NOAA/WDS Paleoclimatology - Central Japan 2,600 Year Composite Tree-Ring Oxygen Isotope Data

Oxygen isotope ratios (d18O) of tree-ring cellulose are a novel proxy for summer hydroclimate in monsoonal Asia. In central Japan, we collected 67 conifer wood samples, mainly Chamaecyparis obtusa, with ages encompassing the past 2600 years. The samples were taken from living trees, archeological wood, architectural wood, and buried logs. We analyzed stable isotope ratios of oxygen (d18O) and hydrogen (d2H) in tree-ring cellulose in these samples (more than 15 000 rings in total) without using a pooling method and constructed a statistically reliable tree-ring cellulose d18O time series for the past 2500 years. However, there were distinct age trends and level offsets in the d18O record, and cellulose d18O values showed a gradual decrease as an individual tree matures. This suggested it is difficult to establish a cellulose d18O chronology for low-frequency signals by simple averaging of all the d18O time series data. In addition, there were opposite age trends in the cellulose d2H, and d2H gradually increased with tree age. There were clear positive correlations in the short-periodicity variations between d18O and d2H, probably indicating a common climate signal. A comparison of the d18O and d2H time series in individual trees with tree-ring width suggested that the opposite age trends of d18O and d2H are caused by temporal changes in the degree of post-photosynthetic isotope exchange with xylem water (physiological effect), accompanied by changes in stem growth rate that are influenced by human activity in the forests of central Japan. Based on the assumptions that cellulose d18O and d2H vary positively and negatively with constant proportional coefficients due to climatological and physiological effects, respectively, we solved simultaneous equations for the climatological and physiological components of variations in tree-ring cellulose d18O and d2H in order to remove the age trend. This enabled us to evaluate the climatic record from cellulose d18O variations. The extracted climatological component in the cellulose d18O for the past 2600 years in central Japan was well correlated with numerous instrumental, historical, and paleoclimatological records of past summer climate at various spatial and temporal scales. This indicates that integration of tree-ring cellulose d18O and d2H data is a promising method to reconstruct past summer climate variations on annual to millennial timescales, irrespective of the growth environment. However, analytical and statistical methods need to be improved for further development of this climate proxy.