[HCG30-01] Oxygen isotope dendrochronology-Its principle, challenge and future-
Keywords:tree ring, oxygen isotope ratio, cellulose, dating, Paleoclimatology, excavated wood
The high inter-tree correlations of tree-ring δ18O are due to the fact that only two meteorological factors, precipitation δ18O and relative humidity, determine water δ18O in leaf where raw material of cellulose is produced, and by this mechanism, it becomes a good proxy of summer climate, important for understanding climate-society relationship in the past. In order to promote wood dating and climate reconstruction, it is necessary to establish master chronologies of tree-ring δ18O as long as possible in many regions. So far, they have been extended to about 5,000 years ago over Japan, and successfully applied to date excavated wooden artifacts and old architectural woods from various areas and periods. During the process, various analytical methods have been developed to extract tree-ring cellulose from highly degraded excavated woods quickly and safely for successful dating of many archaeological samples such as rice paddy piles, waterway boards and pit dwelling pillars. However, there have remained big challenges like reconstructing of long-term climate variation and dating of numerous small woods with small ring numbers.
Tree rings generally record past climate in annual resolution, while centennial or millennial variations are more useful for archaeological studies. However, it was not easy to reconstruct long-term climate variation by tree ring because biological age trends in tree rings usually interfere long-term climate signals. Recently, a novel method to reconstruct past climate seamlessly from annual to millennial scales has been developed by integrating tree-ring oxygen and hydrogen isotope ratios (Nakatsuka et al., 2020: https://doi.org/10.5194/cp-2020-6). On the other hand, cellulose isotope ratios are being measured at intra-ring levels to clarify seasonal (spring-summer) climate variations. The intra-ring database are now created not only for modern period but also for late Yayoi and early Kofun periods because seasonal signals of isotope ratios may be useful to date small excavated woods with about 10 tree rings.
What is the ultimate goal of oxygen isotope dendrochronology? In addition to wood dating and climate reconstruction, one of the biggest contributions to archaeology must be the quantitative assessment of human activities in pre-historical era. Traditional pottery-based relative chronology in Japan cannot give us quantitative index like “pottery productivity per year”. But, tree-ring δ18O can make yearly histograms of excavated wooden artifacts in a region which reflect past variations of human activity quantitatively. By comparing the data with climate variations, it is possible to elucidate in detail how climate variations did or did not influence human societies in the past.