Characteristics of hydrogen and oxygen stable isotope ratios in precipitation collected in a snowfall region, Aomori Prefecture, Japan

Abstract

Characteristics of δ2H and δ18O in precipitation samples collected at 16- to 17-d intervals at three stations at the northern end of Honshu Island, Japan, were investigated. Rokkasho and Ajigasawa stations are approximately 100 km apart, on the Pacific Ocean coast and the Sea of Japan coast, respectively, of Aomori Prefecture. Hakkoda station is between Rokkasho and Ajigasawa stations at 1334 m ASL. Precipitation samples were continuously collected during 2000–2011 at Rokkasho, and during 2003–2006 at Ajigasawa and Hakkoda. At all stations, δ2H and δ18O of the precipitation samples showed weak seasonality, with higher values in summer and lower values in winter. In the summer (June, July, and August), the intercept of the regression line between δ2H and δ18O, that is, the local meteoric water line (LMWL), was lower than that of the global meteoric water line (GMWL) with similar slope, but in winter (December, January, and February), the LMWL intercept was higher than the GMWL intercept with similar slope. Temporal variations of δ18O at the two coastal stations were similar, although the timing of precipitation events was different. At all stations, d-excess values showed clear seasonality, with high values in winter and low values in summer, indicating a seasonal change in the source of the water vapor in air masses arriving over northeast Japan. In winter, d-excess values were higher at Hakkoda than at the other stations, suggesting a precipitation at Hakkoda was derived from water vapor from a remote area of the Sea of Japan. The long-term precipitation data set for δ2H and δ18O obtained in this study will be useful for investigations of regional hydrology and validation of numerical models.