1. Geology Department, Arizona State University, Tempe, Arizona 85287-1404, Earth and Environmental Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ England, Energy and Environment Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Bodega Marine Laboratory, University of California Davis, P.O. Box 247, Bodega Bay, California 94923