Archaeology: An Introduction - 4th Edition 2002
The Online Companion: updated November 2007


CHAPTER 4 : Dating the Past

>> CHAPTER OVERVIEW

4.1. BACKGROUND

4.2. TYPOLOGY AND CROSS-DATING

4.3. HISTORICAL DATING

4.4. SCIENTIFIC DATING TECHNIQUES

4.5. ABSOLUTE TECHNIQUES

4.6. DERIVATIVE TECHNIQUES

4.7. THE AUTHENTICITY OF ARTEFACTS

 

 


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4.1. BACKGROUND

It is increasingly difficult for prehistorians working in the twenty-first century to conceptualise the problems experienced by their predecessors, and approaches to interpretation before the 1960s are consistently criticised. Culture history and diffusionism may - with hindsight - seem excessively preoccupied with classification and social evolution, and to have applied unsophisticated historical interpretations instead of asking fundamental questions about human behaviour.

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4.2. TYPOLOGY AND CROSS-DATING

4.2.1. Sequence dating and seriation

 

4.2. TYPOLOGY AND CROSS-DATING

It must be made clear at the outset that typology is not, strictly speaking, a dating method, but a means of placing artefacts into some kind of order. Classification divides things up for the purposes of description, whereas typology seeks to identify and analyse changes that will allow artefacts to be placed into sequences.

4.2.1. Sequence dating and seriation

These techniques both place assemblages of artefacts into relative order. Petrie used sequence dating to work back from the earliest historical phases of Egypt into pre-dynastic Neolithic times, using groups of contemporary artefacts deposited together at a single time in graves. Seriation was developed in the USA to place in order finds from strata or other kinds of assemblages such as potsherds collected from the surface of sites.

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4.3. HISTORICAL DATING

4.3.1. Applying historical dates to sites

 

4.3. HISTORICAL DATING

Prehistorians sometimes overestimate the accuracy and detail of frameworks based on historical evidence; in practice, early written sources may provide little more information than a scatter of radiocarbon dates. The extent of documentation varied considerably in 'historical' cultures and the information that survives is determined by a variety of factors.

4.3.1. Applying historical dates to sites

If a context containing burnt debris and broken artefacts is excavated on a site from a historical period, it is tempting to search the local historical framework for references to warfare or a disaster in the region, and to date the excavated context accordingly.

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4.4. SCIENTIFIC DATING TECHNIQUES

4.4.1. Geological time-scales

4.4.2. Climatostratigraphy

:: Seabed deposits

:: Ice cores

4.4.3. Varves (and Tephrochronology)

4.4.4. Pollen

4.4.5. Dendrochronology (tree-ring dating)

:: The application of tree-ring dating

 

4.4. SCIENTIFIC DATING TECHNIQUES

The transformation of archaeological dating that began around 1950 continues, but archaeologists may overlook the revolution in scientific dating that had already taken place in geology during the first half of the twentieth century; from this wider perspective, the emergence of radiocarbon dating may seem slightly less dramatic

4.4.1. Geological time-scales

Accurate knowledge of the age of the Earth was of little direct help to archaeologists, but it emphasised the potential of scientific dating techniques. The first half of the twentieth century witnessed similar progress that began with the dating of recent geological periods in which early hominids lived, and ended with the introduction of radiocarbon dating.

4.4.2. Climatostratigraphy

While some geologists concentrated on the age of the Earth, others studied distinctive surface traces left behind by changes in the extent of polar ice during the most recent (Quaternary) geological period. They identified a succession of Ice Ages alternating with temperate conditions (glacials and interglacials) which - if they could be dated - would reveal much about the evolution of early humans in the context of changing environmental conditions.

----- Seabed deposits

Cores extracted from ocean floor deposits reveal variations in oxygen isotopes in the shells and skeletal material of dead marine creatures, which reflect fluctuations in global temperature and the volume of the ocean.

----- Ice cores

A datable record of climatic change in relatively recent periods has been recovered from cores, up to 3 km long, extracted from the ice sheets of Greenland and elsewhere.

4.4.3. Varves

Sections cut through lake beds in glacial regions reveal a regular annual pattern of coarse and fine layers, known as varves. Variations in climate produced observable differences in the thickness of sediments, and, like the patterns of variation in tree rings, this allows matches to be made between deposits in separate lake beds.

Tephrochronology

Deposits of volcanic ash encountered in stratified contexts on archaeological sites offer opportunities for dating.

4.4.4. Pollen (NB: pollen analysis has been superceded as a DATING method by radiocarbon since the 1950s)

Microscopic wind-blown pollen grains survive well in many soil conditions, and pollen that has accumulated in deep deposits - such as peat-bogs - can provide a long-term record of changes in vegetation; suitable samples may be collected from soils exposed by excavation, or from cores extracted from bogs.

4.4.5. Dendrochronology

It has been recognised since at least the fifteenth century that trees produce annual growth rings - their physiology was understood by the eighteenth century - and that they could be counted to calculate the age of a tree when it was felled. Because the thickness of these rings is affected by annual climatic factors, distinctive sequences of rings may be recognised in different samples of timber and used to establish their contemporaneity.

----- The application of tree-ring dating

Unfortunately there are many problems in the direct application of dendrochronological dating. Not all tree species are sufficiently sensitive to display distinctive variations in their ring characteristics, particularly when growing in temperate climates. Wood only survives under exceptionally wet or dry conditions, and large timbers must be recovered to provide sufficient rings for valid comparisons because they rely on patterns that accumulated over several decades.

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4.5. ABSOLUTE TECHNIQUES

4.5.1. Radioactive decay

4.5.2. Radiocarbon dating

:: Key factors

4.5.3. Presenting and interpreting a radiocarbon date

:: Radiocarbon samples

:: The impact of radiocarbon dating

4.5.4. Potassium-argon (40K/40Ar) and argon-argon dating (40Ar/39Ar)

4.5.5. Uranium series dating

4.5.6. Fission-track dating

4.5.7. Luminescence dating

4.5.8. Electron spin resonance (ESR)

 

4.5. ABSOLUTE TECHNIQUES

4.5.1. Radioactive decay

The successful development in the early twentieth century of radiometric methods relying upon radioactive decay for dating geological periods offered hope that a similar technique might be found to give absolute dates for prehistoric archaeology.

4.5.2. Radiocarbon dating

Radiocarbon dating was one peaceful by-product of accelerated wartime research into atomic physics and radioactivity in the 1940s. The rate of decay of 14C, which has a half-life of 5730 (40) years, is long enough to allow samples of carbon as old as 70,000 years to contain detectable levels of radioactive emissions, but short enough for samples from periods since the late Stone Age to be measured with reasonable precision.

Key factors

Click for a list of the key factors for Radiocarbon Dating

4.5.3. Presenting and interpreting a radiocarbon date

Because interpretation is so complex, all radiocarbon dates included in an archaeological publication must be presented in a standard format.

----- Radiocarbon samples

Most organic materials are suitable for dating; the lower the carbon content, the larger the sample needs to be.

----- The impact of radiocarbon dating

Radiocarbon dating has grown exponentially, and many problems and inaccuracies have been isolated and examined, some leading to major adjustments of the results. Without doubt, it has made the greatest single contribution to the development of archaeology since geologists and prehistorians escaped from the constraints of historical chronology in the nineteenth century.

4.5.4. Potassium-argon (40K/40Ar) and argon-argon dating (40Ar/39Ar)

Potassium-argon is ideal for dating early hominid fossils in East Africa, for they occur in an area that was volcanically active when the fossils were deposited between one and five million years ago; pioneering results in the 1950s doubled previous estimates of their age.

4.5.5. Uranium series dating

The dating of rocks back to the Pre-Cambrian by measuring the proportions of uranium to lead or uranium to helium was possible because isotopes of uranium remain radioactive for such a long period.

4.5.6. Fission-track dating

This method involves counting microscopic tracks caused by fragments derived from fission of uranium-238 in glassy minerals, whether geological or of human manufacture. In practice the most useful samples come from zircon or obsidian, which was used extensively for making tools.

4.5.7. Luminescence dating

The physical phenomenon of luminescence may be used to date artefacts that were made from (or include) crystalline minerals which have been subjected to strong heating. The first successful application was to clay fired to make pottery, but it is commonly used now for dating flint tools that have been burnt, for example by being dropped accidentally into a fire.

4.5.8. Electron spin resonance (ESR)

Like thermoluminescence, ESR is a 'trapped charge' dating method, but it is applied to different kinds of samples, and the method of measurement is also different. ESR does not release trapped electrons, but subjects them to electromagnetic radiation in a magnetic field, which causes electrons to resonate and absorb electromagnetic power. The strength of resonance reflects the number of electrons that have become trapped since the crystals were formed.

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4.6. DERIVATIVE TECHNIQUES

4.6.1. Protein and amino acid diagenesis dating

4.6.2. Obsidian hydration dating

4.6.3. Archaeomagnetic dating

 

4.6. DERIVATIVE TECHNIQUES

Derivative methods may only be used for dating if their results can be related to a time-scale or reference curve that has been established by absolute dating methods. If it is not affected in any way by its environment the result can be described as absolute. In contrast, dating the change of one form of amino acid to another is derivative because the rate of alteration varies, and is heavily dependent on the temperature and humidity of the context where the sample has been buried.

4.6.1. Protein and amino acid diagenesis dating

Bones, teeth and shells contain proteins that break down after death, and the most commonly investigated products of decomposition are amino acids. Amino acid racemization dating (AAR) measures changes between these amino acids' L- and D-forms; their ratio is an indication of age.

4.6.2. Obsidian hydration dating

Obsidian - a natural volcanic glass - was a popular alternative to flint for making flaked tools in many parts of the world. As soon as a fresh surface of obsidian is exposed, for example during the process of making it into a tool, a microscopically thin hydration rim begins to form as a result of the absorption of water.

4.6.3. Archaeomagnetic dating

The Earth's magnetic field undergoes continuous change. The position of magnetic North wanders around the North Pole, and even reverses completely to the South Pole for extended periods on a geological time-scale. From any reference point its position is measurable in terms of two components: movement up or down (inclination or 'dip') and from side to side (declination).

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4.7. THE AUTHENTICITY OF ARTEFACTS

When major museums buy items for their collections they become involved in expensive commercial dealings in the fine art market. The profits to be made encourage not only illicit plundering of ancient sites but skilful forgeries. Scientific dating techniques can provide reassurance; when what is needed is confirmation that an object is not a modern fake, rather than a precise date, full control of all the variables that affect accuracy is not necessary.

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