So, when they die, they contain the same amount of Carbon 14 as their environment.
(f) A dark brown to black mollisol of YD age at the Flaming Arrow site (locality 40a) is highly bioturbated. (d) The Sunshine Paleoindian site in Nevada (locality 44) contains a black paleosol of YD Age. The lower dark gray paleosol of YD age is the Nall paleosol and underlies the Baker paleosol. †Number in parentheses indicates geomorphic position shown in Fig. Position 6, lake or pond shore, is not shown, nor is position 7 for cave sites. †Number in parentheses indicates geomorphic position shown in Fig. Position 6, lake or pond shore, is not shown, nor is position 7 for cave sites. Ahler SA (1986) The Knife River Flint Quarries: Excavation at site 32DU508.
So, long after an organism dies, scientists can compare the amount of Carbon 14 in a sample to records of atmospheric radiocarbon in sediments of different ages.
They can then use this information to calculate how long ago something like a tree or a human stopped taking in radioactive carbon, which indicates when they stopped living.
The radiocarbon dates reported in this list are almost all based on collagen measurements from human archaeologic bone material.
This collagen was isolated according to the methods developed by Berger, Horney and Libby (1964), Ho, Marcus and Berger (1969), Longin (1971) and Protsch (1973).