1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125;
2. Department of Anthropology and Institute of Mesoamerican Archaeology and Development, Vanderbilt University, Nashville, Tennessee 37235;
3. Department of Geological Sciences and Land Use and Environmental Change Institute, University of Florida, Gainesville, Florida 32611;
4. Middle American Research Institute, Tulane University, New Orleans, Louisiana 70118;