Understanding Radiocarbon Dating

This discussion is a simplified introduction to radiocarbon dating. There are exceptions to the theories and relationships introduced below that are beyond the scope of this discussion.

First, remember that carbon is the basis of life and is in all living beings.

“Radiocarbon” is radioactive carbon. It is also called carbon-14 and is often written in the form “14C”. It is present in all living things in tiny, tiny amounts. Since it is radioactive, it gradually fades away by radioactive decay until it is all gone. Radiocarbon dating uses carbon-14 to determine the last time something (or someone) was alive.

Carbon-14 originates in the upper atmosphere of the earth. It is created when neutrons originating from solar radiation bombardment collide with nitrogen in the air. A reaction occurs in a tiny number of these collisions converting nitrogen to carbon-14. This carbon-14 immediately starts to radioactively decay, but is constantly being re-created. This leaves the amount in the air relatively constant. It immediately reacts with oxygen in the air forming carbon dioxide (CO2). This carbon dioxide rapidly mixes throughout the atmosphere, where at ground level it is taken in by plants during photosynthesis. This process is constantly ongoing, so that at any point in time, the amount of carbon-14 in living plants is the same as the amount of carbon-14 in the air around them.

Living plants are an active component of the overall food chain. Animals eat the plants, and animals eat animals, and we eat plants, and we eat animals. Therefore all living plants, animals and human beings have the same amount of carbon-14 in their bodies at the same time. Their bodies are said to be in “equilibrium” with carbon-14 in the air. Although the carbon-14 is radioactively decaying away in the body, it is constantly being replaced by new photosynthesis or the ingestion of food, leaving the amount relatively constant.

When a plant stops photosynthesizing carbon dioxide, or when an animal or human being stops eating, the ingestion of carbon-14 also stops and the equilibrium is disrupted. From that time forward, the only process at work in the body is the radioactive decay. Eventually, all the carbon-14 in the remains will disappear. This principal applies equally to a person dying, a corn stalk being cut down, or to a soybean plant being pulled out of the ground. When they stop living they stop taking in carbon-14 from the air around them, and the amount of carbon-14 in the remains gradually disappears.

A radiocarbon dating laboratory is able to measure the amount of carbon-14 remaining in a fossil. It then uses this information to determine the last time the fossil was respiring carbon (i.e. eating or photosynthesizing). It is able to do this using the known “half-life” of carbon-14. The half-life of carbon-14 is the amount of time it takes for ½ of the original amount to disappear by radioactive decay. It is about 5700 years and means that every 5700 years the amount of carbon-14 in a fossil is only ½ of what it was 5700 years ago. It also means that if a radiocarbon dating laboratory sees that a dead plant has 50% as much carbon-14 in it than in a living plant, the dead plant was last living about 5700 years ago.

After 50,000 years a fossil won’t have any carbon-14 left in it. It will have all disappeared by radioactive decay. When a radiocarbon dating laboratory doesn’t see any carbon-14 in a fossil, it knows the fossil is more than 50,000 years old. Petroleum and dinosaur bones are examples of fossil materials that don’t have carbon-14 remaining in them. They are from more than 50 million years ago.

Radiocarbon dating is applicable to biobased content measurements in manufactured products because those products contain some combination of recently living materials and fossil materials. The recently living materials (the biobased component) have a lot of carbon-14 in them relative to the fossil materials (derived from petroleum) which have no carbon-14 in them. This means that all the carbon-14 in the product comes from the biobased component. In the case of a product containing both crop-derived components and petrochemical components, the biobased content analysis will use the carbon-14 content to calculate how much of the product is derived from the plant components vs. the petroleum derived components.