1. Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A
2. Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309, U.S.A
3. Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A
4. U.S. Geological Survey, Box 25046, Federal Center, Denver, Colorado 80225, U.S.A