著者

東田 盛善 隈田 昌良 佐竹 洋

出版者

一般社団法人日本地球化学会

雑誌

地球化学 (ISSN:03864073)

巻号頁・発行日

vol.40, no.3, pp.111-123, 2006-08-25 (Released:2017-01-26)

参考文献数

30

被引用文献数

2

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One hundred and eight samples of daily precipitation on Ishigaki Island were collected from Dec. 4, 1997 to Dec. 5, 1998 and were analysed for chemical and isotopic compositions. δD and δ18O values of daily precipitation ranged from -82 to 11‰ and -12.1 to -0.5‰, respectively. These values were higher in winter than in summer. The d value (d=δD-8δ18O) ranged from 7 to 35 and showed seasonal variation in which it was high in winter and low in summer. This indicated that precipitation of the winter season on Ishigaki Island is derived from the continental cold airmass. Amount effects of precipitation on isotopic composition were observed throughout the year. Furthermore two trends of amount effects were observed in summer. High concentrations of chemical components were found in precipitation with high wind velocity caused by typhoons and the seasonal winds in winter on Ishigaki Island. In particular, the typhoons significantly increased the concentration of chemical components. Annual average of Cl- concentration was nearly equal to the Cl- concentration in winter precipitation along the Sea of Japan side on Honshu Island, and the annual deposition rate of NaCl was about 7 times larger than the national average. The calcium ion of non sea salt origin in precipitation is probably supplied through the dust containing CaCO3 from the limestone-rich soil of Ishigaki Island and NH4+ by chemical fertilizers used in the island. The annual average value of the NO3-/nss SO42- ratio, 0.64, was twice as larger as the ratio of Tokyo (0.35). It may be caused by photochemical reaction forming nitric acid in Ishigaki Island under the strong insolation of the subtropical climate. High concentrations and large deposition rates of nss SO42- were observed in fall to winter, when d values of precipitation were high. The ratio of NO3-/nss SO42- was observed to be low in winter on Ishigaki Island. This indicated that the sulfuric acid was transported from the Asian Continent to Ishigaki Island by the continental cold airmass. The pH ranges from 4.1 to 6.9 and its annual mean was 5.4. This mean value of pH was higher than those of Honshu Island (e.g., Tokyo 4.7). Nss Ca2+/NH4+ ratio, 1.7, was much higher than those of Honshu Island (e.g., Tokyo 0.33). This suggests that nss Ca2+ is main acid neutralizer rather than NH4+. This result reflects the environmental characteristics of Ishigaki Island which is covered with limestone-rich soil.