Keywords:Tree-ring cellulose, Stable oxygen isotope ratio, Intra-ring fluctuation, Relative humidity, The three great famines in the Edo period, Historical daily weather record
Stable oxygen isotope ratio (δ18O) of tree-ring cellulose is known to reflect well relative humidity at the time of ring formation. In this study we attempt high-resolution reconstruction of climates under the three great famines (Kyoho, Tenmei, Tempo) in the Edo period (1603-1867) by analyzing intra-ring data of oxygen isotope ratio of tree-rings. Tree-ring samples used in this study were taken from two hinoki (Chamaecyparis obtusa) trees growing at the Mt.Tanakami site, Shiga Prefecture, central Japan. We extracted cellulose from the wood samples by the “cross-section” method and divided into 12 (or 2, 6, 24, depending on the ring width) segments for each ring using a scalpel. Isotopic measurement was carried out using a continuous flow system with a pyrolysis-type elemental analyzer and an isotope ratio mass spectrometer (TCEA-Delta V Advantage). By comparing measured intra-ring data of cellulose δ18O and observational data of relative humidity at the adjacent meteorological station for the mid- to late 20th century, intra-ring δ18O was found to reflect variation of humidity at sub-seasonal (monthly or finer) temporal resolution in the growth season of the tree (May-Jul. or May-Sep. for our samples). According to this result, we reconstructed variations of humidity for the pre- to early Baiu rainy season (May-Jun.) and the mid- to late Baiu season (Jun.-Jul) usingδ18O chronologies of early part and middle or late part of each ring (Figure). For the pre- to early Baiu season, highly variable δ18O was found in the Kyoho period (1716-1736), implying periodical occurrence of dry early summer caused by delayed onset of the Baiu season. We can also recognize significant decrease in δ18O (increase in humidity) in the Tempo period (1831-1845), probably due to abnormally early onset of the Baiu season. Contrarily, variability of δ18O is relatively small for the mid- to late Baiu season.