Friday, November 13, 2009

Celebrating 60 Years of Using the Carbon 14 Dating Method

Projectile point excavated from the Central Builders site, Northumberland County.
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 1949, we would not know if this spear point was part of the same cultural tradition as similar spear points found below the Mason-Dixon Line or that this one represents a completely different culture. We could only guess at the age and its relationship to other regions or how fast projectile point styles changed.

The discovery of C-14 dating enabled archaeologists to accurately date for the first time prehistoric archaeological materials in much of the world. It was a spectacular discovery and it revolutionized archaeology. With refinements to the method, especially over the past twenty years, we can now determine the age of objects in years before the present (B.P.); we can accurately determine the age of significant events such as the beginning of agriculture or the entrance of humans into the New World; we can compare cultural sequences in widely separated regions of the world; and, most importantly for archaeology, we can measure the rates of cultural change.

from David Hurst Thomas

How Carbon 14 dating works
Carbon 14 is an isotope that is formed when rays from the sun bombard nitrogen molecules in our atmosphere. It acts like other elements such as oxygen or iron, but it is radioactive and therefore, unstable. It behaves just like the stable or non-radioactive form of carbon. All living things contain the stable form of carbon – carbon 12 and the unstable form of carbon – carbon 14. As long as plants and animals are alive, they absorb carbon 14 thereby introducing it into the cells of the body.

However, when an organism dies, the input of C-14 ceases. At the same time, the stable form of carbon remains unchanged in the body. Because C-14 is unstable, over time it returns to a stable form of nitrogen. In 5,730 years, half of the original amount of carbon 14 in an organism will change back to nitrogen. This is called the carbon 14 half-life. Through Libby and Arnlod's discovery, the ratio of stable carbon to unstable carbon can be measured and a date can be calculated to determine the age of the carbon at the time of death. In essence this is how carbon dating works.

A draw back 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 or pottery and these are, by far, the most common artifacts from prehistory that have survived the vestiges of time. Therefore, archaeologists must date the organic materials directly associated with these non-organic artifacts. Essentially, 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 B.P. (University of Arizona Laboratory #10053). What does this mean? First, the University of Arizona facility performed the analysis and the sample number was 10053. 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 (remember the discovery date of the carbon 14 method was year earlier) as the present. The date of the charcoal is 9165 years before 1950 but this is based on several measurements of the amount of carbon 14 and carbon 12 remaining in the sample. In the University of Arizona sample #10053, the number 9165 is the mean of a number of measurements made by the lab.

This results in a plus or minus factor or standard deviation produced by the laboratory calculations. Therefore, using one standard deviation, there is a 68% chance that the actual date falls between 9375 and 8955 B.P. Usually, archaeologists are looking for a 95% probability and that means the date is somewhere between 9580 and 8745 B.P. This is a range of 835 years, but as we will see next week, this plus or minus factor has been greatly reduced by refining the radiocarbon dating method.

For more information, visit or the Hall of Anthropology and Archaeology at The State Museum of Pennsylvania .

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