1. A. H. Knoll is at the Botanical Museum, Harvard University, Cambridge, MA 02138, USA. R. K. Bambach is in the Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.
2. D. E. Canfield is at the Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, Bremen 28359, Germany.
3. J. P. Grotzinger is in the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.