In…

Orr, James C. et al (2017). Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP), Geosci. Model Dev., 10, 2169–2199, https://doi.org/10.5194/gmd-10-2169-2017

…a "standard" (14C/C) molar isotopic ratio is given as 1.170e-12.


“For 14C, we adopt as a reference the standard isotopic fractional abundance 14𝐑std (14C/C) of 1.170 × 10⁻¹², which follows from the absolute international standard activity for 14𝐑std of 13.56±0.07 disintegrations per minute (dpm) per g C (Karlen et al., 1965) and a radiocarbon half-life of 5700±30 years (Audi et al., 2003; Bé et al., 2013). For comparison, Karlen et al. (1965) used the now outdated value for the half-life (5730±40 years, Godwin, 1962) to infer that 14𝐑std = 1.176×10⁻¹² …”