Age determinations of fossil corals using²³⁰Th/²³⁴Th and²³⁰Th/²²⁷Th

The growth toward radioactive equilibrium of 230Th with 238U and of 231Pa with 235U has successfully been used to determine the ages of fossil corals. The chemical procedures employed in these measurements can be simplified dramatically if only thorium isotopes need be measured. Our method uses 234Th as a measure of 238U and uses 227Th as a measure of 231Pa to calculate the age of the coral. In addition to reducing the length of time required for an analysis by more than one half, our method does not rely on spikes but on a calibration standard that is counted to determine relative alpha and beta counting efficiencies. Ages of fossil corals from Jamaica correlate with the 120,000- to 135,000-yr-high sea stand reported by other workers.