Carbon has a weight of twelve atomic mass units (AMUs), and is the building block of all organic matter (plants and animals).A small percentage of carbon atoms have an atomic weight of 14 AMUs. Carbon-14 is an unstable, radioactive isotope of carbon 12.This carbon-14 dioxide then ends up in humans and other animals as it moves up the food chain. Libby introduced the carbon-14 dating method at the University of Chicago.There is then a ratio of carbon-14 to carbon-12 in the bodies of plants, humans, and other animals that can fluctuate, but will be fixed at the time of death. The premise behind the method is to determine the ratio of carbon-14 left in organic matter, and by doing so, estimate how long ago death occurred by running the ratio backwards.There are a few reasons to believe this assumption is erroneous.The industrial revolution greatly increased the amount of carbon-12 released into the atmosphere through the burning of coal.This would have resulted in an enormous amount of carbon-12 being released into the oceans and atmosphere.The effect would be not unlike opening a can of soda and having the carbon dioxide fizzing out.
The carbon-14 dioxide is utilized by plants in the same way normal carbon dioxide is.For every one trillion carbon-12 atoms, you will find one carbon-14 atoms.The carbon-14 that results from the reaction caused by cosmic radiation quickly changes to carbon dioxide, just like normal carbon-12 would.The formation of carbon-14 increases with time, and at the time of creation was probably at or near zero.Since carbon-14 is radioactive, it begins to decay immediately as its formed.
Also, the atomic bomb testing around 1950 caused a rise in neutrons, which increased carbon-14 concentrations.