Radiocarbon Dating: A look at the last 80 years

One of the fundamental concepts in archaeology is the Law of Superposition, which states that older material is located underneath more recent material. So generally, on an archaeological site, the deeper you dig, the older the materials will be. It’s a simple concept that at times can require a good amount of interpretation and investigation. But for archaeologists before the mid-20^th century, the law of superposition provided the only means of estimating the age of objects at an archaeological site.

Then, in June 1946, Willard Libby introduced the method of radiocarbon dating. The discovery of radiocarbon dating in the late 1940s enabled archaeologists to accurately date, for the first time, archaeological materials in much of the world. It was a spectacular discovery and it revolutionized archaeology and many other scientific fields.

How Does Radiocarbon Dating Work?

There are many very good sources on how radiocarbon dating works online, so we will give a basic explanation here. In short: Radiocarbon (C-14) is created in our atmosphere and it is absorbed by plants. At the same time, plants also take in stable carbon (C-12). When that plant (or the animal that ate the plant) dies, the intake of C-14 and C-12 stops. The C-14 begins to decay right away at a constant rate, but the C-12 doesn’t decay. After 5,730 years, half of the C-14 will have decayed and all of the C-12 remains. Measuring the remaining amount of C-14 compared to the remaining C-12 provides a radiocarbon date.

How old is it?

The above projectile point was excavated from a depth of nine feet below the ground surface along with a small group of stone tools and chips produced from the sharpening of tools. They were recovered next to a small cooking hearth with fire altered rock and charcoal. This point is similar to a style of spear point found throughout Virginia and North Carolina, but it is slightly different. Based on its excavation depth, it is probably very old.

Prior to the discovery of carbon 14 (C-14) dating by Willard Libby and J. R. Arnold in the late 1940s, we would not be able to determine how old this spear point was. We could only estimate its age and its relationship to similar projectile points found in other regions.

A drawback with C-14 dating is that it can only be used on organic material such as wood, bone, or shell –materials that were once parts of plants and organisms. It cannot, unfortunately, be used to directly date stone spear points like this one. Therefore, archaeologists must date organic materials directly associated with these non-organic artifacts. Usually, this means scientists date the charcoal from the cooking hearth or trash pit with which the artifacts are associated, resulting in a proxy date for the non-organic artifact.

The projectile point from the Central Builders site pictured above was associated with charcoal from the cooking hearth that radiocarbon dated to 9165 +/- 210 radiocarbon years B.P. (University of Arizona Laboratory #10053). What does this mean?

The letters “B.P.” are an abbreviation for Before the Present. But that is not exactly correct, because it actually means before the year 1950. Since the present is always changing, Libby and Arnold decided to use a standard date of 1950 as the present. The date of the charcoal is 9,165 radiocarbon years before 1950 and 9,165 is an average of numerous measurements made by the lab. This results in a plus or minus factor (or standard deviation) produced by the laboratory calculations. In this sample, the range of possible radiocarbon age of the charcoal is 210 years before or after 9,165 BP.

Advances in Radiocarbon Dating in the 20^th Century

In the past, C-14 dating required large samples of organic material. But starting in the 1980s a new method, accelerator mass spectrometry (AMS), allowed for extremely small samples to be precisely dated. With AMS methods, samples like seeds or the remains of a burned meal adhering to the inside of a clay cooking pot can be C-14 dated. This newer methodology allows for radiocarbon dates with a plus or minus factor of 40 years or less.

Radiocarbon Dating Challenges

One challenge with C-14 dating is potential contamination of the samples – the addition of old or new carbon. A great deal of care must be used in collecting the samples. Further, charcoal is lightweight and can be moved around by wind and water. The same flood deposits that cover artifacts at a stratified site can bring in old charcoal eroded from a site upstream resulting in a C-14 date that does not accurately date the deposit. This type of contamination can be offset by getting many dates from a site and paying close attention to the context of materials that are analyzed.

Another challenge is equating radiocarbon years to calendar years. As discussed briefly at the beginning, radiocarbon dating is a measure of the amount of C-14 that has decayed using the known half-life of C-14. For radiocarbon dating to work perfectly, a single radiocarbon year would need to be the same as a calendar year. Unfortunately, this isn’t the case, because the amount of C-14 in the atmosphere fluctuates over time. This was discovered through dendrochronology – tree ring dating. Using C-14 dating to date individual trees rings, it has been discovered that C-14 years do not exactly correlate with tree ring dates, which we are sure relate to calendar years.

Radiocarbon Date Calibration

To correct for the discrepancy between radiocarbon years and calendar years, scientists use calibration methods that correlate radiocarbon dates to calendar years by using dendrochronology data. Since the 1960s, scientists have been continually improving calibration methods. Today, calibration programs, like Calib (released in 1993) and OxCal (released in 1994), provide easy access to different calibration curves.

Radiocarbon dates can be re-calibrated at any time after they have been run. So we can calibrate the radiocarbon date above (9,165 +/- 210 radiocarbon years B.P.), using the most recent calibration curve (IntCal20) available via OxCal. Thanks to pairing radiocarbon dating methods with dendrochronology, we can say with confidence (95.4% confidence) that the projectile point found in Northumberland County dates between 9,124 and 7,747 calendar years BC.

Interested in Learning More?

The last 20 years have seen a real proliferation in innovative uses of radiocarbon dating in archaeology all over the world, including approaches that expand beyond dating a single archaeological site. Unfortunately, we don’t have time or space to delve into this here. If you are interested though, these include large scale population studies using datasets of thousands of radiocarbon dates, the introduction of Bayesian statistics for chronological studies and modeling, and large scale efforts to produce databases of radiocarbon dates to facilitate many of these lines of research.

TWIPA Connection

This blog post is a synthesis of multiple posts previously published on the “This Week in Pennsylvania Archaeology” blog. These posts were originally published on November 13^th and 20^th 2009. Content from these posts has been modified and updated to reflect current research and data from archaeological studies in Pennsylvania. Modifications may include editing, removal or addition of text or the removal or addition of images.

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