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

Takeshi Nakatsuka,Noboru Higami,Minoru Sakamoto,Nanae Nakao,Yuki Shigeoka,Kenjiro Sho,Takumi Mitsutani,Zhen Li,Masaki Sano,Misao Yokoyama,Hiromasa Ozaki,Chenxi Xu,Keiko Ohnishi,Akane Tsushima

doi:10.5194/cp-16-2153-2020

Abstract

Abstract. Oxygen isotope ratios (δ18O) 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 (δ18O) and hydrogen (δ2H) 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 δ18O time series for the past 2500 years. However, there were distinct age trends and level offsets in the δ18O record, and cellulose δ18O values showed a gradual decrease as an individual tree matures. This suggested it is difficult to establish a cellulose δ18O chronology for low-frequency signals by simple averaging of all the δ18O time series data. In addition, there were opposite age trends in the cellulose δ2H, and δ2H gradually increased with tree age. There were clear positive correlations in the short-periodicity variations between δ18O and δ2H, probably indicating a common climate signal. A comparison of the δ18O and δ2H time series in individual trees with tree-ring width suggested that the opposite age trends of δ18O and δ2H 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 δ18O and δ2H 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 δ18O and δ2H in order to remove the age trend. This enabled us to evaluate the climatic record from cellulose δ18O variations. The extracted climatological component in the cellulose δ18O 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 δ18O and δ2H 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.

Highlights

It is not straightforward to extract low-frequency climate signals from tree-ring time series because there are typically age trends in tree-ring width
Given that the average correlation coefficient (R bar) amongst the different δ18O time series is ca. 0.6–0.8, the expressed population signal (EPS) (Wigley et al, 1984), defined as N × R/[1 + (N − 1) × R], where the N and R values are the number of time series and R bar, respectively, is > 0.85 for almost all periods during the past 2500 years (Fig. 3b), indicating that the δ18O data in Fig. 3a are sufficient to establish accurate δ18O chronologies
We found that there are distinct age trends in the δ18O time series

Summary

Introduction

It is not straightforward to extract low-frequency climate signals from tree-ring time series because there are typically age trends in tree-ring width. Tree-ring cellulose oxygen isotope ratios (δ18O) are a novel proxy for past summer hydroclimate, and it has been reported that such data are not affected by age trends (Young et al, 2011; Kilroy et al, 2016; Xu et al, 2016; Büntgen et al; 2020) or only show short-lived juvenile effects of higher δ18O values (Szymczak et al, 2012; Duffy et al, 2017). These juvenile effects are interpreted to be related to the lessdeveloped root system of young trees and the evaporative enrichment of 18O in soil water in the near surface. Given that subsequent studies of tree-ring cellulose δ18O have reported that age trends are negligible, the mechanisms responsible for long-term age trends remain unclear

Methods

Results

Conclusion