Francey et al. (1992)
Also: Francey et al. 1992
A 1000-year high precision record of δ13C in atmospheric CO2
Francey, R. J., Allison, C. E., Etheridge, D. M., Trudinger, C. M., Enting, I. G., Leuenberger, M., Langenfelds, R. L., Michel, E. & Steele, L. P. (1992). A 1000-year high precision record of δ 13C in atmospheric CO2. Science. 51(2),170-193. Article
https://doi.org/10.1126/science.256.5053.74
References
All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.