The distribution of rare earth and minor elements in manganese nodules and sediments from the equatorial and S.W. Pacific

Abstract

62 manganese nodules and 17 associated sediments from the equatorial and S.W. Pacific have been analyzed for a number of elements, including the rare earth elements (REE), by instrumental neutron activation. Sc, Co, As, Hf, Th and REE occur in higher concentrations in S.W. Pacific than in equatorial Pacific nodules, reflecting the higher iron content of the S.W. Pacific nodules. These elements are probably incorporated nondiscriminantly into the iron oxyhydroxide structure by direct sorption from seawater because they are too large to be incorporated into the manganese oxide structure. The highest Ce La ratios are also found in the S.W. Pacific nodules. This may result from the fact that these nodules lie beneath the path of the fast flowing, well-oxygenated Antarctic Bottom Water (AABW) which facilitates oxidation of Ce to the tetravalent state. If so, this supports the idea that the Ce La ratio of manganese nodules is an important redox indicator. Negative cerium anomalies were noted for nodules from Areas F and G can be interpreted on the same model. The REE contents in equatorial Pacific siliceous oozes and S.W. Pacific red clays are similar on a carbonate-free basis. The Ce La ratio of the S.W. Pacific red clays is, however, much higher than that of the equatorial Pacific sediments, possibly reflecting the preferential oxidation of Ce to the tetravalent state beneath the AABW. There appears to be no systematic relationship between the REE abundance in the nodules and their associated sediments, although absolute REE abundances are higher in nodules than in their associated sediments. Nodules with the highest Ce La ratios are, however, found on sediments with the highest Ce La ratios. The principal difference in the REE distribution pattern of the nodules and sediments lies in the well-known cerium anomaly which is positive to slightly negative for nodules and always negative for sediments.