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Fission-track analysis, used to map uranium distributions in sections at the ppb to ppm level, revealed considerable intraskeletal heterogeneities in corals and mollusks. In a layer next to the internal surface of the corallite in the corals studied, uranium is enriched in comparison with the inner parts of the skeleton; configuration and the factor of relative uranium enrichment of the layer differ among corals. The branching coral Oculina diffusa, in addition, exhibits alternating bands of relatively high and low uranium contents arranged subparallel with the external surface. In the mollusk shells studied, uranium concentrations vary between different layers of a given shell; further variations exist within a given shell perpendicular to the direction of accretion. The e systematic heterogeneities result from variations in the processes of skeleton formation and growth. In all samples, apparently random variations were observed; they are superimposed on any distributional pattern which may exist. Trace-element distribution in carbonate skeletons is determined primarily by biologic processes and can be expected to change when affected by physicochemical diagenetic influences. Hence, the patterns of uranium distribution shown are potential indicators of uranium mobility during diagenesis and checks on the closed system requirement for uranium series dating of biogenic carbonates. This study also demonstrates that compositional complexities known from major elements are present on the trace-element level in skeletal carbonates.
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