The authors thank M. Nomura, T. Uruga, and K. Okumura for their guidance with the XANES experiments and E. Ohta for carbon analyses. Reviews by Dr. R.W. Murray and an anonymous reviewer have greatly improved this study.
This research used samples 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 Institutions (JOI), Inc. Funding for this research was provided by a grant-in-aid for scientific research from the Ministry of Education, Science, Sports and Culture of Japan awarded to Y.T. and H.S. XANES experiments were performed with the approvals of KEK (proposal no. 2000G267) and JASRI (proposal no. 2001B0393-NX-np).