TY - JOUR

T1 - Reassessment of the C-13/C-12 and C-14/C-12 isotopic fractionation ratio and its impact on high-precision radiocarbon dating

AU - Fahrni, Simon M.

AU - Southon, John R.

AU - Santos, Guaciara M.

AU - Palstra, Sanne W. L.

AU - Meijer, Harro A. J.

AU - Xu, Xiaomei

PY - 2017/9/15

Y1 - 2017/9/15

N2 - The vast majority of radiocarbon measurement results (C-14/C-12 isotopic ratios or sample activities) are corrected for isotopic fractionation processes (measured as C-13/C-12 isotopic ratios) that occur in nature, in sample preparation and measurement. In 1954 Harmon Craig suggested a value of 2.0 for the fractionation ratio b that is used to correct C-14/C-12 ratios for shifts in the C-13/C-12 ratios and this value has been applied by the radiocarbon community ever since. While theoretical considerations suggest moderate deviations of b from 2.0, some measurements have suggested larger differences (e.g. b = 2.3, measured by Saliege and Fontes in 1984). With the high precision attained in radiocarbon measurements today (+/- 2%), even a relatively small deviation of b from 2.0 can impact the accuracy of radiocarbon data, and it is, therefore, of interest to re-evaluate the fractionation corrections. In the present study, the fractionation ratio b was determined by independent experiments on the chemical reduction of carbon dioxide (CO2) to elemental carbon (graphitization reaction) and on the photosynthetic uptake of CO2 by C-3 and C-4 plants. The results yielded b = 1.882 +/- 0.019 for the reduction of CO2 to solid graphite and b = 1.953 +/- 0.025 for the weighted mean of measurements involving C-3 and C-4 photosynthesis pathways. In addition, the analysis of over 9600 full-sized OX-I and OX-II normalizing standards measured between 2002 and 2012 confirms b values lower than 2.0. The obtained values are in good agreement with quantum mechanical estimates of the equilibrium fractionation and classic kinetic fractionation as well as with results from other light three-isotope systems (oxygen, magnesium, silicon and sulfur). While the value of the fractionation ratio varies with the relative importance of kinetic and equilibrium fractionation, the values obtained in the present study cluster around b = 1.9. Our findings suggest that a significant fraction of all samples ("unknowns") would be shifted by 2% (16 radiocarbon years) or more due to this effect: for example, for b = 1.882, between 16.8% and 25.9% of almost 60,000 radiocarbon values measured at the Keck Carbon Cycle AMS facility between 2002 and 2012 would be affected. The implications for radiocarbon dating and its accuracy are discussed. (C) 2017 Elsevier Ltd. All rights reserved.

AB - The vast majority of radiocarbon measurement results (C-14/C-12 isotopic ratios or sample activities) are corrected for isotopic fractionation processes (measured as C-13/C-12 isotopic ratios) that occur in nature, in sample preparation and measurement. In 1954 Harmon Craig suggested a value of 2.0 for the fractionation ratio b that is used to correct C-14/C-12 ratios for shifts in the C-13/C-12 ratios and this value has been applied by the radiocarbon community ever since. While theoretical considerations suggest moderate deviations of b from 2.0, some measurements have suggested larger differences (e.g. b = 2.3, measured by Saliege and Fontes in 1984). With the high precision attained in radiocarbon measurements today (+/- 2%), even a relatively small deviation of b from 2.0 can impact the accuracy of radiocarbon data, and it is, therefore, of interest to re-evaluate the fractionation corrections. In the present study, the fractionation ratio b was determined by independent experiments on the chemical reduction of carbon dioxide (CO2) to elemental carbon (graphitization reaction) and on the photosynthetic uptake of CO2 by C-3 and C-4 plants. The results yielded b = 1.882 +/- 0.019 for the reduction of CO2 to solid graphite and b = 1.953 +/- 0.025 for the weighted mean of measurements involving C-3 and C-4 photosynthesis pathways. In addition, the analysis of over 9600 full-sized OX-I and OX-II normalizing standards measured between 2002 and 2012 confirms b values lower than 2.0. The obtained values are in good agreement with quantum mechanical estimates of the equilibrium fractionation and classic kinetic fractionation as well as with results from other light three-isotope systems (oxygen, magnesium, silicon and sulfur). While the value of the fractionation ratio varies with the relative importance of kinetic and equilibrium fractionation, the values obtained in the present study cluster around b = 1.9. Our findings suggest that a significant fraction of all samples ("unknowns") would be shifted by 2% (16 radiocarbon years) or more due to this effect: for example, for b = 1.882, between 16.8% and 25.9% of almost 60,000 radiocarbon values measured at the Keck Carbon Cycle AMS facility between 2002 and 2012 would be affected. The implications for radiocarbon dating and its accuracy are discussed. (C) 2017 Elsevier Ltd. All rights reserved.

KW - Photosynthesis

KW - Radiocarbon

KW - C-14

KW - AMS

KW - High-precision measurement

KW - Three-isotope system

KW - Isotopic fractionation

KW - Fractionation correction

KW - ACCELERATOR MASS-SPECTROMETRY

KW - SAMPLE PREPARATION

KW - MALONIC-ACID

KW - C-14 DATA

KW - ICP-MS

KW - CARBON

KW - OXYGEN

KW - GRAPHITE

KW - GAS

U2 - 10.1016/j.gca.2017.05.038

DO - 10.1016/j.gca.2017.05.038

M3 - Article

VL - 213

SP - 330

EP - 345

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -