Evolution of Nd and Pb isotopes in Central Pacific seawater from ferromanganese crusts

Hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts. By analysing Nd, Pb and Be isotope profiles within crusts it is possible to reconstruct seawater tracer histories.

Depth profiles of ¹⁰Be/⁹Be 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 of isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 20 Ma. This indicates a constant Pb composition for the erosional sources and suggests further that erosional Nd inputs may have been uniform too. ε_Nd values vary considerably with time and most probably reflect changes in ocean circulation. The ε_Nd values of the crusts not only vary as a function of age but also as a function of water depth. From 25 to 0 Ma, crust VA13/2 from 4.8 km water depth has a similar pattern of ε_Nd variation to the two shallower crusts from 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This suggests that ε_Nd stratification in Pacific seawater, as demonstrated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in ε_Nd from 3–5 Ma to the present, which is interpreted in terms of an increase in the NADW component present in the Pacific. From 10 to 3–5 Ma ago the crusts show an increase in ε_Nd. This suggests a decreasing role for a deep water source with ε_Nd less than circum-Pacific sources. In this regard the Panamanian gateway restriction from ∼ 10 Ma with final closure at 3–5 Ma may have played an important role in reducing access of Atlantic-derived Nd to the Pacific.