1. Department of Earth Science, University of Bergen, PO Box 7803, N-5020 Bergen, Norway
2. Mellomila 2, N-7018 Trondheim, Norway
3. Department of Geosciences, University of Oslo, Sem Sælands vei 1, N-0371 Oslo, Norway
4. British Geological Survey, Environmental Science Centre, Keyworth, Nottingham NG12 5GG, UK