Redundant Th-230/U-234/U-238, Pa-231/U-235 and C-14 dating of fossil corals for accurate radiocarbon age calibration

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Quaternary Science Reviews
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Th-230/U-234/U-238 dating of fossil corals by mass spectrometry is remarkably precise, but some samples exposed to freshwater over thousands of years may gain and/or lose uranium and/or thorium and consequently yield inaccurate ages. Although a delta U-234(initial) value equivalent to modern seawater and modern corals has been an effective quality control criterion, for samples exposed to freshwater but having delta U-234(initial) values indistinguishable from modern seawater and modern corals, there remains a need for additional age validation in the most demanding applications such as the C-14 calibration (Fairbanks et al., 2005. Radiocarbon calibration curve spanning 0 to 50,000 years BP based on paired Th-230/U-234/U-238 and C-14 dates on pristine corals. Quaternary Science Reviews 24(16-17), 1781-1796). In this paper we enhance screening criteria for fossil corals older than 30,000 years BP in the Fairbanks0805 radiocarbon calibration data set (Fairbanks et al., 2005) by measuring redundant Th-230/U-234/U-238 and Pa-231/U-235 dates via multi-collector magnetic sector inductively coupled plasma mass spectrometry (MC-MS-ICPMS) using techniques described in Mortlock et al. (2005. Th-230/U-234/U-238 and Pa-231/U-235 ages from a single fossil coral fragment by multi-collector magnetic-sector inductively coupled plasma mass spectrometry. Geochimica et Cosmochimica Acta 69(3), 649-657.). In our present study, we regard paired Pa-231/U-235 and Th-230/U-234/U-238 ages concordant when the Pa-231/U-235 age (20) overlaps with the associated Th-230/U-234/U-238 age (26). Out of a representative set of 11 FairbankS0805 (Fairbanks et al., 2005) radiocarbon calibration coral samples re-measured in this study, nine passed this rigorous check on the accuracy of their Th-230/U-234/U-238 ages. The concordancy observed between Th-230/U-234/U-238 and Pa-231/U-235 dates provides convincing evidence to support closed system behavior of these fossil corals and validation of their Th-230/U-234/U-238 dates for radiocarbon calibration and Delta C-14 reconstruction. (c) 2006 Elsevier Ltd. All rights reserved.


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DOI 10.1016/j.quascirev.2006.01.025