1. CNRS, Laboratoire de Glaciologie et Géophysique de l'Environnement, BP 96, 38402 Saint-Martin d'Hères Cedex, France, Geophysical Institute, Department of Chemistry, and Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska 99775, and U.S. Army Cold Regions Research and Engineering Laboratory, Fort Wainwright, Alaska 99703