Why does Carbon 12 in dead tissue ''remain'' undisturbed?
That's a pretty short question out of any context. But I'll give it a shot.
The atmosphere contains carbon mostly as CO2, Carbon comes in three isotopes 12C, 13C and 14C (one millionth part) and the rest is mostly stable 12C. 13C is also stable and rare. 14C is unstable with a half time of some 5700 years. It forms when nitrogen (14N) is hit by a neutron created by cosmic radiation. 14C, with a half-life of 5730 years, then it decays back to 14N again. So for the CO2 in the air there is a constant production and decaying of 14C balancing each other more or less (emphasis definitely on more or less).
While an organism is alive and is taking in carbon from the atmosphere, it contains this balanced ratio of 14C. However, after death the growing life processes for exchanging carbon such as feeding, breathing and photosynthesis ceases obviously. The 14C in dead tissues continually decreases by that radioactive decay and it will not be replenished anymore but the 12C (13C) is not affected by that process. It remains basically the same. There are decay processes and eventually perhaps petrification but the ratio between 14C and 12C decreases constantly. So by measuring the 14C ratio and comparing that amount to the original, a date for the death of the organism can be estimated.
There are a lot of complications though, leading to the speculation that 14C carbon dating is worthless. However the methods are so much more elaborated now that all those problems appear to have been overcome. There is a lot more to tell but that would get very technical.
thank you andre
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