Radiocarbon Dating

Radiocarbon dating is one of the most intensively used archeological analyses, as it can place in time organic materials recovered from archeological sites, such as plant materials, shell, and bones. The actual methods are relatively complicated. Briefly, the samples are cleaned in a method called pretreatment, which varies depending on the material type and the environment it was deposited in. Typically, pretreatment includes acid/alkali (base)/acid baths to remove contaminates from the radiocarbon sample. In the case of bones, an extraction with alkali is done to isolate the collagen and remove the mineral fraction (bone apatite). After pretreatment, the sample is burned and the carbon is extracted from the sample material in gas form, and then reduced to pure carbon graphite. This graphite is then measured by conventional methods or AMS (accelerator mass spectrometry). Yes, the size of the sample matters, with large chunks of wood or bone a conventional measurement method can be used, which directly counts radioactive decay. For tiny samples the use of Accelerator Mass Spectrometry (AMS) is best. AMS measures the ratio of radiocarbon and the stable carbon isotopes to calculate a radiocarbon age. The precision of conventional methods and AMS is the same for most materials. See Radiocarbon Dating Understood to learn more.

Radiocarbon is incorporated into plants via photosynthesis, and into animals via the food they eat. Different photosynthetic pathways of plants results in isotopic fractionation (different behavior of carbon isotopes that result from their atomic mass). The standard error on an analyzed 13C/12C value is quite small. These stable carbon isotope ratios (δ13C) are given as per mil (‰) difference from PDB-1 standard.

In the case of the Varga site, 66 radiocarbon samples were run, with multiple samples run for each of the four primary use zones. The large number of assays (measured samples) allows for considerably more precision in correctly identifying the age of the deposits, a greater degree of confidence in the age derived, and a narrowing of the age range for the four periods represented. This project employed two different radiocarbon laboratories, Beta Analytic Inc. in Florida and the Center for Applied Isotope Studies at the University of Georgia in Athens, to make radiocarbon measurements on the samples. All the radiocarbon dates in this study are presented in calibrated years before present (cal BP with present equal to 1950).

As an example, the chronometric age of the Toyah component is derived primarily from radiocarbon dating 21 samples. These samples were recovered from a cultural component across excavation Blocks A and B. Radiocarbon dates on one limestone burned rock (11,690 ± 40 BP) from Feature 38 and two of four dated ceramic sherds (930 ± 40 BP and 26,960 ± 40 BP) are obviously much too old and beyond the realm of any reasonable possibility. These three dates are not acceptable. Three other samples, two charcoal and one ceramic sherd, yielded radiocarbon ages that are less than 200 years old and are also not acceptable for this component.

Two charcoal dates of 920 ± 40 BP and 940 ± 40 BP from the lowest and best-defined stratigraphic context in the Toyah component, that in Feature 36 (a dense, but limited in area occupation zone along the western margin of Block A), are older than the presumed age for Toyah events. They are also some 600 to 700 years younger than the well-dated Late Archaic component that is older than 1700 BP. Therefore, these two roughly 930 BP ages may date an unrecognized Late Prehistoric event or component that dates to slightly earlier than the Toyah occupation. Consequently, 62 percent of the obtained radiocarbon assays that targeted this component provide a time range in which the Toyah events probably occurred. The 13 acceptable radiocarbon assays, all on wood charcoal, reveal a narrow time range of some 370 years between 290 and 660 BP.

The extensive radiocarbon dating of the diverse materials proved invaluable in determining more precise ages for the various use periods, the associated features and cultural assemblages, and internal movement of individual objects and classes of objects up and sown the deposit. Without this intensive dating program, many questions concerning specific material classes, features, and associations would have been left open to question. For example, four dates were obtained from Feature 8 and ranged from 80 BP to 640 BP The 80 BP age is much too recent for this prehistoric component and was rejected. This dated chunk of charcoal is believed to have been intrusive to this feature. However, since multiple dates were obtained from Feature 8, the other three dates can be averaged to derive an approximately age of 600 BP.