Spatial and temporal variations in oxygen isotopic ratios of tree-ring cellulose in Japan during last three centuries

We present here the first result of a network on yearly resolutions of tree-ring cellulose δ18O measurements in Japan, containing 7 sites from sub-boreal (Hokkaido) to sub-tropical (Kagoshima) regions, 4 of which extend over 250 years. Because the tree-ring δ18O directly reflects δ18O of precipitation (water vapor) and local relative humidity without any large biological effects, it is one of the most promising proxies in the tree ring for reconstruction of past water cycles. Although it is necessary to clarify spatial pattern of tree-ring δ18O variations for reconstruction of past water cycles, it has never been tried outside of Europe or North America. Japanese islands are located between Eurasian continents and Pacific Ocean and partly influenced by East Asian Summer Monsoon, in which δ18O of precipitation shows minimum in summer, opposite to high latitudes such as Europe. There are some common characteristics in short and long-term variability of the tree-ring δ18O in Japan. Short-term variations in tree-ring δ18O are well correlated with those of local summer relative humidity near the sample sites, among which southwestern and northeastern Japan can be categorized into different groups according to their inter-annual variability. There is a long-term decreasing trend of the tree-ring δ18O from the 18th century, possibly reflecting the gradual enhancement of East Asian Summer Monsoon after Little Ice Age, but it increased again after late 19th or early 20th centuries at most sites due to regional decreases in relative humidity with global warming. In the 18th century, abrupt decadal-scale drops in the tree-ring δ18O were often observed at some sites only in southwestern Japan, possibly showing short-term enhancements of Asian Summer Monsoon during Little Ice Age.