AN OPEN-SYSTEM MODEL FOR U-SERIES AGE DETERMINATIONS OF FOSSIL CORALS

The source of excess ²³⁴U in fossil corals and its relationship to U-series age determinations has been an outstanding problem in geochronology for more than 20 years. With increasing numbers of U-series isotope measurements in corals, and significant improvements in analytical precision through mass spectrometry, it is increasingly apparent that a substantial fraction of observed isotope ratios cannot be reasonably explained by closed-system decay. Moreover, observations of a positive correlation between ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U ratios in corals from the same terrace are difficult to explain. However, the decay of dissolved uranium and alpha-recoil mobilization of uranium daughters produce particle-reactive ²³⁴Th and ²³⁰Th, whose coupled addition could simultaneously increase coral ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U. Here we present a quantitative model, based on decay-dependent redistribution of ²³⁴Th and ²³⁰Th, permitting calculation of open-system coral ages. Isotopic arrays of Barbados corals, and corals from terraces around the world, are consistent with model predictions suggesting the open-system model is generally applicable. Corals whose extreme isotopic compositions are impossible to produce by closed-system decay are also consistent with the limited range of isotopic compositions predicted by our model. For corals from a single terrace, ²³⁴Th and ²³⁰Th redistribution appears to be a source of significant systematic conventional age error, even for corals with slightly elevated ²³⁴U. However, open-system ages are consistent, even for corals with extremely elevated ²³⁴U. For the youngest three Barbados terraces, mean open-system terrace ages are consistent with mean conventional terrace ages calculated from pristine samples. If the most accurate conventional ages are from corals whose initial ²³⁴U/²³⁸U is identical to modern seawater, then the open-system model will improve the accuracy of coral U-series age determinations and dramatically increase the number of reliable ages.