Carbon dating methodology
Plankton absorbs, Carbon-14 from the ocean much like terrestrial plants absorb Carbon-14 from the air.
So every living thing has a certain amount of radiocarbon within them.
After an organism dies, the radiocarbon decreases through a regular pattern of decay. The time taken for half of the atoms of a radioactive isotope to decay in Carbon-14’s case is about 5730 years.
He had assumed that amounts of Carbon-14 in the atmosphere had remained constant through time.
In fact, levels of Carbon-14 have varied in the atmosphere through time.