1. Department of Earth and Planetary Science, Graduate School of ScienceThe University of Tokyo Tokyo 113‐0033 Japan
2. Geological Survey of JapanNational Institute of Advanced Industrial Science and Technology Tsukuba City Ibaraki 305‐8567 Japan
3. Kochi Institute for Core Sample Research, Japan Agency for Marine‐Earth Science and Technology Nankoku City Kochi 783‐8502 Japan
4. Department of Applied Science, Faculty of ScienceKochi University Kochi City Kochi 780‐8520 Japan
5. Research and Development Center for Ocean Drilling ScienceJapan Agency for Marine‐Earth Science and Technology Yokohama City Kanagawa 236‐0001 Japan