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Using this technique, almost any sample of organic material can be directly dated. There are a number of limitations, however.
- First, the size of the archaeological sample is important. Larger samples are better, because purification and distillation remove some matter. Although new techniques for working with very small samples have been developed, like accelerator dating, these are very expensive and still somewhat experimental.
- Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon. For each sample, clean trowels should be used, to avoid cross contamination between samples. The samples should be packaged in chemically neutral materials to avoid picking up new C-14 from the packaging. The packaging should also be airtight to avoid contact with atmospheric C-14. Also, the stratigraphy should be carefully examined to determine that a carbon sample location was not contaminated by carbon from a later or an earlier period.
- Third, because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor (the standard deviation) may be larger than the date obtained. The practical upper limit is about 50,000 years, because so little C-14 remains after almost 9 half-lives that it may be hard to detect and obtain an accurate reading, regardless of the size of the sample.
- Fourth, the ratio of C-14 to C-12 in the atmosphere is not constant. Although it was originally thought that there has always been about the same ratio, radiocarbon samples taken and cross dated using other techniques like dendrochronology have shown that the ratio of C-14 to C-12 has varied significantly during the history of the Earth. This variation is due to changes in the intensity of the cosmic radiation bombardment of the Earth, and changes in the effectiveness of the Van Allen belts and the upper atmosphere to deflect that bombardment. For example, because of the recent depletion of the ozone layer in the stratosphere, we can expect there to be more C-14 in the atmosphere today than there was 20-30 years ago. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years. When reading archaeological reports, be sure to check if the carbon-14 dates reported have been calibrated or not.
- Finally, although radiocarbon dating is the most common and widely used chronometric technique in archaeology today, it is not infallible. In general, single dates should not be trusted. Whenever possible multiple samples should be collected and dated from associated strata. The trend of the samples will provide a ball park estimate of the actual date of deposition. The trade-off between radiocarbon dating and other techniques, like dendrochronology, is that we exchange precision for a wider geographical and temporal range. That is the true benefit of radiocarbon dating, that it can be employed anywhere in the world, and does have a 50,000 year range. Using radiocarbon dating, archaeologists during the past 30 years have been able to obtain a much needed global perspective on the timing of major prehistoric events such as the development of agriculture in various parts of the world.
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