1. Department of Geosciences and Earth and Environmental Systems Institute, Pennsylvania State University, Deike Building, University Park, PA 16802, USA.
2. Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA.
3. Alfred-Wegener-Institut für Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, Germany.
4. Departement Geografie, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium.
5. Polar Science Center, Applied Physics Lab, University of Washington, 1013 NE 40th Street, Seattle, WA 98105, USA.