We thank Ms. Noriko Edasawa (Hokkaido University) for analytical assistance in the laboratory. Special thanks are due to Professors Masao Minagawa, Noriyuki Suzuki (Hokkaido University), Asahiko Taira, and Hiroshi Kitazato (Japan Marine Science and Technology Center) for their valuable inputs. This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institution (JOI), Inc.