1. Environmental Bio-Geochemistry Group, LAFQA, Instituto de Geografía, National Autonomous University of Mexico, Circuito Exterior, Ciudad Universitaria, México, Coyoacán, 04510, D.F., México, Stanford Synchrotron Radiation Laboratory, SLAC, P.O. Box 4349, Stanford, California 94309, and Division of Ecosystem Sciences, University of California, Berkeley, California 94720-3110