Variations of element distribution in ferromanganese nodules and its bearing on growth rates

Mineralogical, chemical, and radiochemical analyses of nodules sampled at various distances from the East Pacific Rise indicate that the mineralogical composition is not clearly linked to distance, but chemical composition seems to be. The pelagic nodules accrete at a lower rate and are, on the whole, richer in manganese, uranium, and thorium and poorer in lead and silicon than nodules from the ridge. The deep-seated nodules display a marked decrease in thorium concentration toward their centers, which can be explained in several ways. One hypothesis involves a decrease in the growth rate as the plate moves away from the ridge and manganese flux diminishes. As suggested in previous studies, some dissolution of the bottom of the nodules may occur and contribute to abnormal element distributions.