1. Quaternary Research Center and Department of Earth and Space Sciences, Box 351360, University of Washington, Seattle, WA 98195–1360, USA.
2. Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK.
3. Center for Accelerator Mass Spectrometry, L-397 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
4. Geology Department, Colorado College, Colorado Springs, CO 80903, USA.