Lesson 3 Enrichment

Using Radionuclides to Date Materials

As scientists have come to understand the various aspects of different radionuclides, they have found many practical uses for this knowledge. One of the practical applications is in the area of scientific dating of old materials. You have probably heard of carbon-14 dating, but do you know how, and why, it works?

This enrichment reading and exercise will help you understand how knowledge of radionuclides is very useful for scientists in many different fields. Scientists apply known facts about radionuclides to understand much about was happening on Earth at various times in the past.

Radiocarbon (Carbon-14) Dating: A Scientific Tool

As we’ve learned elsewhere in this unit, most carbon atoms on Earth are stable. Carbon atoms all have 6 protons, and the most abundant isotope has 6 neutrons. Carbon-12 is the predominant isotope of carbon on our planet. There are other isotopes of carbon, however — carbon-13 and the radioactive carbon-14. Carbon-11 is another radioisotope, but has very short half-life, allowing it to be used in nuclear medicine but not in natural systems studies.

When cosmic radiation enters the Earth’s atmosphere, energetic neutrons collide with nitrogen-14 atoms in the air. (Neutron radiation is part of cosmic radiation, which is in the form of both rays and particles.) When hit by an ionizing neutron from the cosmic radiation, a nitrogen- 14 atom becomes one carbon-14 atom and one hydrogen atom. Eventually the carbon-14 decays back into stable nitrogen-14.

Carbon-14 is radioactive. By volume, about 78 percent of our planet’s air is made up of nitrogen, so cosmic radiation produces a substantial number of carbon-14 atoms as it penetrates the atmosphere. Like other carbon isotopes, the carbon-14 atoms combine with oxygen atoms in the air to make molecules of carbon-dioxide gas.

Carbon-dioxide is naturally absorbed by Earth’s plants. Through photosynthesis, plants incorporate the carbon-14 atoms in the carbondioxide into their own fibers. Thus, plants contain radioactive carbon- 14 along with stable carbon, and when humans and animals eat the plants — or when they eat other animals that have eaten plants or animals — they absorb carbon-14 into their own bodies through the natural food chain. Every living thing on Earth is naturally radioactive because of the absorption of carbon-14 (and other radionuclides) through the food chain.

Some carbon-dioxide, of course, is formed of molecules made of oxygen and stable isotopes of carbon. By weight, human beings are composed of about 23 percent carbon, both stable and radioactive isotopes. Remember, atoms of the same element have exactly the same chemical properties and reactivity, whether or not they are radioactive. So carbon-dioxide gas can have stable carbon atoms, or carbon-14 atoms, or both. It’s still carbon-dioxide.

The existence of both radioactive and non-radioactive isotopes of carbon in the food chain and in the air, and thus in all living matter, provides scientists with a particularly useful tool for determining the age of many formerly living materials.

When an organic entity dies, it stops eating and breathing (or otherwise respiring if it doesn’t “breathe” in the terrestrial way). When the organism stops eating and breathing, it stops taking in new carbon, including carbon-14.

The carbon-14 in its body before it died continues to decay at the steady, predictable half-life of carbon-14. With no new carbon-14 being added, eventually the remains will lose their radioactive carbon. The amount of stable carbon in the organic entity, however, does not change after it dies. The stable carbon remains constant.

Like other radionuclides, carbon-14 has its own unique half-life — approximately 5,730 years. By measuring the amount of stable carbon- 12 against the level of carbon-14 still present in the remains, scientists can determine the ratio of carbon-14 to carbon-12 in the material. They then compare that ratio to the ratio in a living organism, and thereby determine how long ago the organism died.

In all living beings on our planet, the ratio of carbon-14 to carbon-12 is essentially constant. So any measurable decrease in the ratio indicates how much carbon-14 has decayed and, thus, when the organism died.

This technique allows scientists to calculate, often to within a few decades, the age of dead plant, animal, insect, or human remains. Radiocarbon dating, however, is only effective for organisms that have been dead for several hundred years or more — because enough carbon-14 must decay for the decrease in the ratio to be detectable. At the same time, of course, once all the carbon-14 has decayed away, radiocarbon dating is no longer possible. Based on the half-life of carbon-14, this means scientists can date organic material as old as about 50,000 years.

Organic material can dissolve and work its way into a mixture or matrix of other materials and substances. If organic material can be found with non-organic materials, the organic material can tell scientists much about its environs.

Carbon-14 dating can be applied to the remains of shell fish found on underwater rocks or along seashores, for example. Radiocarbon dating can tell us much about an ecosystem’s overall history. Carbon- 14 dating can even be applied to carbonate deposits from fresh water lakes and rivers to date changes in the shape or direction of the particular body of water over the previous 50,000 years.

Ever since radiocarbon dating was discovered in the late 1940s, the technique has been used around the world as a standard scientific dating methodology. Archeologists, anthropologists, geologists, zoologists, biochemists, ecologists, and many other scientists use the technique.

Radiocarbon Dating and Ancient Civilizations

Carbon-14 dating is especially useful in studying ancient human civilizations. Recorded human history, after all, goes back only about 10,000 to 12,000 years, putting all of it well within the dating range of carbon-14. For periods before we started consciously recording our own history, radiocarbon dating is one of our best methods of reading our own past.

Even after soft tissues and fibers have degraded too much to provide a useable sample, the teeth and bones of human beings and animals still contain carbon-14 from when the organism was alive (unless it’s more than 50,000 years old). And, under the right circumstances, even soft tissues and plant fibers are preserved for very long times.

Before the twentieth century’s artificial fabrics (e.g., nylon, polyester, rayon), human beings made most of their clothing out of some kind of animal- or plant-derived material (e.g., wool, fur, skins, leathers, cotton, flax, straw, leaves and other vegetation).

Often, ancient human beings’ tools, utensils, buildings, and artistic items were also made partly of organic material — or decorated with paints and stains made out of plant or animal materials. In such cases, carbon-14 dating can even be used to date many non-organic cultural artifacts, even when the object itself was never alive, or has no carbon in it.