Radiocarbon calibration and comparison to 50 kyr BP with paired C-14 and Th-230 dating of corals from Vanuatu and Papua New Guinea

We calibrated portions of the radiocarbon time scale with combined Th-230, Pa-231, C-14 measurements of corals collected from Espiritu Santo, Vanuatu and the Huon Peninsula, Papua New Guinea. The new data map C-14 variations ranging from the current limit of the tree-ring calibration (11,900 calendar years before present [cal BP], Kromer and Spark 1998, now updated to 12,400 cal BP, see Kromer et al., this issue), to the C-14-dating limit of 50,000 cal BP, with detailed structure between 14 to 16 cal kyr BP and 19 to 24 cal kyr BP. Samples older than 25,000 cal BP were analyzed with high-precision Pa-231 dating methods (Pickett et al. 1994; Edwards et al. 1997) as a rigorous second check on the accuracy of the Th-230 ages. These are the first coral calibration data to receive this additional check, adding confidence to the age data forming the older portion of the calibration. Our results, in general, show that the offset between calibrated and C-14 ages generally increases with age until about 28,000 cal BP, when the recorded C-14 age is nearly 6800 yr too young. The gap between ages before this time is less; at 50,000 cal BP, the recorded C-14 age is 4600 yr too young. Two major C-14-age plateaus result from a 130 parts per thousand drop in Delta C-14 between 14-15 cal kyr BP and a 700 parts per thousand drop in Delta C-14 between 22-25 cal kyr FIR In addition, a large atmospheric Delta C-14 excursion to values over 1000 parts per thousand occurs at 28 cal kyr BP. Between 20 and 10 cal kyr BP, a component of atmospheric Delta C-14 anti-correlates with Greenland ice 8180, indicating that some portion of the variability in atmospheric Delta C-14 is related to climate change, most likely through climate-related changes in the carbon cycle. Furthermore, the 28-kyr excursion occurs at about the time of significant climate shifts. Taken as a whole, our data indicate that in addition to a terrestrial magnetic field, factors related to climate change have affected the history of atmospheric C-14.