THE AGE AND ORIGIN OF THE "YOUNGER DRYAS CLIMATE EVENT" IN GREENLAND ICE CORES

Th-230/U-234 and C-14 dating of Barbados corals has extended the calibration of C-14 years B. P. to calendar years B. P. beyond the 9200 year tree ring series (Bard et al., 1990). This now permitst he conversioonf C-14 chronozones, which delimit major climate shifts in western Europe, to calendar years. The Younger Dryas chronozone, defined as 11,000 to 10,000 C-14 years B. P., corresponds to 13,000 to 11,700 calendar years B. P. This calibration affects the interpretation of an intensely studied example of the "Younger Dryas climate event," the delta O-18 anomaly between 1785 and 1793 m in Dye 3 ice core. The end of the delta O-18 anomaly in Dye 3 ice core has been dated by measurements of C-14 in air bubbles(Andree et al., 1984, 1986) and by annual layer counting (Hammer et al., 1986). The older C-14 dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the Barbados Th-230/U-234 calibration. T he C-14 corrected age for the end of the delta O-18 event is 10,300+/-400 calendar years B. P. compared to the annual layer counting age of 10,720 +/- 150 years B. P. Thus, the " Younger Dryas" event in the Dye 3 ice core ends in the Preboreal chronozone (11,700 to 10,000 calendar years B. P.) and is not correlative with the end of the Younger Dryas event identified in pollen records marking European vegetation changes. The end the Dye 3 delta O-18 event is, however, correlative with the end of meltwater pulse IB (Fairbanks, 1989), marking a period of intense deglaciation with meltwater discharge rates exceedinf 13,000 km(3)/yr.