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Radiocarbon Calibration by Means of Mass Spectrometric 230Th/234U and 14 C Ages of Corals: An Updated Database Including Samples from Barbados, Mururoa and Tahiti

Abstract : As first shown by Bard et al. (1990a), high-precision 230Th-234U ages can be used successfully to calibrate the radiocarbon time scale beyond the high-precision tree-ring calibration that now reaches 11,900 cal BP (Kromer and Spurk 1998). Using mass spectrometric techniques, we measured 14C and 230Th ages on new samples collected from boreholes drilled off the islands of Tahiti and Mururoa (French Polynesia) in order to complement the database previously obtained on Barbados corals (Bard et al. 1990a, 1993). METHODS New 230Th/234U ages for Tahiti and Mururoa samples (Table 1) were measured with a VG-54-30 thermal ionization mass spectrometer (TIMS) fitted with an ion-counting Daly detector, at CEREGE (Aix-en-Provence). The chemical separations were similar to those previously described (Bard et al. 1990b). The 26 precision of the 230Th ages ranges from 30 to 60 yr, for ages between 8000 and 14,000 230Th yr BP.4 This represents an improvement by a factor of 2 to 3 over the performance obtained on Barbados corals by single collection and analog Daly detector on an MM30 mass spectrometer (Table 1; Bard et al. 1990a,b). The precision of the ages was checked by measuring numerous replicates (Bard et al. 1996). In particular, we performed five analyses of different pieces of the same coral specimen (sample P7-7: 10,995 ± 40, 11,005 ± 30, 11,025 ± 30, 10,995 ±30 and 10,995 ± 30 230Th yr BP; ages are rounded to the nearest 5 yr). The five 230Th/234U ages agree with each other within the 26 uncertainties, with an overall 2c uncertainty on the mean of 12 yr and a maximum difference between replicates of ca. 30 yr. Following Ludwig et al. (1992) and Stirling et al. (1995), the 229Th/ 33U ratio of our mixed spike was calibrated against the uraninite standard HU1 assumed to be at exact secular equilibrium. This calibration was shown to be accurate within 5% by means of gravimetric U and Th standards. This agreement is satisfactory if one takes into account the overall uncertainty on the half-lives of 234U and 230Th (2%o and 8%o, respectively; see Ludwig et al. 1992). Th samples are loaded with colloidal graphite on single-zone refined Re filaments and U samples on Re-Ta triple filaments. 234U/238U ratios are measured in dynamic multicollection mode: 233U, 234U and 235U ion beams are measured with the Daly ion counting detector, whereas 235U and 238U ion currents are measured with Faraday cups. Correction for isotopic fractionation is performed by normalizing the measured 238U/235U atomic ratio to the natural value (137.88). Faraday/Daly gain is monitored with 235U signals during each measurement block in order to correct for possible shifts of the gain. The external precision on individual values of b234U; (= [initial234U /238U-1 ] x 1000) is on the order of 2% (at 2a), as shown by repeated measurements of standards. The accuracy has been checked by ages expressed here as "BP" are relative to a fixed present of 1950.
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Edouard Bard, Maurice Arnold, Bruno Hamelin, Nadine Tisnerat-Laborde, Guy Cabioch. Radiocarbon Calibration by Means of Mass Spectrometric 230Th/234U and 14 C Ages of Corals: An Updated Database Including Samples from Barbados, Mururoa and Tahiti. Radiocarbon, University of Arizona, 1998, 40 (3), pp.1085-1092. ⟨10.1017/S0033822200019135⟩. ⟨hal-02497211⟩

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