1. National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK (Present address: Scottish Association for Marine Science, Oban, PA37 1QA, UK) E-mail: mvz@noc.ac.uk
2. Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK E-mail: P.Plucinski@bath.ac.uk, A.C.Y.Dartiguelongue@bath.ac.uk
3. British Geological Survey, Environmental Science Centre, Keyworth, Nottingham, NG12 5GG, UK E-mail: plusty@bgs.ac.uk