A NEW MODEL FOR THE ORIGIN OF THE ANOMALOUS RADIOACTIVITY IN NIUE-ISLAND (SOUTH-PACIFIC) SOILS

The anomalous natural radioactivity of Niue Island is due to large excesses of both Th-230 and Pa-231 compared with their Parent radionuclides. It is suggested that material in Niue Island soils, consisting mainly of goethite and gibbsite partially trapped within porous calcite, was exposed to seawater a few hundred thousand years B.P. and the U content of the sea-water adsorbed onto the soil minerals. The island then rose and remained essentially above sea-level until present. The daughter products of the U isotopes grew into radioactive equilibrium with the U parents, but then the U parents were leached away some time in the span 100-70 ka B.P., leaving large amounts of orphan daughters in the soils without supporting parents.