The Radiocarbon Revolution Since its development by Willard Libby in the s, radiocarbon 14C dating has become one of the most essential tools in archaeology. Radiocarbon dating was the first chronometric technique widely available to archaeologists and was especially useful because it allowed researchers to directly date the panoply of organic remains often found in archaeological sites including artifacts made from bone, shell, wood, and other carbon based materials.
In contrast to relative dating techniques whereby artifacts were simply designated as "older" or "younger" than other cultural remains based on the presence of fossils or stratigraphic position, 14C dating provided an easy and increasingly accessible way for archaeologists to construct chronologies of human behavior and examine temporal changes through time at a finer scale than what had previously been possible. The application of Accelerator Mass Spectrometry AMS for radiocarbon dating in the late s was also a major achievement.
Compared to conventional radiocarbon techniques such as Libby's solid carbon counting, the gas counting method popular in the mids, or liquid scintillation LS counting, AMS permitted the dating of much smaller sized samples with even greater precision.
Regardless of the particular 14C technique used, the value of this tool for archaeology has clearly been appreciated. However, as with any dating technique there are limits to the kinds of things that can be satisfactorily dated, levels of precision and accuracy, age range constraints, and different levels of susceptibility to contamination. Probably the most important factor to consider when using radiocarbon dating is if external factors, whether through artificial contamination, animal disturbance, or human negligence, contributed to any errors in the determinations.
For example, rootlet intrusion, soil type e. Bioturbation by crabs, rodents, and other animals can also cause samples to move between strata leading to age reversals. Shell may succumb to isotopic exchange if it interacts with carbon from percolating ground acids or recrystallization when shell aragonite transforms to calcite and involves the exchange of modern calcite. The surrounding environment can also influence radiocarbon ages.
The introduction of "old" or "artificial" carbon into the atmosphere i. This is a major concern for bone dates where pretreatment procedures must be employed to isolate protein or a specific amino acid such as hydroxyproline known to occur almost exclusively in bone collagen to ensure accurate age assessments of bone specimens.
Alone, or in concert, these factors can lead to inaccuracies and misinterpretations by archaeologists without proper investigation of the potential problems associated with sampling and dating. To help resolve these issues, radiocarbon laboratories have conducted inter-laboratory comparison exercises see for example, the August special issue of Radiocarbon , devised rigorous pretreatment procedures to remove any carbon-containing compounds unrelated to the actual sample being dated, and developed calibration methods for terrestrial and marine carbon.
Shells of known age collected prior to nuclear testing have also been dated http: What can we date with radiocarbon dating? Radiocarbon dating can be used on either organic or inorganic carbonate materials. However, the most common materials dated by archaeologists are wood charcoal, shell, and bone. Radiocarbon analyses are carried out at specialized laboratories around the world see a list of labs at: How do we measure 14C?
In brief, radiocarbon dating measures the amount of radioactive carbon 14 14C in a sample. When a biological organism dies, the radioactive carbon in its body begins to break down or decay.
This process of decay occurs at a regular rate and can be measured. By comparing the amount of carbon 14 remaining in a sample with a modern standard, we can determine when the organism died, as for example, when a shellfish was collected or a tree cut down. However, there are a number of other factors that can affect the amount of carbon present in a sample and how that information is interpreted by archaeologists.
Thus a great deal of care is taken in securing and processing samples and multiple samples are often required if we want to be confident about assigning a date to a site, feature, or artifact read more about the radiocarbon dating technique at: In addition, click here to see short movie clips on how radiocarbon is produced in the atmosphere, a decay profile, and how it is analyzed by a lab: