ABSTRACT

The hydrogen and oxygen isotopic characteristics of precipitation in coastal areas of Japan were explained on the basis of a long period (23 years) observation of hydrogen and oxygen isotopic ratios (δD and δ¹⁸O) of precipitation collected monthly at Akita as well as a short period (1 or 2 years) observation at other localities. Coastal precipitation is characterized by a high d-value (=δD-8δ¹⁸O) in winter and a low d-value in spring to fall, and the d-value of annual mean precipitation depends on the mixing ratio of these two types and is generally higher (15-18) on the Japan Sea side and lower (11-12) on the Pacific Ocean side. The average values of δD and δ¹⁸O of monthly precipitation in winter are in ranges of -50 to -30‰ in δD and -9.0 to -6.5‰ in δ¹⁸O from northern Honshu to southern Kyushu, and the values in spring to fall are in the ranges of -55 to -45‰ in δD and -8.0 to -7.0‰ in δ¹⁸O. These variations seem to be mainly due to differences in sea surface temperature of the Japan Sea or Pacific Ocean where source vapor of precipitation is supplied. The values of δD and δ¹⁸O of annual mean precipitation are in the ranges of -55 to -40‰ and -8.6 to -6.7‰, respectively, between northern Honshu to southern Kyushu. The slopes of the δD and δ¹⁸O relationship are almost the same in winter and spring to fall and are close to 8 of Craig's meteoric line. This suggests that δD and δ¹⁸O in these two seasons are controlled by the same isotopic fractionation processes in source vapor supply except for the kinetic effect and in cloud forming processes. Therefore, if the kinetic effect is eliminated, δD and δ¹⁸O show similar seasonal variation patterns.