is sometimes not possible. The new approach described here, developed through research supported by the Leverhulme Trust, presents a complementary technique that can be applied to just such samples.
Published in the Journal of Quaternary Science in August 2019 (https://doi.org/10.1002/jqs.3115), this method uses oxygen isotope signals created by summer precipitation, which are also recorded in tree-rings. Importantly, this signal is recorded regardless of whether the trees have undergone environmental stresses. Through testing it was determined that tree-ring stable isotope values could be used successfully to date timbers with fewer rings than are needed for ring-width dendrochronology, and to date those with invariant or disturbed sequences. This method therefore has the potential to widen the range of wooden structures and artefacts suitable for precision dating.
By using isotopic ratios in oak samples from central England, researchers from Swansea University and Oxford University have now developed a master chronology spanning AD 1200-2000. They applied this and the new technique to three samples taken from a portcullis windlass mechanism at the Tower of London, which ring-width-based dendrochronology had previously failed to date. The portcullis protects the Byward Tower, the Tower of London’s public gateway, which was constructed between AD 1275 and 1281, and is believed to have undergone several phases of modification between the 13th and 18th centuries. The portcullis itself has been radiocarbon dated to AD 1236-1302, indicating that it is the original gate, but it had previously been suggested that the windlass – used to raise and lower the portcullis – is a later enhancement.
Two samples of oak and one of elm, taken from the gear-wheel arm, underwent stable isotope dating, and the combined isotopic series from the two oak samples produced a date of AD 1656, while the last ring of the elm sample dated to AD 1648 (it was not possible to assign a felling date for this sample, as the bark edge was not preserved). When analysis of all three timbers was combined, it indicated that the trees were felled in the winter of AD 1656/1657. (Ring-width dendrochronology was also carried out on eight other samples but, individually, these failed to produce conclusive results.)
This result suggests that the windlass was constructed c.AD 1657, confirming that the mechanism had been replaced even though the original portcullis survives. These new dates offer an insight into a part of the Tower of London’s history that was previously unknown – the results point to the Commonwealth or Interregnum period, when record-keeping was especially poor.
These findings, published in the Journal of Archaeological Science (https://doi.org/10.1016/j.jas.2020.105103), confirm that oxygen isotope dendrochronology can be used successfully to date even short or invariant sequences of oak. They also demonstrate the technique’s potential for dating elm and other non-oak species using an isotopic master chronology that is composed entirely of oak.