Carbon-14 Dating:

Background Information

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Carbon dating is used to determine the age of biological artifacts up to 50,000 years old. This technique is widely used on recent artifacts, but teachers should note that this technique will not work on older fossils (like those of the dinosaurs which are over 65 million years old). This technique is not restricted to bones; it can also be used on cloth, wood and plant fibers. Carbon-14 dating has been used successfully on the Dead Sea Scrolls, Minoan ruins and tombs of the pharohs among other things.

 

What is Carbon-14?

Carbon-14 is a radioactive isotope of carbon. Its has a half-life of about 5,730 years. The short half-life of carbon-14 means its cannot be used to date extremely old fossils. Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 remains undecayed).

 

How is Carbon-14 formed?

Carbon-14 is created from nitrogen-14 in the upper atmosphere of the earth. Radiation from the sun collides with atoms in the atmosphere. These collisions create secondary cosmic rays in the form of energentic neutrons. When these neutrons collide with nitrogen-14 in the atmosphere carbon-14 can be created. Nitrogen normally occurs in a seven proton, seven nuetron, nitrogen-14 state. When it collides with an energetic neutron it becomes carbon-14, with six protons and eight neutrons and gives off a hydrogen atom with one proton and zero neutrons.

 

How is Carbon-14 used to date artifacts?

Most of the carbon on Earth exists as carbon-12. Carbon-14 is an isotope of carbon, which exists only is small amounts in the environment (1 in one trillion carbon atoms is carbon-14). The ratio of carbon-14 to carbon-12 in the atmosphere and on earth is nearly constant, although there has been some change in carbon-14 concentration over the last 10,000 years.

Carbon-14 formed in the atmosphere can combine with oxygen to form carbon dioxide (CO2). This carbon-14 labeled carbon dioxide is taken up by plants in their normal process of photosynthesis. Animals and people that eat these plants take the carbon-14 into their bodies as well.

Carbon-14 in the bodies of animals and plants is constantly decaying. However, the decaying carbon-14 is constantly being replaced as the plant or animal consumes more carbon-14 from the air or through its food. At any given moment all living plants and animals have approximately the same percentage of carbon-14 in their bodies.

When a plant or animal dies it stops bringing in new carbon-14. However, the carbon-14 already in the organism's body continues to decay at a constant rate. Therefore, the amount of carbon-14 in an artifact decreases at a predictable rate while the amount of carbon-12 remains constant. By comparing the ratio of carbon-14 to carbon-12 in an artifact to the ratio of carbon-14 to carbon-12 in living organisms scientists can determine the age of an artifact.

 

So how do I use my carbon-14 data?

A formula used in carbon dating is:

t = [ln (Nf/No) / (-0.693)] * t1/2

where ln (Nf/No) = the natural logarithm of the percent carbon-14 in the sample compared to the percent carbon-14 in living tissue, and t1/2 = the half-life of carbon-14 = 5,700 years.

If you wanted to date a fossil, first you would determine the percent carbon-14 it contained compared to a living sample. Imagine your sample contained 20% of the carbon-14 found in a living plant leaf. Then,

t = [ln (0.2) / (-0.693)] * 5,700 years

t = [(-1.609) / (-0.693)] * 5,700 years

t = [2.322] * 5,700 years

t = 13,238 years old

 

Will Carbon-14 dating work on all artifacts?

No. There are a few categories of artifacts that cannot be dated using carbon-14.

First, carbon-14 cannot be used to date biological artifacts of organisms that did not get thier carbon dioxide from the air. This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock. The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating. This restriction extends to animals that consume seafood in their diets, as well.

Carbon dating also cannot be used on artifacts over about 50,000 years old. These artifacts have gone through many carbon-14 half-lives and the amount of carbon-14 remaining in them is miniscule and very difficult to detect.

Carbon dating cannot be used on most fossils, not only because they are almost always too old, but also because they rarely contain the original carbon of the organism. Also, many fossils are contaminated with carbon from the environment during collection or preservation proceedures.

 

How do we know Carbon-14 dating is accurate?

Scientists check the accuracy of carbon dating by comparing carbon dating data to data from other dating methods. Other methods scientists use include counting rock layers and tree rings.

When scientists first began to compare carbon dating data to data from tree rings, they found carbon dating provided "too-young" estimates of artifact age. Scientists now realize that production of carbon-14 has not been constant over the last 10,000 years, but has changed as the radiation from the sun has changed. Carbon dates reported in the 1950s and 1960s should be questioned, because those studies were conducted before carbon dating was calibrated by comparision with other dating methods.

Nuclear tests, nuclear reactors and the use of nuclear weapons have also changed the composition of radioisotopes in the air over the last few decades. This human nuclear activity will make precise dating of fossils from our lifetime very difficult due to contamination of the normal radioisotope composition of the earth with addition artificially produced radioactive atoms.

 

How do scientists date older fossils?

Although the half-life of carbon-14 makes it unreliable for dating fossils over about 50,000 years old, there are other isotopes scientists use to date older artifacts. These isotopes have longer half-lives and so are found in greater abundance in older fossils.

Some of these other isotopes include:

 

Links to more Carbon-14 Dating Information:

Marshall Brain's How Stuff Works: a clear concise explanation, but not as detailed.

Carbon Dating Home Page: another breif summary.

Carbon-14 Dating: this page includes pictures of carbon-14 formation and a more in-depth explanation of the formation process.

 

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