Uranium - 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 234U/238U ratios in corals. Samples older than 150 kyr B.P. are shown to be characterized by significant excesses of 234U relative to the uranium isotopic composition expected if the corals grew in present-day sea water. Assuming that the 230Th-ingrowth ages are accurate, these anomalies translate into high initial 234U/238U 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 234U/238U composition of sea water through time. The 234U anomalies cast doubt on the accuracy of the classical 230Th-ingrowth dating method in old corals, and in particular for the use of measured 234U/238U ratios alone to date corals older than 150 kyr.