International Radiocarbon Intercomparison (IRI)
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| International Radiocarbon Intercomparison (IRI) | |
|---|---|
| Name | International Radiocarbon Intercomparison |
| Discipline | Radiocarbon dating |
| Established | 1960s |
| Founders | International Atomic Energy Agency |
| Participants | Laboratories worldwide |
International Radiocarbon Intercomparison (IRI) is a coordinated series of proficiency tests and calibration exercises that benchmark radiocarbon measurement accuracy across laboratories and institutions. Conceived amid concerns about interlaboratory variability, the program produced standardized sample sets and protocol comparisons to harmonize results from accelerator mass spectrometry, gas proportional counting, and liquid scintillation counting. IRI activities influenced calibration curves and consensus standards used by archaeological, geological, and environmental sciences communities.
Background and Objectives
The initiative arose as part of broader efforts by the International Atomic Energy Agency and collaborating bodies to resolve discrepancies highlighted in comparisons involving the University of Cambridge, University of California, Berkeley, Max Planck Society, Harvard University, and national metrology institutes such as the National Institute of Standards and Technology and Physikalisch-Technische Bundesanstalt. Objectives included quantifying interlaboratory bias, establishing proficiency-testing procedures analogous to intercomparisons organized by the Bureau International des Poids et Mesures and the International Organization for Standardization, and informing calibration efforts like those leading to the IntCal radiocarbon calibration curves. Stakeholders ranged across institutions including the Smithsonian Institution, British Museum, CNRS, Australian National University, and the Russian Academy of Sciences.
Organization and Governance
IRI campaigns were coordinated by consortia composed of representatives from the International Union of Geological Sciences, International Union for Quaternary Research, and agencies such as the World Meteorological Organization and the United Nations Educational, Scientific and Cultural Organization. Governance drew on models used by the Intergovernmental Panel on Climate Change and advisory panels with expertise from the Royal Society, National Academy of Sciences (United States), Deutsches Archäologisches Institut, and metrology bodies like the International Laboratory Accreditation Cooperation. Working groups included specialists affiliated with the University of Oxford, University of Cambridge (England), ETH Zurich, and the Scripps Institution of Oceanography.
Participants and Sample Materials
Participants comprised national and university laboratories including the Australian Nuclear Science and Technology Organisation, Centre for Isotope Research (CIO), Radiocarbon Laboratory, Waikato University, University of Arizona, University of Bern, University of Groningen, University of Copenhagen, Universidade de São Paulo, Leibniz Institute for Applied Geophysics, Uppsala University, and others. Sample materials included certified reference materials drawn from collections at the British Geological Survey, Natural History Museum, London, National Museums of Scotland, and archives at the Library of Congress, with matrices such as tree-ring cellulose from chronologies used by A.E. Douglass, peat from sites studied by Gustav Olaf Ahlmann, carbonate samples from corals linked to studies by Charles D. Keeling, and marine sediments relevant to work by Wallace S. Broecker. Reference materials paralleled standards like those promulgated by the International Bureau of Weights and Measures and preserved by repositories such as the Smithsonian Institution and the Vlaams Instituut voor het Zeewezen.
Methods and Protocols
Protocols compared measurement modalities including Accelerator mass spectrometry (AMS) systems developed at institutions like McMaster University and Lawrence Livermore National Laboratory, gas proportional counting techniques refined at Brookhaven National Laboratory and Australian Nuclear Science and Technology Organisation, and liquid scintillation counting practices used by laboratories at Columbia University and University of Tokyo. Pretreatment chemistries referenced methods from laboratories associated with Gordon G. Shrum, Willard Libby legacy procedures, and contaminant-removal protocols adopted in studies by Gosta Holmgren. Quality assurance and uncertainty estimation used statistical approaches consistent with guidance from the International Organization for Standardization and the Joint Committee for Guides in Metrology.
Intercomparison Campaigns and Results
IRI rounds produced multi-center datasets revealing systematic offsets traced to sample pretreatment, conversion efficiencies, and instrument calibration tied to secondary standards maintained by the National Physical Laboratory (United Kingdom), Physikalisch-Technische Bundesanstalt, and the National Research Council (Canada). Campaign reports referenced calibration efforts such as the IntCal13 and IntCal20 projects and informed revisions to age models used in high-profile studies at institutions like the Max Planck Institute for Evolutionary Anthropology, Smithsonian Tropical Research Institute, University of Washington, and Karlsruhe Institute of Technology. Results highlighted successful harmonization among clusters of laboratories including networks centered on European Commission research infrastructures and consortia supported by the European Research Council and National Science Foundation.
Impact on Radiocarbon Dating Standards
Findings from IRI informed adoption of internationally accepted reference materials and consensus protocols adopted by museums such as the British Museum and archives including the National Archives (United States). Corrections identified through IRI underpinned improvements to calibration curves produced by collaborators at Queen's University Belfast, University of Arizona Lunar and Planetary Laboratory, and laboratories contributing to the IntCal Working Group. The program influenced accreditation criteria used by national agencies such as the Food and Drug Administration for tracer studies and guided best practices cited in methodological chapters of monographs from publishers like Cambridge University Press and Oxford University Press.
Challenges and Future Directions
Ongoing challenges include maintaining long-term reference materials in repositories like the Natural Environment Research Council collections, addressing matrix-specific contamination identified in studies by University of Helsinki and Peking University, and integrating novel measurement platforms emerging from research at Lawrence Berkeley National Laboratory and GNS Science. Future directions emphasize coordination with the IntCal Working Group, expansion of intercomparisons to include emerging laboratories in regions served by institutions such as Indian Institute of Science, Universidad Nacional Autónoma de México, and University of Cape Town, and incorporating data harmonization frameworks used by the Global Carbon Project and Paleoclimate Model Intercomparison Project.