We thank W.F. Agena, J.J. Miller, and P. Nelson for their comments on earlier version of this paper. We wish to thank Andrew Gorman, Anne Tréhu, and Warren Wood for many helpful suggestions. 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 Institutions (JOI), Inc. This work was partially funded by the U.S. Department of Energy under interagency agreement DE-A121-92 MC29214.