Radiocarbon Dates
What is a radiocarbon date?
Once living materials (wood, charcoal, bone) and belongings made of once living materials such as cloth take in naturally occurring carbon during their lifetimes. The radioactive form of carbon, known as carbon-14 decays after the death of the living material and the remaining carbon-14 can then be used to estimate the age of the material; this is known as a radiocarbon date.
While a plant or animal is alive, it takes in carbon from the environment, including a small amount of radioactive carbon called carbon-14. After it dies, it stops taking in carbon, and the carbon-14 slowly breaks down over time. By measuring how much carbon-14 remains, scientists can estimate how long ago the organism died.
A radiocarbon date of 3,000 years BP (Before Present) means the organism died about 3,000 years ago. Scientists use 1950 as the standard starting point so dates can be compared consistently. They can also use calibration tools to convert radiocarbon dates into approximate calendar years.
Indigenous knowledge systems consider all earth as living organisms and indeed the soil (earth) also contains living organisms that take in radioactive carbon; therefore, soil can also be used to estimate age. When soil is dated, scientists must consider that living organisms are constantly adding and moving carbon through the soil, which can sometimes make the soil appear younger than it really is.
Any living materials that might have been incorporated into rocks are typically far too old to be measured using this technique as the limit for radiocarbon dating is around 50,000 years. At that point, the amount of radioactive carbon remaining is too small to be measured.
You said that a radiocarbon date is an estimate of age. What does the number after the +/- mean?
When scientists measure the amount of radioactive carbon-14 left in the sample, they compare it with other samples that also have the same amount of radioactive carbon remaining. Some of these samples have known calendar ages. These dates provide a statistical data set that can be used to produce other age estimates.
The charcoal from the bone at âsowanânihk came back with a reported age of 8,244 +/- 16 BP. This means that there is a 68 percent chance that the real date of the sample falls between 8,228 and 8,260 years ago. The calibration results show that there is a 92.5% chance that the age falls between 9,303 and 9,126 calendar years ago.
You may have heard that the calendar age should be the radiocarbon age minus 1950. However, this is not accurate because the amount of radiocarbon in the atmosphere has changed over time due to factors such as climate change and volcanic eruptions. Scientists use calibration methods to account for these changes and determine a more accurate calendar age.
Why is the radioactive carbon in the atmosphere important?
Living organisms take in both stable and radioactive carbon while they are alive. Plants take in this carbon during photosynthesis. Because the amount of carbon-14 in the atmosphere has changed over time, plants living thousands of years ago may have absorbed more or less carbon-14 than plants today. If scientists do not account for these changes, a sample with extra carbon-14 could appear younger than it really is because it would still contain more carbon-14 after some has decayed.
Digging Deeper
There are many fine websites describing radiocarbon techniques. Below, we provide links to two laboratories that describe the techniques on their webpages: