A 2600-year summer climate reconstruction in central Japan by integrating tree-ring stable oxygen and hydrogen isotopes

Abstract. Oxygen isotope ratios ( δ18 O) 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 ( δ18 O) and hydrogen ( δ2 H) 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 δ18 O time series for the past 2500 years. However, there were distinct age trends and level offsets in the δ18 O record, and cellulose δ18 O values showed a gradual decrease as an individual tree matures. This suggested it is difficult to establish a cellulose δ18 O chronology for low-frequency signals by simple averaging of all the δ18 O time series data. In addition, there were opposite age trends in the cellulose δ2 H, and δ2 H gradually increased with tree age. There were clear positive correlations in the short-periodicity variations between δ18 O and δ2 H, probably indicating a common climate signal. A comparison of the δ18 O and δ2 H time series in individual trees with tree-ring width suggested that the opposite age trends of δ18 O and δ2 H 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 δ18 O and δ2 H 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 δ18 O and δ2 H in order to remove the age trend. This enabled us to evaluate the climatic record from cellulose δ18 O variations. The extracted climatological component in the cellulose δ18 O 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 δ18 O and δ2 H 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.