1Department of Geology, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands
2Department of Earth Sciences, University of Aarhus, C.F. Møllers Allé, DK-8000 Århus-C, Denmark
3Department of Biogeology, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands