1. Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637;
2. School of Earth and Ocean Sciences, University of Victoria, British Columbia, V8W 3P6 Canada
3. Max Planck Institute for Meteorology, 20146 Hamburg, Germany
4. School of Geographical Sciences, University of Bristol, BS8 1SS England
5. Department of Global Ecology, Carnegie Institution, Stanford, California 94305
6. Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455
7. LPAP—Astrophysique/Géophysique, Université de Liège, B-4000 Liège, Belgium