Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals. It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14.
Later, when radiometric absolute dating methods were developed, they still were not applicable to sedimentary layers.
Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events.
In archaeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history).
This is a radiometric technique since it is based on radioactive decay.
Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide.