1491: New Revelations of the Americas Before Columbus

by Charles C. Mann

Along the way, Libby realized, the cascade of interactions creates a trickle of carbon-14 (C14), a mildly radioactive form of carbon that over time disintegrates—decays, as scientists say—into a form of nitrogen. Libby determined that the rate at which cosmic rays create C14 is roughly equal to the rate at which it decays. As a result, a small but steady percentage of the carbon in air, sea, and land consists of C14. Plants take in C14 through photosynthesis, herbivores take it in from the plants, and carnivores take it in from them. In consequence, every living cell has a consistent, low level of C14—they are all very slightly radioactive, a phenomenon that Libby first observed empirically.

When people, plants, and animals die, they stop assimilating C14. The C14 already inside their bodies continues to decay, and as a result the percentage of C14 in the dead steadily drops. The rate of decline is known precisely; every 5,730 years, half of the C14 atoms in

nonliving substances become regular carbon atoms. By comparing the C14 level in bones and wooden implements to the normal level in living tissues, Libby reasoned, scientists should be able to determi...