Application of the 227Th/230Th method to dating Pleistocene carbonates and comparison with other dating methods

The 227Th/230Th dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the 231Pa/235U and 230Th/234U methods. The 227Th/230Th ages are found to be critically dependent on corrections for decay of 227Th prior to alpha counting and ingrowth of daughter isotopes of 232Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess 227Th in the calcite of samples younger than ∼ 50 ky, possibly due to the coprecipitation of 231Pa during formation. Calculated 'negative' 227Th/230Th ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the 227Th/230Th is found to be a useful alternative dating technique for carbonates which are between ∼ 50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods. (author)