FERROMANGANESE CRUSTS AS INDICATORS FOR PALEOCEANOGRAPHIC EVENTS IN THE NE ATLANTIC

Hydrogenetic ferromanganese crusts reflect the chemical conditions of the seawater from which they formed. Fine-scale geochemical analysis o f crust layers in combination with age determinations can therefore be used to investigate paleoceanographic changes which are recorded in g eochemical gradients in the crusts. At Tropic seamount (off northwest Africa), uniform crust growth influenced by terrigenous input from the African continent occurred during approximately the past 12 Ma. Phosp hatization of these crusts is minor. In contrast. crusts from Lion sea mount, located between Madeira and the Portuguese coast, display a muc h more variable growth history. A pronounced increase in Ni, Cu, and Z n is observed in some intervals of the crusts, which probably reflects increased surface productivity. A thick older phosphatized generation occurs in many samples. Hydrographic profiles indicate that Mediterra nean outflow water (MOW) may play an important role in the composition of these crusts. Be-10 dating of one sample confirms that the interru ption of the MOW during the Messinian salinity crisis (6.2-5 Ma ago) r esulted in changes in element composition. Sr-isotope dating of the ap atite phase of the old crust generation has been carried out to obtain a minimum age for the older generation of Atlantic crusts and to dete rmine whether crust phosphatization in the Atlantic can be related to phosphatization episodes recorded in Pacific crusts. The preliminary d ata show that the old phosphatized crust generation might be as old as approximately 30-40 Ma.