Friday, November 20, 2009

Celebrating 60 years of using the carbon 14 dating method: Part II

Refinements in the method and improvements in procedures

In the past, C-14 dating required large samples of organic material. New processes, such as accelerator mass spectrometry (AMS) allow for extremely small samples to be accurately dated such as seeds or the remains of a burned meal adhering to the inside of a clay cooking pot. For example, the plus or minus factor for the above date would be reduced to 40 years or less.

A problem with C-14 dating is 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 light in weight 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. C-14 dates are relatively inexpensive (approximately $300 for standard dates and $600 for AMS dates) and presently it is a common practice, where the charcoal is available, to get over twenty dates from the same site. Archaeological analysis is a process of identifying patterns and C-14 dates are part of the patterning. If all of the artifacts and stratigraphy points to a date of 5000 B.P. and the C-14 date is 20,000 B.P., there is probably something wrong with the C-14 date.

Changes in the intensity of the sun, the burning of fossil fuels and the testing of nuclear weapons has had an effect on the accuracy of carbon 14 dating. This has resulted in two problems. First, dates less than 300 years old are not very dependable and other methods must be used to date artifacts from this period. Second, it turns out that the amount of C-14 in the atmosphere has changed over time. This was discovered through dendrochronology – tree ring dating. Using AMS 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. For example, C-14 dates of around 3500 B.P. are several hundred years too old. On the other hand, C-14 dates of 11,000 B.P. are almost 2000 years too young.

Formulas are being developed to convert radio carbon years into calendar years but the system still needs to be refined. In the meantime, archaeologists are using both systems - calendar years (cal yr B.P.) and radio carbon years (14C yr B.P.). This is probably going to be resolved in the next few years but in the mean time it’s confusing for both professional archaeologists and the general public. However, as one archaeologist, (David Hurst Thomas) has put it, “radiocarbon dating is the workhorse of archaeology”. It produces reasonably accurate dates, to within a few decades and it allows us to compare a variety of significant technological and cultural events.

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

No comments:

Post a Comment