Can we assume constant C14 to C12 ratio in living tissues?

[wywong]

Plants prefer C12 to C13 during photosynthesis, so that the C13 to C12 ratio in plants is less than that in the atmosphere (by around 20% for most plants, but less difference in C4 plants like maize). Do plants discriminate against C14 too? If so, won't that cause a considerable uncertainty in carbon-14 dating?

 

[jim mcnamara]

Per https://www.irsn.fr/EN/Research/pub...s-sheets/environment/Documents/Carbone_UK.pdf

C¹⁴ discrimination is minimal

 

[wywong]

Thanks jim mcnamara for your reply.

I find it hard to believe the same photosynthesis process discriminates against C13 but not against C14 which is heavier than C13. I did a google search and found this article:

https://www.cambridge.org/core/jour...e-for-13c12c/7034794E608EB5B6A3F8ED31D4FA38AD

If I understand correctly, the discrimination against C14 is more than twice that against C13, but the former can be more or less corrected for based on the later. That answers my original question.

 

[Anthropocene]

I think the the fractionation is about 2% (15 to 27 per mil). For 14C it is twice as much. In Radicarbon dating (and other 14C applications) , the 13C/12C ratio is used to correct for this effect.

 

[jjoensuu]

Well, looking at articles such as this one...
https://phys.org/news/2021-02-earth-magnetic-field-broke-years.html

...it seems that Earth's magnetic field has went through some tumultuous instability in the past. What happens when it weakens is that it allows a lot more of cosmic radiation to penetrate lower altitudes, and this again will increase the amount of atmospheric C14.

So there are points in the past where C14 cannot be assumed to have been whatever it is assumed to be today due to environmental reasons.

 

[Anthropocene]

The 14C fraction of carbon varies. The cosmonenic production changes (it is about 5 kilogram 14C per year) and the reservoir (the amount of carbon) changes. For instance by burning fosil fuel the amonunt of carbon as CO2 in the atmosphere increases and the 14C/12C ratio drops. This is the fossil fuel effect. The famous journal Radiocarbon has many hundreds of research articles dealing with this interesting question.