1. Atmosphere and Ocean Research Institute; University of Tokyo; Kashiwa; 277-8564; Japan
2. Geological Survey of Japan; National Institute of Advanced Industrial Science and Technology; Tsukuba; 305-8567; Japan
3. Minnesota Isotope Laboratory, Department of Geology and Geophysics; University of Minnesota-Twin Cities; Minneapolis; Minnesota; 55455; USA
4. Department of Nuclear Engineering and Management, School of Engineering; University of Tokyo; Tokyo; 113-8656; Japan
5. Faculty of Education; Okayama University; Okayama; 700-8530; Japan
6. Department of Applied Chemistry, Faculty of Science and Technology; Keio University; 3-14-1 Hiyoshi, Kouhoku-ku,; Yokohama, Kanagawa; 223-8522; Japan