1. Max Planck Institut f¨ur Biogeochemie, Postfach 100164, D07701 Jena, Germany;
2. IPSL, Case 101, U.P.M.C., 4 Place Jussieu, 75252 Paris Cedex 5, France;
3. CSIRO Atmospheric Research, PMB 1 Aspendale, Victoria 3195, Australia;
4. Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA;
5. ExxonMobil Research and Engineering Company, Route 22E, Annandale, NJ 08801, USA;
6. UPMCINRA-CNRS Biochimie Isotopique UMR 7618, 4 Place Jussieu, 75252 Paris Cedex 5, France