1. Division of Marine Biogeochemistry, NIOZ, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands
2. Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, United Kingdom
3. Current address: Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom