1. Laboratoire de Géochimie Isotopique, URA-CNRS 1763, Université de Montpellier II, Place E. Bataillon, Case courrier 066, 34095 Montpellier Cedex 05, France, Department of Geology, Southampton Oceanography Centre, Southampton, SO14 3ZH, U.K., and Department of Geography & Earth Sciences, Brunel University, Borough Road, London TW7 5DU, U.K.