Wood14C Dating with AixMICADAS: Methods and Application to Tree-Ring Sequences from the Younger Dryas Event in the Southern French Alps

International audienceThe AixMICADAS facility is in part dedicated to research on radiocarbon (14C) calibration by meansof various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In thecurrent study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimizationbased on 14C ages, 13C/12C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleachingpretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanksequivalent to an age of 51,300 14C BP with a standard deviation of 1500 yr based on 25 analyses. The seven woodsamples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verificationof the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossilpines (Barb12 and Barb17) collected in the southern French Alps. New 14C analyses were performed at high resolution(every third year) and are shown to agree well with results obtained previously by high precision β-counting on CO2from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new14C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively correspondsto the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The 14C comparison betweenthe Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the 14CInter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout thestudied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ14Crise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere