We thank S. O'Connell and A. Klaus for corrections and comments. We greatly appreciate the assistance of the scientific party and crew of the JOIDES Resolution in the acquisition of these data. We thank N. Maestro and B. Paracuellos for their help in the laboratory. J.M. Anguita drew the final figures. This work was supported through the "DGICYT" of Spain under grant PB91-0080-C02-02.