Dating Techniques in Archaeology

Preface

This exercise is designed to introduce you to basic techniques used in determining the age of archaeological materials and sites. There are two general categories of methods for dating in archaeology. One is Relative Dating, and the other is Absolute Dating. In general, archaeologists use both methods in conjunction. Absolute dating to determine the actual age of an object or stratigraphic layer, and relative dating to tie associated artifacts and layers into the sequence provided by the absolute dates.

Contents

The methods that we will be exploring here are listed in the table below. You may click on any topic to jump immediately to that topic.

Superposition

One of the most fundamental principles of archaeology is the Law of Superposition. The law states that strata that are younger will be deposited on top of strata that are older, given normal conditions of deposition. This law is the guiding principle of stratigraphy, or the study of geological or soil layers. Stratigraphy is still the single best method that archaeologists have for determining the relative ages of archaeological materials.

Stratigraphy

Stratigraphy is the study of strata, or layers. Specifically, stratigraphy refers to the application of the Law of Superposition to soil and geological strata containing archaeological materials in order to determine the relative ages of layers. In addition, stratigraphy can tell us much about the processes affecting the deposition of soils, and the condition of sites and artifacts. These are called postdepositional processes, and their study is part of Middle Range Theory.

As this example has shown you, post depositional processes, both natural and human, can result in very complex stratigraphy. Although stratigraphy and the Law of Superposition can help us determine the relative ages of occupations, one must be very alert to alterations in stratigraphy that may throw chronological reckoning off. Various phenomena, such as hole digging or mudslides, can completely reverse stratigraphy. Thus, long profiles or profiles from a number of units are necessary to avoid misinterpretation.

Cross-Dating

Cross-dating is a technique used to take advantage of consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology. A specialized form of cross-dating, using animal and plant fossils, is known as biostratigraphy. The following animation will provide you with an example of how cross-dating is used. Click on the button below to start the animation.

Artifacts of Known Age

Relative dating is an invaluable tool, but does not tell us WHEN an event occurred, just the ORDER in which events occurred. The oldest technique for establishing the actual ages of deposits is to use artifacts of a known age. These can be coins with minting dates stamped on them, writings with dates included, or objects that we know were only manufactured during a certain time.

Dendrochronology

Dendrochronology is another traditional technique for establishing the abolute date of events. This is also called Tree-Ring Dating. Tree-Ring dating is based on the principle that the growth rings on certain species of trees reflect variations in seasonal and annual rainfall. Trees from the same species, growing in the same area or environment will be exposed to the same conditions, and hence their growth rings will match at the point where their lifecycles overlap.

Limitations of Dendrochronology

There are limitations on dendrochronology. Some of those limitations include:

• 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!

Radiocarbon Dating

Radiocarbon, or Carbon-14, dating is probably one of the most widely used and best known absolute dating methods. It was developed by J. R. Arnold and W. F. Libby in 1949, and has become an indispensable part of the archaeologist's tool kit since. It's development revolutionized archaeology by providing a means of dating deposits independent of artifacts and local stratigraphic sequences. This allowed for the establishment of world-wide chronologies.

Where does C -14 Come From?

Radiocarbon dating relies on a simple natural phenomenon. As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).

The unstable isotope is brought to Earth by atmospheric activity, such as storms, and becomes fixed in the biosphere. Because it reacts identically to C-12 and C-13, C-14 becomes attached to complex organic molecules through photosynthesis in plants and becomes part of their molecular makeup. Animals eating those plants in turn absorb Carbon-14 as well as the stable isotopes. This process of ingesting C-14 continues as long as the plant or animal remains alive.

C-14 Decay Profile

The C-14 within an organism is continually decaying into stable carbon isotopes, but since the organism is absorbing more C-14 during its life, the ratio of C-14 to C-12 remains about the same as the ratio in the atmosphere. When the organism dies, the ratio of C-14 within its carcass begins to gradually decrease. The rate of decrease is 1/2 the quantity at death every 5,730 years. That is the half-life of C-14. The animation provides an example of how this logarithmic decay occurs. Click on the "Show Movie" button below to view this animation.

C-14 Decay Profile

How is a C-14 Sample Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a C-14 sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

C-14 Processing

The Limitations of Carbon 14 Dating

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 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 infallable. 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 radicarbon dating, that it can be employed anywhere in the world, and does have a 50,000 year range. Using radicarbon 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 varous parts of the world.

Potassium-Argon Dating

Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium-40 (K-40) ,decays to the gas Argon as Argon-40 (Ar-40). By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.

How Does the Reaction Work?

Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass). One out of every 100 Potassium atoms is radioactive Potassium-40 (K-40). These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron. With 18 protons and 22 neutrons, the atom has become Argon-40 (Ar-40), an inert gas. For every 100 K-40 atoms that decay, 11 become Ar-40.

How is the Atomic Clock Set?

When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.

The Decay Profile

In this simulation, a unit of molten rock cools and crystallizes. The ratio of K-40 to Ar-40 is plotted. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C-14 graph. Click on the "Show Movie" button below to view this animation.

K-Ar Decay Profile

How are Samples Processed?

Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date. This is actually a mini-simulator, in that it processes a different sample each time and generates different dates.

K-Ar Processing

Limitations on 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.
  
  
• As the simulation of the processing of potassium-argon samples showed, 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 4.3 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.

Conclusion

There are several other dating techniques employed in archaeology. Some of these include: fission-track dating, paleomagnetic and archaeomagnetic dating, obsidian hydration dating, and thermoluminescence dating. Many of these techniques are still experimental, or have not found wide acceptence in archaeology yet, hence they have not been discussed here. For more information on dating techniques consult:

You may now return to the Exercise menu by clicking on the "Main" button below. Please refer to the next section of your Study Guide for the next assignment.

Department of Anthropology, UCSB

Brian M. Fagan and George H. Michaels
Comments to web author: michaels@id-archserve.ucsb.edu

All contents copyright © 1995, The Regents of The University of California. All rights reserved.
Revised: December 9, 1995
URL: http://id-archserve.ucsb.edu/www/Anth3/Exercises/Dating/DatingTech.html