1. U.S. Geological Survey, Denver, Colorado 80225
2. Department of Geological Sciences, University of Washington, Seattle 98195
3. College of Ocean and Fisheries Science and Quaternary Research Center, University of Washington
4. Instituto Nacional de Pesquisas da Amazônia, 69,000 Manaus, Brazil
5. Centro de Energia Nuclear na Agricultura, Universidade de Sāo Paolo, 13,400 Piracicaba, Brazil