Wait, so how does carbon dating work?
First, 14C is the isotope that decays and is measured in carbon dating. Secondly, the level of 14C in the atmosphere has remained constant, as it is continually replenished in the atmosphere by the bombardment of nitrogen with cosmic rays. This means that as “old” 14C decays, “new” 14C takes it’s place. This is the process that allows for carbon dating of living things. The bone you mention stopped taking up 14C 20,000 yrs ago, therefore would have gone through ca. 4 half lives since its formation. The leather in your jacket was from a cow killed relatively recently, and would still have the present level of 14C. This difference allows for the determination of the age of the different items.
