230Th-234U and 14C Ages Obtained by Mass Spectrometry on Corals

In 1988, Fairbanks conducted a drilling expedition off the south coast of Barbados to recover submerged corals contemporaneous with the last deglaciation. Core recovery was excellent and >30 different samples were dated by conventional β-counting techniques (Fairbanks 1989). At about the same time, we developed, at Lamont, the thermal ionization mass spectrometry (TIMS) technique to obtain precise U-Th ages (Edwards 1988), and to compare them with the 14C estimates measured on the same samples. A surprising result was that the discrepancy between 14C and U-Th ages increased through time to ca. 3000–3500 yr at ca. 15,000 14C BP (Bard et al. 1990a). Because the three youngest samples yielded U-Th ages in agreement with their calibrated 14C ages, we concluded initially that the TIMS U-Th determinations were not only precise, but also accurate, and that the 14C vs. U-Th data set could be used for a first-order 14C calibration.