This work was supported in part by a United States Science Support Program postcruise fellowship to R.E. Blake and by the Max Planck Society to M.E. Böttcher. We are grateful to H. Bao and T. Vennemann for providing thoughtful and constructive reviews of the manuscript. This research used samples and/or 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.