Corrections for initial isotopic disequilibrium in the speleothem U-Pb dating method

The accuracy of ages produced by the U-Pb speleothem chronometer is influenced by initial disequilibrium effects resulting from weathering and transit of water through the epikarst. If some residual disequilibrium remains at the time of measurement, corrections can be made. However, once the system reaches equilibrium, the initial state has to be estimated thus propagating significant uncertainty into the age calculation. As an alternative approach, here we explore the possibility of calculating [U-234/U-238](1) initial values by forcing concordance in the U-238 to Pb-206 and U-235 to Pb-207 decay schemes. Whether this method can be successfully implemented relies heavily on the quality of the linear regression enacted, particularly with respect to the U-235-Pb-207 decay scheme. We therefore also explore the effect of Pb-208-based normalisation in order to optimise isochron construction. An unexpected consequence of this analysis is the observation that Pb-208/Pb-204 ratios may aid in the identification of heterogeneous initial Pb isotopic compositions in speleothems. Both methods have the potential to generate improved isochron fits which ultimately produce improved disequilibrium corrected speleothem ages. Initial disequilibrium values obtained in this way can be applied to other samples where disequilibrium information is unavailable.