EVOLUTION OF ND AND PB ISOTOPES IN CENTRAL PACIFIC SEAWATER FROM FERROMANGANESE CRUSTS

Hydrogenetic ferromanganese crusts incorporate elements from ambient s eawater during their growth on seamounts. By analysing Nd, Pb and Be i sotope profiles within crusts it is possible to reconstruct seawater t racer histories. Depth profiles of Be-10/Be-9 ratios in three Pacific ferromanganese crusts have been used to obtain growth rates which are between 1.4 and 3.8 mm/Ma. Nd and Pb isotopes provide intact records o f isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 2 0 Ma. This indicates a constant Pb composition for the erosional sourc es and suggests further that erosional Nd inputs may have been uniform too. epsilon(Nd) values vary considerably with time and most probably reflect changes in ocean circulation. The epsilon(Nd) values of the c rusts not only vary as a function of age but also as a function of wat er depth. From 25 to 0 Ma, crust VA13/2 from 4.8 km water depth has a similar pattern of epsilon(Nd) variation to the two shallower crusts f rom 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This sug gests that epsilon(Nd) stratification in Pacific seawater, as demonstr ated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in epsilon(Nd) from 3-5 Ma to the present, whi ch is interpreted in terms of an increase in the NADW component presen t in the Pacific. From 10 to 3-5 Ma ago the crusts show an increase in epsilon(Nd). This suggests a decreasing role for a deep water source with epsilon(Nd) less than circumpacific sources. In this regard the P anamanian gateway restriction from similar to 10 Ma with final closure at 3-5 Ma may have played an important role in reducing access of Atl antic-derived Nd to the Pacific.