A preliminary evaluation of uranium series inequilibrium as a tool for absolute age measurement on marine carbonates

Experimentally determined ratios of Ra226 to U238 for marine carbonates are compared with ratios predicted on the basis of the age of the sample as determined by radiocarbon. The following assumptions are made in computing these ratios: (1) the initial concentrations of Ra226 and Th230 are too small to make a significant contribution to the Ra226 currently in the sample, (2) the U234 concentration is initially 15 per cent in excess of its equilibrium value, and (3) the samples act as closed systems for U, Th, and Ra. Three major conclusions may be drawn: (a) fossil mollusks contain appreciably more uranium than their contemporary equivalents, (b) acid leach fractions have considerably higher Ra226/U238 ratios than the residual material and, (c) the Ra226/U238 ratios are either within experimental error of, or in excess of, the predicted ratios (hence ages computed from the Ra226/U238 ratio would be equal to or greater than the corresponding C14 ages). Measurements of Th230, Th232, U234, and Pa231 are necessary before any elaborate interpretation of the observed anomalies is attempted.