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

We calibrated portions of the radiocarbon time scale with combined (super 230) Th, (super 231) Pa, (super 14) C measurements of corals collected from Espiritu Santo, Vanuatu and the Huon Peninsula, Papua New Guinea. The new data map (super 14) C variations ranging from the current limit of the tree-ring calibration (11,900 calendar years before present [cal BP], Kromer and Spurk 1998, now updated to 12,400 cal BP, see Kromer et al., this issue), to the (super 14) C-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 (super 231) Pa dating methods (Pickett et al. 1994; Edwards et al. 1997) as a rigorous second check on the accuracy of the (super 230) Th 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 (super 14) C ages generally increases with age until about 28,000 cal BP, when the recorded (super 14) C age is nearly 6800 yr too young. The gap between ages before this time is less; at 50,000 cal BP, the recorded (super 14) C age is 4600 yr too young. Two major (super 14) C-age plateaus result from a 13 ppm drop in Delta (super 14) C between 14-15 cal kyr BP and a 700 ppm drop in Delta (super 14) C between 22-25 cal kyr BP. In addition, a large atmospheric Delta (super 14) C excursion to values over 1000 ppm occurs at 28 cal kyr BP. Between 20 and 10 cal kyr BP, a component of atmospheric Delta (super 14) C anti-correlates with Greenland ice Delta (super 18) O, indicating that some portion of the variability in atmospheric Delta (super 14) C 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 (super 14) C.