Educational Resources Lets Talk Science Challenge participants

Several longhouse and other stone structures on Pamiok Island, in Ungava Bay, Quebec. Lkovac via Wikimedia Commons

Several longhouse and other stone structures on Pamiok Island, in Ungava Bay, Quebec. Lkovac via Wikimedia Commons

STEM in Context

The Dating Habits of Archaeologists

Stephanie Halmhofer

Summary

Radiocarbon dating is a method of radiometric dating. Learn how archaeologists use carbon isotopes to find the age of organic materials at archaeological sites.

For decades, people assumed that our ancestors started to make stone tools 2.4 million years ago. But a 2015 study found something different. Stone tools discovered in Kenya were actually 3 million years old! This discovery pushed the development of stone tools back by 700 000 years. It changed the whole story of human evolution!
Knowing the age of an artifact can be key to understanding the history of a site, a group, a region, or even human history in general. People who study human history through physical remains are called archaeologists.

Some of the stone tools found in Kenya. Reprinted by permission from Springer Nature: Nature.
Some of the stone tools found in Kenya. Reprinted by permission from Springer Nature: Nature. (3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya. Sonia Harmand, Jason E. Lewis, Craig S. Feibel, Christopher J. Lepre, Sandrine Prat et al.), (2015).

But how do archaeologists figure out how old something is? The answer depends on whether the material being tested is organic or inorganic. Organic material comes from living things. Food or plants are examples. Inorganic things do not. Stone is an example.

Most archaeological sites do contain organic materials. The most common method for determining their age is radiocarbon dating. For very old objects, archaeologists might use potassium-argon dating. Let’s learn about these methods in more detail.

What is radiocarbon dating?

Accelerator mass spectrometer at Lawrence Livermore National Laboratory in California
Accelerator mass spectrometer at Lawrence Livermore National Laboratory in California (Source: US Department of Energy via Wikimedia Commons).

Radiocarbon dating tests the level of the unstable isotope carbon-14 (14C) in organic materials. Living organisms like plants, animals, and humans all absorb 14C from the atmosphere and the foods they eat. After organisms die, the 14C in their bodies begins to break down and turn into the stable carbon-12 (12C). The half-life of 14C is approximately 5 730 years. This means that 5 730 years after something dies, its bones and tissues will only contain 50% of the 14C it had when it died.

Did you know?

Archaeologists often use the growth rings of trees to study human or ecological history. This is called dendrochronology.

Imagine you’re working on an archaeological dig and you find some elk bones. You could use these elk bones to figure out how old the site is. First, you would take a small sample of bone and carefully clean it to remove anything that doesn’t belong, like contaminants, tree roots or soil. Once the sample is clean, you can run it through an accelerator mass spectrometer (AMS).

This highly sensitive machine measures the amount of 14C and 12C left in the bone. Then you can compare these levels to existing benchmarks and figure out when the elk was eaten. This will give you a very good idea of how old the site is. For example, if there was 50% 14C and 12C left, the bone - and the site - would be about 5 730 years old.

The percentage of carbon-14 in bone decreases by half with each half-life of 5 730 years
The percentage of carbon-14 in bone decreases by half with each half-life of 5 730 years (© 2019 Let’s Talk Science).

Did you know?

The first results of radiocarbon dating were published in 1949 by an American chemist named Willard Libby. He received the 1960 Nobel Prize in chemistry for this work. 

What is potassium-argon dating? 

Radiocarbon dating can be used to date objects that are up to 50 000 years old. Anything older won’t have any 14C left in it. So what about fossils and inorganic materials like those 3-million-year-old stone tools found in Kenya? Archaeologists can date these materials using a similar method called potassium-argon dating. When rocks, including fossils, are created, potassium-40 (40K) becomes trapped inside and slowly decays into argon-40 (40Ar). The half-life of 40K is 1.3 billion years. That means measuring the ratio of 40K to 40Ar can provide dates for objects that are millions of years old.

What is the marine reservoir effect?

Unfortunately, radiocarbon dating and related techniques are not perfect. For example, archaeologists must take the marine reservoir effect into account. 14C levels vary over time. This is because time brings changes in the atmosphere where the 14C is produced. However, oceans are so deep that the 14C from the atmosphere takes a very long time to reach the bottom. Any location in the ocean that is deeper than 10 metres has 14C levels that can be up to 400 years older than on the surface.

Archaeologist working at the site where Richard III was discovered
Archaeologist working at the site where Richard III was discovered (Source: Sue Hutton [CC BY-SA 2.0] via Wikimedia Commons).

What does this mean for archaeologists? They have to remember that the remains of humans and animals with diets rich in seafood will have 14C levels that suggest they are  older than they actually are. Archaeologists take the marine reservoir effect into account by making small corrections to test results. For example, historical records indicate that King Richard III of England died and was buried in 1485. When his skeleton was discovered, radiocarbon dating suggested it had been buried sometime between 1412 and 1449. However, other tests showed that the skeleton contained high levels of marine protein. After taking the marine reservoir effect into account, radiocarbon dating actually pointed to a burial date between 1475 and 1530. These corrected results supported the theory that the skeleton belonged to Richard III.

Summing up...

Radiocarbon dating was first used at the end of the 1940s. Since then, it has remained one of the most important tools used by archaeologists. It is only one of many different dating methods. However, it’s the most commonly used. That’s because most archaeological sites contain organic materials. But for really old artifacts, archaeologists can use potassium-argon dating. 
Radiometric dating can help archaeologists understand all kinds of aspects of the past. It can help them understand technological advances, determine when a group of people moved to a certain region, or even identify a famous king. It’s an archaeologist’s best friend! 

STARTING POINTS

Connecting and Relating
  • Are there any archaeological excavations happening in or near your community? If so, describe what they are investigating.
  • Have you ever visited an archaeological excavation? Explain.
  • Would you like to spend your summer vacation working on an ancient settlement alongside professional archaeologists and university students? Why/why not?
  • If you found an object you thought was very old, would you keep it as a souvenir or would you donate it to a suitable place? Explain.
Relating Science and Technology to Society and the Environment
  • How important is it that the federal government protect and manage the archaeological heritage under its jurisdiction? Explain.
  • Ground-penetrating radar is a tool that allows scientists to see underground without doing any digging. Should the study of ancient burial sites be limited to the data that can be obtained this way? Why/why not?
  • Should some archaeological sites have greater protections than others? Explain.
Exploring Concepts
  • In your own words, explain the concept of half-life. Use carbon-14 to illustrate your explanation.
  • What factor(s) reduce the rate of accuracy for radiocarbon dating?
  • A radioactive isotope has a half-life of 5 000 years. A recently discovered fossil contains 1⁄8 of the original isotope. Calculate the age of the fossil.
  • What information is provided by an accelerator mass spectrometer when used for radiocarbon dating?
  • Can the radiocarbon dating method be used to determine the age of dinosaur bones? Why or why not?
Nature of Science/Nature of Technology
  • What are the limitations of radiocarbon dating?
  • Studies have indicated that the burning of fossil fuels is altering the ratio of carbon in the atmosphere and may cause objects tested using radiocarbon dating in the coming decades to seem hundreds or thousands of years older than they actually are. Should we be concerned that radiocarbon dating will no longer be a technique that can be utilized? Why/why not?
Teaching Suggestions
  • This article supports teaching and learning in Chemistry, Biology and Earth & Environment related to radiometric dating, isotopes and the carbon cycle. Key concepts introduced include archaeologist, radiocarbon dating, isotope, accelerator mass spectrometer, potassium-argon dating and marine reservoir effect.
  • After reading the article and to help students consolidate their learning,  teachers could have students conduct a Write Around strategy to answer the question, “How does an archaeologist date samples?”. Download ready-to-use reproducibles using the Write-Around-Discussion Learning Strategy for this article in [Google doc] and [.pdf]
  • To conclude and assess comprehension, teachers could provide students with an Exit Slip. Download ready-to-use reproducibles using the Exit Slip Learning Strategy for this article in [Google doc] and [.pdf].

Learn more

Willard Libby and Radiocarbon Dating - American Chemical Society - An interesting and detailed history of how Libby discovered this process and how he worked out how to improve it.

Radiocarbon Dating Explainer - The Conversation - Starts by explaining how it works, but goes on to discuss more archaeological cases when it was useful as well as recent advances

Time-lapse Recording of the Archaeological Dig at the Richard III Burial Site (2013) - University of Leicester - This 11-minute time-lapse recording shows the four-week excavation at the Richard III Burial site. It is narrated by archaeologists Matthew Morris.

References

Burleigh, R. (1974). Radiocarbon dating: Some practical considerations for the archaeologist. Journal Of Archaeological Science, 1(1), 69-87. doi: 10.1016/0305-4403(74)90018-1

Taylor R.E. (2014) Radiocarbon Dating in Archaeology. In: Smith C. (eds) Encyclopedia of Global Archaeology. Springer, New York, NY. doi: 10.1007/978-1-4419-0465-2_325