Record #77, shown in Fig. 4, is an example of a record dated by six methods, with up to five age determinations for each method. Note that ages are concordant within each method but mostly discordant between methods. The FT age is significantly lower than the ages yielded by the other methods. Consequently, this record has a concordance score of only 0.23. By contrast, record #558, shown in Fig. 5, is one of only two records in the “Three Methods Comparisons” subset that displayed concordance. This record has an overall concordance score of 0.92 (33 out of 36 comparisons are concordant). C. “U-Th-Pb Comparison” The database contained 998 records (5.4%) with ages calculated using at least three of the four U-Th-Pb methods. The concordance scores for this dataset are shown in Table 5. The scores were calculated using all the U-Th-Pb and non-U-Th-Pb ages in each record included in the dataset. The distribution of concordance scores for this subset of records (Fig. 6) more closely matches the distribution of scores for the entire database than for the “Three Methods Comparisons.” Many more “U-Th-Pb Comparisons” than “Three Methods Comparisons” had a concordance score of 1.00 (28.7% vs. 2.9%). The average concordance score for our “U-Th-Pb Comparisons” is 0.57, which is lower than the database average of 0.64 but higher Figure 5. Scatterplot of radioisotope age determinations for Record #558. Radioisotope ages are plotted against the present half-lives of the parent radioisotopes, with any specified error bars shown. The color of each data point represents the atomic weight of each parent isotope, as given by the legend on the right-hand side of each plot. Diamond-shaped data points represent α-decay, circle-shaped data points represent β-decay, and plus-shaped points represent nuclear fission. than the average of 0.39 for our “Three Methods Comparisons.” The average concordance score was 0.46 for the 369 records that used all four U-Th-Pb methods; for the remaining 629 records using three out of the four methods the average concordance score was 0.64. There is substantial variation in concordance between the U-Th-Pb methods, as shown in Table 2. The highest degree of concordance (0.79) is between 235U-207Pb and 238U-206Pb, and the lowest (0.18) is between Pb-Pb and 232Th-208Pb. The remaining four comparisons fall within the range 0.43-0.55. Overall, the U-Th-Pb methods are more concordant with each other than they are with the other methods (and generally than the other methods are to each other). D. Systematic Discordances The results of our “Two Methods Comparison” revealed a clear and systematic pattern of radioisotope age discordances. See the results in Table 3. For the same rock units, radioisotope dating methods tended to yield ages from oldest to youngest in the following order: Pb-Pb > Rb-Sr > 235U-207Pb > 238U-206Pb > 232Th-208Pb > K-Ar > FT. This compares to the pattern of systematic discordances reported by the RATE group, in which Sm-Nd > U-Pb > Rb-Sr > K-Ar (Austin 2005; Snelling 2005). V. DISCUSSION BEACHY, KINARD AND GARNER How often do radioisotope ages agree? 2023 ICC 394
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