1. IUEM-UBO, UMR CNRS 6539; Technopôle Brest-Iroise; Place Copernic, F-29280 Plouzané France
2. IRD-CRHMT; UR THETIS; Avenue Jean Monnet BP 171, 34203 Sète France
3. Department of Geological and Environmental Sciences; Stanford University; Stanford California 94305-2115 USA
4. UMR 10, IFREMER/CNRS; CREMA-L'Houmeau; 17137 L'Houmeau Cedex France