1. Environmental Change Research Centre, Department of Geography; University College London; London; WC1E 6BT; UK
2. Department of Marine Organic Geochemistry; Royal Netherlands Institute for Sea Research; NL-1790 AB; Den Burg; Netherlands
3. Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry; University of Bristol; Bristol; BS8 1TS; UK