We thank M. Hullar and M. Brainard for assisting with sample preparation and data analysis. J. Kaufman, J. McCarthy, and M. Hullar provided valuable comments on early drafts of this paper, and M. Voss provided very helpful input in preparation of the final version of this paper. This project was supported by grants from ODP (Grant #418923-BA200) and the Harvard College Research Program (to JCM).