Above: Remains of a Viking Age site used by fishers and sealers near Bjuröklubb, Sweden. Brown bear bones found inside the stone ring have been dated to 779-968 CE (Wikimedia Commons/Krister Hägglund)

Did you know? Along with radiocarbon and potassium-argon dating, dendrochronology (tree-ring dating) is a common technique used by archaeologists.

As soon as archaeologists discover something, they usually ask “How old is this?” 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. For example, Homo habilis (the “Handy Man”), a 2.4 million-year-old evolutionary ancestor of modern humans, was discovered in 1960. For decades, it was assumed that this was the first species to make stone tools. However, stone tools recently discovered in Kenya have turned out to be 3 million years old. This pushes the development of stone tools back by 700,000 years and changes the whole story of human evolution!

But how do archaeologists figure out how old something is? The answer depends on whether the material being tested is organic (like food or plants) or inorganic (like stone). However, most archaeological sites contain organic materials. And the most common method for determining their age is radiocarbon dating.

14C Remaining

Age of sample (years)

100%

0

50%

5730

25%

11,460

12.5%

17,190

6.25%

22,920

0%

57,300

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 you die, the 14C in your body begins to break down and turn into the stable carbon-12 (12C). The half-life of 14C is approximately 5730 years. This means that 5730 years after you die, your bones and tissues will only contain 50% of the 14C they had at the time of your death.

You can determine how old an object is by testing the ratio of 14C to 12C in the organic material it contains. For example, imagine you’re working on an archaeological dig and you find some elk bones in a hearth. You will need to take a small sample of bone and carefully clean it to remove any possible contaminants, like tree roots or soil. Once the sample is clean, it can be run through an accelerator mass spectrometer (AMS). This highly sensitive machine measures the amount of 14C and 12C left in the bone. Finally, you can compare these levels to existing benchmarks and figure out when the elk was eaten. This will also give a very good idea of how old the site is.

Did you know? The first results of radiocarbon dating were published in 1949 by Willard Libby. He received the 1960 Nobel Prize in chemistry.

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 the 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). Because the half-life of 40K is 1.3 billion years, measuring the ratio of 40K to 40Ar can provide dates for objects that are millions of years old.

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 because of 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.

Accelerator mass spectrometer at Lawrence Livermore National Laboratory in California (Wikimedia Commons/US Department of Energy)

This means is that when humans and animals whose diets are rich in seafood will have 14C levels that provide older radiocarbon dates. 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 in 2012, 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.

Since it was first used at the end of the 1940s, radiocarbon dating has remained one of the most important tools used by archaeologists. Although it is only one of many different dating methods, it is commonly used to determine the age of artifacts because most archaeological sites contain organic materials. Radiocarbon dating can help 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!

Learn More!

Scientific articles on the history and methods of radiocarbon dating:

From revolution to convention: the past, present, and future of radiocarbon dating (2015)
Rachel Wood, Journal of Archaeological Science 56
Link to abstract. Registration of subscription required to view full text.

Teaching radioisotope dating using the geology of the Hawaiian islands (2009)
Timothy J. Moran, Journal of Geoscience Education 57

Radiocarbon age calibration of marine samples back to 9000 cal yr BP (1986)
M. Stuiver, G. W. Pearson & T. Braziunas, Radiocarbon 28

Website on the discovery and analysis of the remains of Richard III, including a description of radiocarbon dating techniques:

The Discovery of Richard III
University of Leicester

Stephanie Halmhofer

Originally from Steveston, BC, I completed an Associate's degree in Criminology from Kwantlen Polytechnic University in 2009.  During this time I was introduced to the fascinating fields of archaeology and forensic anthropology and knew what I wanted to do with my life! I began my undergraduate degree at the University of British Columbia, specializing in the field of osteoarchaeology, or the study of human skeletal remains from archaeological sites. I completed my degree in 2012 at the University of Alberta, with a minor in First Nations studies, and have been working as an osteoarchaeologist ever since, first in Edmonton, AB, and most recently on the gorgeous Sunshine Coast of BC.

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