Nuclei of 14C experience beta decay. The product of the decay is 14N, some unimportant particles and energy. Lets suppose that your sample is perfectly closed system where nothing can enter or ecape it, there are no chemical reactions with surroundings. Furthermore, lets suppose that at the moment when the sample was created there were N0 particles of 14N and C0 particles of 14C, and L is 14C decay constant. Than at the time "t" there should be C0.Exp[-Lt] particles of 14C and N0+C0(1-Exp[-Lt]) particles of 14N in the sample. In this idealised closed system ratio 14C/14N should be time dependent as folows: C0.Exp[-Lt]/(N0+C0(1-Exp[-Lt])). Since this point everything seems fine - if 14C/14N ratio deviates from expected value at the time "t" something happened to the sample and dating isn t reliable.

But I see several problems with this theory:

So I would say 14C/14N is not suitable indicator of sample reliability.

JS

Dear Jan

Thank You for the very clear and interesting answer. If I correctly understand You, the problem is that we would need to have information on the past atmospheric C14/N14 ratio changes - a sort of N14 calibration curve. Is it correct? 

No, this is not the major problem. Imagine that you know very precisely what was the atmospheric 14C/14N ratio in the arbitrary time.

Since metabolic chemical processes in each organism/plant/... are unique, it utilise carbon and nitrogen at various rate. I would expect that for example oak tree gathers nitrogen faster than elephant (this information is without guarantee of course). Moreover, amount of utilised nitrogen and carbon could possibly depend on local conditions, too (food, soil, etc...)

Therefore before the death of the organism ratio 14C/14N can vary. Furthermore, even after burial the sample is in contact with water, soil and so on - and ratio 14C/14N can be modified.

But no matter how carbon - saturated is food or soil, carbon 12, 13, 14 will enter all the chemical pathways in the (nearly) same rate. I used term nearly because heavier carbons aren t so mobile than lighter one, what slightly reflects to the chemical reaction kinetics. So the ratio 14C/12C or 13C/12C depends only on the natural occurence in the environment and is suitable indicator of sample isotopic fractionation

Thank You. But what are the main issues in using delta 13C ratio to understand possible alteration effects?in other words - how many pre-and-post depositional effects may alter delta 13C in samples dated? 

13C/12C ratio is an usefull tool to reveal wheather your sample experienced isotopic fractionation or not. As I wrote, all of carbon isotopes enter the same chemical reactions. But the heavier atom, the lower mobility and worse reaction kinetics. In standard chemistry there s no reason to worry about such a negligible effects, but since we are talking about radiocarbon dating we are dealing with delicate measurements where even such a weak effects can make a great time shift.

Samples can be isotopicaly fractionated mainly during chemical preparation before measurement (used chemical preparation depends on 14C activity measurement method - scintilation chambers, AMS, ...). This way the sample can experience enrichment or depletion in 14C and 14C/12C ratio may be deviated from original value. This finally imply wrong calculated age.

Since 14C decays (and its amount in sample depends on time and fractionation too), 13C doesn t. It s a stable carbon isotope. So if ratio 13C/12C differs from standard, it indicates that sample was fractionated. The isotopic fractionation rate can be calculated and therefore also 14C activity can be corrected - and therefore you get more reliable age.

Thank You. However, if i do correctly understand, 13C concentration in the environment changes through time and local weather, moreover different living organism can absorb higher or lower 13C quantity for photosynthesis (i am thinking for example, a C3 against a C4 plant). So 13C/12C ratio is used to check for isotopic fractionation in laboratory treatment, but may it also by used for paleoenvironmental changes riconstruction and/or pre-depositional radiocarbon alteration (reservoir effects etc...)?

I don t think there is a simple answer to your question. First of all I d need to know very precisely what is the aim of your research and how do you want to achieve the goals. Maybe the most important is how far to the history would you like to go, which reservoirs are you interested in, what is your experimental setup, ...

Good question, You're right. You can find the case study described at

https://www.researchgate.net/publication/234154045_Mangroves_Shell-middens_and_ancient_seascapes_of_the_northern_coast_of_the_Arabian_Sea_%28Pakistan%29

we had several samples of mangrove shells (mostly adult terebralia palustris specimens, with a few telescopium telescopium) which shown Delta 13C ratio ranging from about +1.30 to -6 and could only tentatively attribute this difference as sign of climatic stress tied to the long run changes in monsoonal cycles, that lead to episode of exhaustion/rejuvenation of the mangrove swamps in the area (which is now a desert). However, we couldn't go much far in verifying this hypothesis!

Data Mangroves, Shell-middens and ancient seascapes of the northe...

Hmmm. This is quite a hard issue for me, but I suppose there is a solution.

Mixing time within single Earth hemisphere is about 2 years (not sure if i remember it correctly) and exchange time between hemispheres is little bit more (maybe 10 years). So if something happen to the some gasseous substance in the atmosphere, after not too long time it should affect the whole planet atmosphere.

If something happened with 13C (especially 13CO2) concentration, it should be possible to trace it (with time delay of several years due to atmospheric mixing). 6 kY is long time, so tree rings apparently are not the option. But what about air bubbles in arctic glaciers? Have you already tried this?

Ideally that would be good, but we don't know how to take in account possible local marine and mixed freshwater/marine reservoirs....