Archaeology
ANT 207
Spring 2012

Some Limitations on Absolute Dating Techniques in Archaeology

Dendrochronology

• In some areas of the world, particularly in the tropics, the species available do not have sufficiently distinct seasonal patterns that they can be used.
• Where the right species are available, the wood must be well enough preserved that the rings are readable. In addition, there must be at least 30 intact rings on any one sample.
• There also must be an existing master strip for that area and species. There is an absolute limit on how far back in the past we can date things with tree rings. Although bristle cone pine trees can live to 9,000 years, this is a very rare phenomenon. As we try to push our matching of archaeological specimens beyond the range for which we have good control data, our confidence in the derived dates diminishes.
• Finally, the prehistoric people being studied had to have built fairly substantial structures using wood timbers. In most of the world that did not begin to happen until about 4,000 to 5,000 years ago!

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Carbon 14 Dating

Using this technique, almost any sample of organic material can be directly dated. There are several limitations, however.

• 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.
• 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.
• 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 radation 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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Potassium-Argon (K-Ar) Dating

• The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution. As with any dating technique, there are some significant limitations.
• The technique works well for almost any igneous or volcanic rock, provided that the rock gives no evidence of having gone through a heating-recrystallization process after its initial formation. For this reason, only trained geologists should collect the samples in the field.
• This technique is most useful to archaeologists and paleoanthropologists when lava flows or volcanic tuffs form strata that overlie strata bearing the evidence of human activity. Dates obtained with this method then indicate that the archaeological materials cannot be younger than the tuff or lava stratum. Because the materials dated using this method are NOT the direct result of human activity, unlike radiocarbon dates for example, it is critical that the association between the igneous/volcanic beds being dated and the strata containing human evidence is very carefully established.
• The standard deviations for K-Ar dates are so large that resolution higher than about a million years is almost impossible to achieve. By comparison, radiocarbon dates seem almost as precise as a cesium clock! Potassium-argon dating is accurate from 4.6 billion years (the age of the Earth) to about 100,000 years before the present. At 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40, pushing the limits of present detection devices. Eventually, potassium-argon dating may be able to provide dates as recent as 20,000 years before present.

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Obsidian Hydration Dating

Using this technique, any sample of obsidian can be dated. There are several limitations, however.

• The rate of hydration is not uniform throughout the world. Variations exist in temperature over time from site to site. Temperature effects are particularly difficult to evaluate. Variations also exist in sample chemical composition. Samples from different obsidian sources hydrate at different rates. Moisture is another source of variability. The amount of moisture present at a site can affect the hydration rate of an obsidian sample. It is really necessary to produce a calibration curve for each archaeological site or area being studied, and this is not always possible.
• Artifact reuse may lead to an erroneous date. For example, one person fashions a tool out of an obsidian nodule and uses it to skin a deer. Once that person finishes using the tool, it is discarded. Several hundred years later, a second person finds the tool, resharpens it, uses it to shave the bark off of a tree branch, and then later discards it as well. Several thousand years later, an archaeologist discovers the tool and takes it to a laboratory to be dated. The archaeologist found the tool at a site that was an arrowshaft workshop. However, instead of dating the surface on the tool that was used to shave bark, the surface that was used to skin the deer several hundred years earlier is dated. The archaeologist would be lead to believe by this erroneous date that arrow production started several hundred years earlier than what was expected.

George H. Michaels and Brian M. Fagan