U-234 Anomalies in Corals Older Than 150,000 Years

We present new precise U-Th ages of well-preserved coral specimens collected from the island of Barbados, West Indies, and the atoll of Mururoa, French Polynesia. Our new data confirm the ages attributed to oxygen isotope stage 7 in the framework of the Milankovitch theory (BERGER, 1978; MARTINSON et al., 1987). By using thermal ionization mass spectrometry (TIMS), it is also possible to quantify precisely the U-234/U-238 ratios in corals. Samples older than 150 kyr B.P. are shown to be characterized by significant excesses of U-234 relative to the uranium isotopic composition expected if the corals grew in present-day sea water. Assuming that the Th-230-ingrowth ages are accurate, these anomalies translate into high initial U-234/U-238 ratios: about 1.2 at 200 kyr and up to 1.5 at about 450 kyr B.P. We propose that the anomalies result from both diagenetic addition and replacement of U and possibly from global changes in the U-234/U-238 composition of sea water through time. The U-234 anomalies cast doubt on the accuracy of the classical Th-230-ingrowth dating method in old corals, and in particular for the use of measured U-234/U-238 ratios alone to date corals older than 150 kyr.