Cerium and neodymium isotope ratios and REE patterns in seawater from the North Pacific Ocean
Résumé
Depth profiles of cerium and neodymium isotopic ratios and REE abundances are reported for three water columns (DE-4, MF52-59, and MF62-67) of the North Pacific Ocean. REEs in seawater of the DE-4 samples are collected by coprecipitation with Fe(OH)3, while others are by in situ direct adsorption onto MnO2-impregnated fiber. Abundance ratios of (HREE)/(LREE) are lower for the MF samples than for the DE-4 samples, indicating lower recoveries of HREEs in the MnO2-fiber method than in the Fe(OH)3 coprecipitation method. In the epsilon(Nd) depth profile for the DE-4 site, the surface water shows less radiogenic epsilon(Nd) of -8, suggesting large contribution of aeolian dust input. The deeper waters have epsilon(Nd) = -5 similar to other Pacific deep waters. This profile proves that bottom water keeps epsilon(Nd) value around -5 at the North Pacific Ocean even if surface waters show extremely low epsilon(Nd) values due to local source-inputs. Epsilon(Ce) values for seawater samples studied range from -1.21 to +1.82, showing chiefly positive values. The positive epsilon(Ce) values for most of seawaters contrast with the negative epsilon(Ce) values for most of Pacific ferromanganese nodules. The positive epsilon(Ce) and negative epsilon(Nd) values for most of the Pacific seawaters can be interpreted by the mixing model of continental and mantle-like oceanic/hydrothermal sources. Only surface waters have MORB-like negative epsilon(Ce) values comparable to nodules, perhaps suggesting preferential photo-reduction of MORB-derived water-borne Ce to continental-derived airborne Ce. This photo-reduction, together with a large aeolian-dust input, is suggested to be responsible for surface maxima of Ce anomaly observed for the DE-4 surface samples.