Dating ancient wood by high-sensitivity liquid scintillation counting and accelerator mass spectrometry—Pushing the boundaries

It is critical that a comprehensive terrestrial radiocarbon (14C) calibration curve is developed beyond 26 ka for high-precision calibration and correlation of climatic, environmental and archaeological records. Abundant sub-fossil New Zealand kauri (Agathis australis) wood, preserved in Oxygen Isotope Stage-2 and 3 peat swamps, provides an important resource for 14C calibration; nowhere else in the world does such an extensive collection of ancient wood exist. Although finite 14C ages beyond 50 ka are becoming routinely reported, few attempts have been made to demonstrate their accuracy or precision. Finite ages beyond 50 ka require optimization of all elements involved in sample preparation and 14C analysis. Here we discuss the methodology employed for optimizing the 14C dating of near-background wood samples by both benzene synthesis for liquid scintillation counting (LSC) or graphite synthesis for accelerator mass spectrometry (AMS). We report the mean background blank activities for both methods and present a statistical model for assigning blank standard errors when blank activity variation is over and above counting statistics. We also present duplicate analyses (using LSC and AMS) of nine successive samples of wood obtained from a sub-fossil kauri log near-background in age to investigate the significance of the measured blank levels and variability.