We measure the current ratio of C-14/C-12 and compare it to naturally occurring ratio.It does not depend on the amount of carbon in the environment or even the amount in the sample. Most scientists today believe that life has existed on the earth for billions of years.This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.Once the material solidifies the decay product will thereafter be fixed in place with the radioactive material.This is the date we determine with radiometric dating.Radiometric dating works by measuring how much a radioactive material has decayed, and using its known decay rate to calculate when the material was solidified.
The relative abundance of carbon-14 varies slightly with solar flares, magnetic field fluctuations, etc.
They all have six protons since they are carbon, and therefore they are identical chemically.
The body cannot tell them apart, so they maintain approximately the same relative abundance in living tissue as it does in the atmosphere.
Of course, there are many problems with such dating methods, such as parent or daughter substances entering or leaving the rock, as well as daughter product being present at the beginning.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers.