How accurate are the U-Th ages obtained by mass spectrometry on coral terraces

International audiencePrecise U-Th ages in corals from Barbados have been measured by means of thermal ionization mass spectrometry technique (TIMS). The Barbados reefs analysed so far can be correlated with the classical Emiliani’s stages based on the oxygen isotopic variations in benthic foraminifera (Emiliani 1955). The available coral samples are representative of the following δ$^{18}$O stages: 1, 2, 3, 4, 5.1, 5.3, 5.5, 7.3, 7.5 and it can be shown that the age results are in broad agreement with the Milankovitch theory. These data also enable to refine the calculation of lags between the extraterrestrial forcing and the climatic response. In particular, it is shown that the available ages obtained on corals from the Last Interglacial period range between 120 and 133 Kyr.Nevertheless, the actual limit of U-Th dating in old corals is no longer due to the precision of the measurements but rather to the presence of important isotopic anomalies of uranium-234. Indeed, it is shown that all samples older than 50 kyr have initial $^{234}$U/$^{238}$U ratios higher than present-day sea water, in contrast to modern or recent corals (< 30 kyr). This uncertainty on the initial ratio raises some doubt about the real accuracy of the age determination, despite the increased precision obtained by this new technique