We are indebted to M. Bohn, E. Boeuf, A. Cossard, and R. Derval for their technical skill, to J. Gotten (Brest University), M. Polve (Toulouse University), and C. Monnier (Nantes University) for geochemical analyses. We also thank C. Mével and M. Cannai for their careful reviews. This work was supported by CNRS grants (Geoscience Marine).