石垣島安良崎のハマサンゴ骨格の酸素・炭素安定同位体比の環境プロキシとしての評価

抄録

The chemical and isotopic compositions of coral skeleton can be used as past climate proxies for tropic and subtropic regions. We have collected a coral core representing 170 years of continuous growth from Ishigaki Island, the Ryukyu archipelago, Japan and conducted detailed analysis of skeletal oxygen and carbon isotope ratios (δ¹⁸Oc and δ¹³Cc) for the upper portion of the sample. The isotope ratios, measured along the major growth axis of the coral skeleton, showed clear periodic variations. A significant linear correlation was found between δ¹⁸Oc and sea surface temperature (SST) at Ishigaki Port, especially in winter data. Thus, wintertime δ¹⁸Oc is a good proxy for winter SST. In contrast, no significant correlation was observed in summer data. Although summertime δ¹⁸Oc is considered to be under influence of the fluctuation of δ¹⁸Osw caused by local climate such as precipitation, no significant correlation was found between gridded sea surface salinity data and summertime δ¹⁸O of seawater (δ¹⁸O_SW) estimated from δ¹⁸Oc and SST. Further examination of coral based SST of Ishigaki Island (SSTIc) revealed that SSTIc can serve as warming trend observed in SST at Ishigaki Port past 40 yeas, and winter SSTIc was significantly affected by East Asian Winter Monsoon from 1958 until 1988/1989 climate regime shift. The δ¹³Cc was significantly correlated to the seasonal variation of solar radiation. Although δ¹³Cc seemed to vary with other factors such as biological effects, the δ¹³Cc can serve as indicators of age and season.