gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1998.62A.3.158 Biological Versus Abiological Mechanisms Explaining Isotopically Heavy Isua Organic Matter van der Meer M. T. J. Schouten S. de Leeuw J. W. Netherlands Institute for Sea Research, Department of Marine Biogeochemistry and Toxicology, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands Ward D. M. Montana State University, Department of Microbiology, MT 59717-0352, Bozeman, Montana, USA 1998 62A 3 1573 1574 Copyright © 1998, The Mineralogical Society 1998