A 10 gram sample of U-238Now that has changed, and some important discoveries are being made.
The atmosphere contains many stable carbon atoms and relatively few radiocarbon atoms.
The raw radiocarbon date of any sample can then be converted to true date by using this calibration table.
This calibration step eliminates any concern about fluctuations in historic radiocarbon to stable carbon ratios or decay rates.
Thus, the ratio of radiocarbon to stable carbon in a living plant is the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.
Animals (and humans) get their carbon atoms primarily from what they eat (i.e., plants).