1. Department of Geography and Geology, Eastern Michigan University, Ypsilanti, Michigan 48197, USA
2. Geology Department, University of Vermont, Burlington, Vermont 05405, USA
3. Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, Vermont 05405, USA
4. Department of Earth and Environmental Sciences, Norwich University, Northfield, Vermont 05663, USA
5. Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK, and Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550, USA
6. Consulting Geologist, Rockland, Maine 04841, USA