U/TH AND C-14 AGES OF CORALS FROM BARBADOS AND THEIR USE FOR CALIBRATING THE C-14 TIME SCALE BEYOND 9000 YEARS BP

The mass spectrometry of Th-230 and U-234 has been developed on a thermal ionization mass spectrometer (VG Micromass 30). It is shown that the mass spectrometric determination is significantly more precise and reproducible than the classical radioactive method (alpha-decay). The age errors obtained by this new technique of U-Th measurement are now comparable to that achieved by the C-14 method which will allow us to pursue the C-14 calibration on carbonates. The comparison between the U/Th and C-14 ages obtained on the Holocene samples demonstrate that the U/Th ages are accurate because they are in agreement with the dendrochronological calibration. Beyond 9000 yr B.P. it is shown that the C-14 ages are systematically younger than the U/Th ages with a maximum difference of about 3000 yr reached at about 20 000 yr B.P. It is also shown that the two main problems are the thorium ionization efficiency and the mass separation of the magnet sector for the U-234/U-238 ratio measurement. Significant improvements can thus be expected from mass spectrometry through the use of sputtering ion sources for optimizing the ionization efficiency, additional magnetic and electrostatic sectors to improve the abundance sensitivity and ion counting detectors to increase the measurement statistics.