Physics, Universität Bremen, Bremen and Alfred-Wegener-Institute for Polar and Marine Research, Bremerhaven 2nd Reviewer: Prof. Dr. Reiner Schlitzer, Alfred-Wegener-Institute for Polar and Marine Research [...] schools. The Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brazil) and Alfred- Wegener-Institut Helmholtz-Zentrum für Polar- u. Meeresforschung (AWI) for the financial support of my