Protactinium 231 method of dating coral from Barbados Island

Abstract

To investigate the feasibility of the Pa231 growth method of dating marine carbonates, measurements of protactinium, as well as of uranium and thorium isotopes, have been made on unaltered coral samples taken from the Island of Barbados. Because analyses of a living species from this island indicate the presence of an insignificant amount of initial Pa231, ages of the fossil samples can be calculated from the extent to which Pa231 has grown toward secular equilibrium with its radioactive parent U235. The three lowest coral reef terraces on the island have Pa231/U235 ages close to 80,000 years, 105,000 years, and 120,000 years. These ages are consistent with the ages estimated independently from the Th230/U234 ratios. This agreement suggests that the Pa231 growth method has the potential of being a useful geochronometer as well as an independent check for the C14 and Th230 dating methods for marine carbonates in the age range of 10,000 to 120,000 years. The Pa231/U235 and Th230/U234 ages (taking t½ Th230 = 75,200 years) are in better accord if one uses 34,300 years rather than 32,480 years as the half-life for the nuclide Pa231.