1. Department of Earth and Planetary Science, University of California, Mulford Hall no. 3114, Berkeley, California 94720-3114, Department of Natural Resources and Environmental Sciences, University of Illinois, 372 National Soybean Research Center, 1101 West Peabody Drive, Urbana, Illinois 61801, and Department of Natural Resources and Environmental Sciences, University of Illinois, W-321 Turner Hall, 1102 South Goodwin Avenue, Urbana, Illinois 61801