1. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA
2. Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, University of GlasgowRankine Avenue, East Kilbride G12 8QQ, UK
3. Department of Earth and Environmental Sciences, University of St Andrews, St Andrews KY16 9AJ, UK
4. Present address: Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77004, USA
5. School for the Environment, University of Massachusetts at Boston, Boston, MA 02125, USA
6. Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK
7. Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, 69117 Heidelberg, Germany