1. Department of Geological Sciences, University of Colorado Boulder, Boulder, Colorado 80309, U.S.A.
2. Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.
3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.
4. Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, U.S.A.
5. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, U.S.A.
6. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, U.S.A.
7. School of Earth Sciences, University College Dublin, Dublin, Ireland
8. Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, U.S.A.