1. 1Department of Geology, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), Ap. 644, E-48080 Bilbao, Spain
2. 2Permafrost Laboratory, Department of Geography, University of Sussex, Brighton BN1 9QJ, UK
3. 3Departamento de Ciencias de la Tierra-Instituto de Ciencias Ambientales (IUCA), Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, E-50009 Zaragoza, Spain