Be lO AND U-SERIES ISOTOPES IN MANGANESE NODULES FROM THE

The exponential decreases with depth of Be lO , Th 230 and Pa 23l in two manganese nodules from the central North Pacific is interpreted in terms of slow growth of the nodules: at average rates of millimeters per million years. We observe in one speci­ men (Mn 139): Belo-based rate < Th230-based rate < Pa 23l -based rate. This suggests that diffusion-mixing may have modified the depth gradients of these nuclides, such that the true rate would be close to or even lower than the observed Be lO rate of 1.3 mm/ 10 6yr. Quantitative assessment using a diffusion-decay model indicates that in Mn 139, Th 230 and Pa 23l could have been sub­ jected to diffusion-like transport with an effective diffusion coefficient of ~ 1x10- 8 cm2/yr. After correction for this effect, the Th 230 and Pa 23l data give a concordant rate of 1.9 mm/10 6yr, about 2-3 times lower than their uncorrected values. The small and variable amounts of integrated dpm/cm 2 of Be lo , Th~~o and Pa~~l found in nodules are best explained by their incorporating only a fraction of the nuclides supplied due to frequent coverage by sediments. The alternative explanation of young "exposure" ages is refutable on several grounds. Among these, we show that diffusion of nuclides required by the exposure-age concept yields depth profiles that are not of the commonly observed exponential form.