Tree ring oxygen isotope dating of wood recovered from a canal in the ancient capital of Japan

Abstract

Trees growing in the lowlands of Japan are not climatically stressed; consequently, ring-width patterns are weakly correlated between different trees. Therefore, dendrochronological studies in Japan based on ring widths rely largely on climatically stressed conifers growing in mountainous regions and/or at higher latitudes. In contrast, oxygen isotope ratios (δ18O) are well-matched between trees, irrespective of climatic stress, indicating that the huge amounts of wood excavated in Japan could potentially be dated. Here, we tested oxygen isotope dating using unearthed stumps (Abies sp., Quercus sp., and Zelkova serrata) from Japan, which historical documents and archaeological evidence suggest date from 708 to 715 CE. A new 2470-year master chronology based on Japanese cypress samples (mainly Chamaecyparis obtusa) was developed, and our dating method was evaluated using modern samples collected from several species (Tsuga, Picea, Taxus, Pinus, and Quercus) across Japan and Korea. Our chronology was then used to date stumps recovered from the bank of a canal, which was constructed when Nara became the new capital city of Japan (Heijō-kyō in Japanese). Even though the samples had only 30–60 rings and comprised multiple species, including broad-leaf trees, their tree ring δ18O patterns were successfully cross-matched. The resulting site chronology was robustly dated against the master chronology, indicating that the felling dates of the stumps ranged from winter 697/8 to 709/10 CE. Overall, the dates are consistent with previous findings, but our study provides exact calendar dating related to the construction of the new capital, and demonstrates that oxygen isotope dendrochronology works well in Japan.