Oxygen isotope dating of oak and elm timbers from the portcullis windlass, Byward Tower, Tower of London

A new isotopic method for dating oak and non-oak (elm) timbers is applied to samples from the portcullis windlass mechanism in the Byward Tower, Tower of London. This structure was previously sampled for ring width dendrochronology but failed to date. Successful application of the stable oxygen isotope dating method returns a felling date of winter AD 1656/7 for the oak timbers used to construct the gear wheel and a date of after AD 1648 (terminus post quem) for an associated elm shaft. This is the first reported application of oxygen isotope dendrochronology to date elm and confirms the significant potential of the method for dating species other than oak. •Stable isotope dendrochronology dates historic timbers from the Tower of London.•Samples were undateable by ringwidth dendrochronology.•.A felling date of winter AD 1656/57 returned for the portcullis windlass.•Original mechanism replaced during the Commonwealth/Interregnum period.•Successful application of oxygen isotope dendrochronology to date elm (Ulmus sp.)