1. Department of Geochemistry, Institute of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, The Netherlands, Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands, Federal Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236, and BDM Petroleum Technologies, P.O. Box 2543, Bartlesville, Oklahoma 74005