1. British Geological Survey National Environmental Isotope Facility Keyworth NG12 5GG Nottingham UK
2. Smithsonian Tropical Research Institute Apartado 0843–03092 Balboa, Ancon Panama
3. Rothamsted Research North Wyke, Okehampton EX20 2SB Devon UK
4. ETH Zurich Institute of Agricultural Sciences 8315 Lindau Switzerland