1. Institute of Geology and Mineralogy; University of Cologne; Zuelpicher Strasse 49a, D-50935 Cologne Germany
2. Alfred-Wegener-Institute for Polar and Marine Research; Am Alten Hafen 26, D-27568 Bremerhaven Germany
3. Helmholtz-Institut fuer Strahlen- und Kernphysik; University of Bonn; Nußallee 14-16, D-53115 Bonn Germany
4. Department of Earth Sciences; University College London; Gower Street, London WC1E 6BT UK