As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.
Carbon-14 is most abundant in atmospheric carbon dioxide because it is constantly being produced by collisions between nitrogen atoms and cosmic rays at the upper limits of the atmosphere.
Secondly, these purported variations are very dubious.
While the lighter isotopes C has decayed that what remains can no longer be measured. In 5,730 years half of the C in the atmosphere, and therefore in plants and animals, has not always been constant.In 2010, Purdue University published a research paper stating that their researchers had detected slight fluctuations in radioactive isotope decay rates "in synch with the rotation of the sun's core." The article also stated: Has there been any further research on this, and has it been found to affect carbon dating techniques or other archeological dating methods? Graven, "Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century," Proceedings of the National Academy of Sciences 112.31 (2015): 9542-9545.Are the fluctuations large enough to call into question currently accepted geological dates? From this article, "By 2050, fresh organic material could have the same $^$C/C ratio as samples from 1050, and thus be indistinguishable by radiocarbon dating. Along with hydrogen, nitrogen, oxygen, phosphorus, and sulfur, carbon is a building block of biochemical molecules ranging from fats, proteins, and carbohydrates to active substances such as hormones.All carbon atoms have a nucleus containing six protons.