Uranium-234 anomalies in corals older than 150,000 years

Details Uranium-234 anomalies in corals older than 150,000 years Uranium-234 anomalies in corals older than 150,000 years

Aug 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.2385b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 55, Issue 8, pp.2385-2390

Computer Science

12

Scientific paper

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; Mar-TINSON et al., 1987). By using thermal ionization mass spectrometry (TIMS), it is also possible to quantify precisely the 234 U / 238 U ratios in corals. Samples older than 150 kyr B.P. are shown to be characterized by significant excesses of 234 U relative to the uranium isotopic composition expected if the corals grew in present-day sea water. Assuming that the 230 Th-ingrowth ages are accurate, these anomalies translate into high initial 234 U / 238 U 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 234 U / 238 U composition of sea water through time. The 234 U anomalies cast doubt on the accuracy of the classical 230 Th-ingrowth dating method in old corals, and in particular for the use of measured 234 U / 238 U ratios alone to date corals older than 150 kyr.

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