Findings from an in-Depth Annual Tree-Ring Radiocarbon Intercomparison

Lukas Wacker ,Jesper Olsen ,Alan G Hogg ,Toshio Nakamura ,Martin Seiler ,Silvia Bollhalder ,John Southon ,Margot Kuitems ,Gregory W L Hodgins ,Fusa Miyake ,Douglas J Kennett ,Alexander Cherkinsky ,Alex Bayliss ,David Brown ,Marie Josee Nadeau ,Ganesh Prasad ,E M Scott ,Fiona Petchey ,Brendan J Culleton ,David Chivall ,Richard A Staff ,Ronny Friedrich ,Christopher Bronk Ramsey ,J P Naysmith ,Timothy Knowles ,Todd Lange ,Michael W Dee ,Takayuki Omori ,Édouard Bard ,Michael Friedrich ,Manuela Capano ,Bente Philippsen ,Thibaut Tuna

doi:10.1017/rdc.2020.49

Abstract

ABSTRACTThe radiocarbon (14C) calibration curve so far contains annually resolved data only for a short period of time. With accelerator mass spectrometry (AMS) matching the precision of decay counting, it is now possible to efficiently produce large datasets of annual resolution for calibration purposes using small amounts of wood. The radiocarbon intercomparison on single-year tree-ring samples presented here is the first to investigate specifically possible offsets between AMS laboratories at high precision. The results show that AMS laboratories are capable of measuring samples of Holocene age with an accuracy and precision that is comparable or even goes beyond what is possible with decay counting, even though they require a thousand times less wood. It also shows that not all AMS laboratories always produce results that are consistent with their stated uncertainties. The long-term benefits of studies of this kind are more accurate radiocarbon measurements with, in the future, better quantified uncertainties.

Highlights

The results show that accelerator mass spectrometry (AMS) laboratories are capable of measuring samples of Holocene age with an accuracy and precision that is comparable or even goes beyond what is possible with decay counting, even though they require a thousand times less wood
It shows that not all AMS laboratories always produce results that are consistent with their stated uncertainties
The offset is on average 16 ± 4 14C yr, this offset varies over the 21 yr supplied

Summary

MOTIVATION

The tree-ring section of the radiocarbon (14C) calibration curve is overwhelmingly based on measurements made by decay counting in the 1980s and 1990s (all but 56 of the 4314 measurements on tree rings included in IntCal were made in this way). As the aim of the exercise was to test the laboratories systematically for their ability to produce accurate, high-precision 14C measurements, 21 continuous single tree-ring samples were prepared for each time period. While many laboratories calculate their uncertainties on the larger of the counting statistic uncertainty or the internal variability of a sample measurement (variability of submeasurements of a single sample over time), other laboratories base their calculations on counting statistics where they add, in quadrature, an additional uncertainty (of typically 1–1.5‰), before adding an uncertainty for standard normalization and blank correction. The error weighted mean for replicate measurements by an individual laboratory on each annual ring was calculated, assuming a systematic uncertainty of 1‰ from sample preparation (Sookdeo et al 2019 in this issue) that is already included in the uncertainties given by the laboratories. The errors given by a laboratory (uncringi ) are compared with the observed deviations of the ring arithmetic mean value adjusted for the laboratory offset: Pn xring χ 2 redlabseries ring ;i xringi offlab2 uncringi 2 n

RESULTS AND DISCUSSION

90 Series H all

CONCLUSIONS