Ignoring Suess Dilution Effect:

  source for logc14.jpg was http://2.bp.blogspot.com/-G79oXdgIZC4/UnteTCVaGGI/AAAAAAAAAA0/AbSzY3s5ZP0/s1600/logc14.jpg

  See also logc14_two_half-lives.png (my annotation of it, showing half-life = 11.5 years).

  See also ndp057_Fruholmen_d14CO2_hairlines2.png (different dataset, showing half-life of 10.5 years).


For newer/better analyses, see

  https://sealevel.info/14C/


https://www.sciencedirect.com/science/article/abs/pii/0160412079900059#:~:text=The%20Suess%20Effect%20is%20a%20term%20which%20has%20come%20to%20signify%20the%20decrease%20in%2014C%20in%20atmospheric%20CO2%20owing%20to%20admixture%20of%20CO2%20produced%20by%20the%20combustion%20of%20fossil%20fuels
says:

"The Suess Effect is a term which has come to signify the decrease in 14C in atmospheric CO2 owing
to admixture of CO2 produced by the combustion of fossil fuels."


https://www.un.org/en/observances/end-nuclear-tests-day/history#:~:text=Atmospheric%20testing%20was,China%20in%201980
says:

"Atmospheric testing was banned by the 1963 Partial Test Ban Treaty. Negotiations had largely responded
to the international community's grave concern over the radioactive fallout resulting from atmospheric
tests. The United States, the Soviet Union and the United Kingdom became Parties to the Treaty; France
and China did not. France conducted its last atmospheric test in 1974, China in 1980."


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.

Excerpt:

“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 (1965) used the now outdated value for the half-life (5730±40 years, Godwin, 1962) to infer that 14𝐑std = 1.176×10⁻¹² …”