1. Department of Earth and Planetary Sciences Kyushu University 744 Motooka, Fukuoka 819‐0395 Japan
2. Center for Advanced Marine Core Research Kochi University B200 Monobe, Nankoku Kochi 783‐8502 Japan
3. Atmosphere and Ocean Research Institute The University of Tokyo 5‐1‐5 Kashiwanoha, Kashiwa, Chiba 277‐8564 Japan