A COMPARISON OF RAPID-SCREENING 14C DATES AND U-TH DATES FROM FOSSIL CORALS: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTION
We compare 69 rapid-screening 14C dates with new and published U/Th dates from fossil corals spanning the last 7,000 years from the Line Islands (~4N, 160W; Cobb et al., 2013). Results show that rapid-screening 14C dates are reproducible and differ from high-precision 14C dates by 1-4% (N=3). Sixty-seven of the 69 rapid-screening 14C dates agree with the U/Th dates within the relatively large uncertainties of the calibrated 14C dates. However, two samples with U/Th ages of ~6,500 years have 14C ages ~1000 years younger. Duplicate samples from these corals yield differing U/Th ages by ~300 years, indicating geochemical heterogeneity consistent with open-system behavior. Specifically, 14C addition (via precipitation of younger carbonates) and/or U removal (via preferential leaching of U during freshwater diagenesis) could explain the observed discrepancies. While the precision of the rapid-screening 14C dates translates into multi-century age errors for samples from the last millennium, this efficient dating technique could likely replace costly and time-consuming U/Th dates for older samples. At the very least, concordance between paired 14C and U/Th dates for fossil corals provides valuable confirmation of closed-system behavior.