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

Abstract

The AixMICADAS facility is in part dedicated to research on radiocarbon (14C) calibration by means of various archives. For this purpose, we are improving upon the capacity to accurately date subfossil wood. In the current study, nine chemical pretreatment protocols are tested on six wood samples of known ages. The optimization based on 14C ages, 13C/12C ratios, carbon % and overall mass yield % leads us to favor the acid-base-acid-bleaching pretreatment (ABA-B). This efficient method is shown to provide a residue of holocellulose with optimal blanks equivalent to an age of 51,300 14C BP with a standard deviation of 1500 yr based on 25 analyses. The seven wood samples from the Sixth International Radiocarbon Intercomparison (SIRI) are then analyzed as a further verification of the accuracy of our method. As a first scientific contribution, we studied two tree-ring sequences from subfossil pines (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 CO2 from large samples at lower resolution for Barb17 and accelerator mass spectrometry (AMS) data for Barb12. The new 14C series are then matched to the Kauri and YDB chronologies: the new sequence of Barb12-17 tentatively corresponds to the interval between 12,836 and 12,594 cal BP within the Younger Dryas cold period. The 14C comparison between the Barb12-17 sequence from France and the Kauri sequence from New Zealand allows calculating the 14C Inter-Hemispheric Gradient (IHG), with an average value of ca. 57 yr. The IHG stayed relatively high throughout the studied period. Interestingly, the IHG exhibits a transient maximum value (ca. 100 yr) during the period of rapid Δ14C rise (12,750–12,720 cal BP), a behavior that could be due to a delayed response of the Southern Hemisphere.

The Influence of Calibration Curve Construction and Composition on the Accuracy and Precision of Radiocarbon Wiggle-Matching of Tree Rings, Illustrated by Southern Hemisphere Atmospheric Data Sets from AD 1500–1950

“…Pretreatment effectiveness is influenced by sample age, preservation environment, species and sample size and what is effective in one study may not be suitable for another. It is important the labs undertaking 14 C measurements for calibration purposes initially determine appropriate pretreatment methods (see Southon and Magana, 2010;Capano et al 2018 for examples). Furthermore, tree-ring 14 C measurements can be made by both AMS and radiometric methods, with both having their strengths and weaknesses.…”

Onset of the Younger Dryas Recorded with 14C at Annual Resolution in French Subfossil Trees

“…The last ring of the youngest tree of the sequence (G5) has been dated in several works to between ca. 12,500 and 12,844 ± 32 cal BP (Hughen et al 2000;Muscheler et al 2008;Hua et al 2009;Bronk Ramsey et al 2012;Hogg et al 2016b;Capano et al 2018).…”

“…• The Barb12-17 sequence, a 240-yr chronology belonging to the French Barbiers chronology that has been sampled at annual resolution for 14 C measurements (Capano et al 2018). Preliminary 14 C dating of the Barb12-17 sequence between ca.…”

“…Preliminary 14 C dating of the Barb12-17 sequence between ca. 12,836 to 12,594 cal BP (Capano et al 2018) was based on visual matching of wiggles against the kauri sequence from New Zealand (Hogg et al 2016a(Hogg et al , 2016b).…”

“…Dryas (YD) gap prevents the Bølling-Allerød floating sequences from being connected with the absolute chronology (Kaiser et al 2012;Reinig et al 2018). Capano et al (2018) analyzed 14 C in two trees of the Barbiers chronology from the southeast French Alps and reconstructed a continuous 240-yr chronology that was used to tentatively link the absolute calibration curve (Kaiser et al 2012) with the floating CELM (Central European Lateglacial Master) chronology for the Allerød period. In this follow-up study, we present an augmented 416-yr tree-ring chronology based on five trees from the Barbiers site, annually 14 C dated to the onset of the YD period.…”