Archive 2408

Book 6. Dating

Physical Evidence

Chronometric Techniques

Often the most precise and reliable chronometric dates come from written records. The earliest writing anywhere in the world only goes back about 5000 years. Most of the chronometric dating methods in use today are radiometric. Radioactive isotopes decay at different rates, and these methods depend on these rates of decay or on cumulative changes in materials caused by radioactivity. The measure of a rate of decay is the half-life, the time it takes for half of a radioactive substance to decay by radioactivity. The numbers of atoms remaining unchanged falls by a half in every half-life. The rate of decay is unaffected by changes in the environment of the sample, such as by intense heat, cold, pressure, or moisture.

Relative and Absolute Dating

Archaeologists collecting artifacts on a given site must use both relative and absolute dating techniques. Absolute dating can find an exact date of how old a specific object is, whereas relative dating simply puts discoveries on the site in an order depending upon stratum and context, without giving a definite date. The law of supposition states that lower layers of earth or artifacts are older than those which lay on top.

Seriation uses the distribution law to distinguish strata from the frequency that an artifact occurs. Objects are slowly introduced into a culture, gradually increase in popularity and then declines in use and disappear. At first they are novel and rare, but as they come into more common use, the number of them found grows. Based upon the frequency of occurence in the strata, a chart or timeline of usage can be drawn and used later to give estimates of the relative position of strata within the period under consideration. By using seriation and stratigraphy an archaeologist can ascertain the phases an artifact has gone through.

Pottery is extremely valuable in this respect because it is long-lived and characteristic in shape and decoration. The use of ceramics in the construction of chronologies has a long history in archaeological studies, extending back to the works of the late nineteenth century. Using seriation, archaeologists can examine changes in ceramic form over time using only the vessels themselves, largely independent of their context of recovery. Seriation has been defined as the procedure of working out a chronology by arranging local remains of the same cultural tradition in the order that produces the most consistent patterning of their cultural traits.

Pollen grains can be used similarly to date artifacts. Since the pollen grain wall is tough, pollen from 400 million years ago can be found today. Each pollen grain is different in morphology, its structure and shape, and can therefore be identified and studied as its frequency and morphology change over geological time. As the microclimate changes, through weather conditions or human intervention, the balance of vegetation changes, and so the frequency of pollen types. These changes yield up a sequence that can be used in that period and locality to date other finds.

Radiocarbon Dating

The most commonly used radiometric dating method is radiocarbon dating, carbon-14 or C-14 dating, used to date organic materials such as charcoal, wood, bone and antler, or marine and fresh-water shell, or any tissue that was once alive. The method was developed after World War II by Willard F—Libby.

C-14 is made by the interaction of cosmic rays with nitrogen in the upper atmosphere. The radioactive carbon quickly bonds with atmospheric oxygen to form carbon dioxide, which is absorbed by green growing plants during photosynthesis. Animals eat plants or other animals that have eaten them, and the C-14 soon spreads through all living things. The concentration of C-14 in the atmosphere is fairly constant and while organisms are alive, they contain C-14 in their tissues in the same ratio as in the air because it is constantly being replaced by the process of living. This process obviously ceases at death and the C-14 is no longer replenished, and decays according to its half-life. The ratio of C-14 to the stable carbon-12 depends on how long the organism has been dead, and so measure the time since its death.

It is necessary that:

The half-life of carbon-14 is 5730 ± 40 years. Modern C-14 has a beta radiation count of about 15 (electrons) per minute per gram of carbon, but C-14 that is 5730 years old has a count of only 15 electrons every two minutes. Such a sample can be deduced to be 5730 years old. Beyond 60,000 years, there is not enough carbon-14 left to measure this way.

Using an accelerator mass spectrometer (AMS) involves actually counting individual carbon-14 atoms, and therefore gives precise answers to the relative amounts of C-12 and C-14, which allows older and smaller samples to be dated, but it is expensive.

Sample contamination by older or younger carbon is the problem, and might be caused by careless technique (unforgiveable), or contamination in situ. Furthermore, the assumption of constant generation of C-14 by cosmic radiation is not true and has to be compensated for by plotting calibration curves from samples of known date, often by dendrochronology.

Dendrochronology

One of the most reliable chronometric dating techniques is based on the fact that annual growth rings on shallow rooted trees vary in width with the amount of water available each season and with temperature fluctuations from winter to summer. Dendrochronology is the study and comparison of tree ring growths, which can provide very accurate dates about the wood itself or artifacts found in close proximity to it. Clark Wissler of the American Museum of Natural History first recognized the potential for using tree rings as a dating method, and worked with A E Douglass on Pueblo Bonito, a pre-historic Native American settlement in New Mexico.

Each spring or summer a new layer of xylem is formed, producing the rings we can count. In the early growing season thin walled cells are laid down. Thicker walled cells, the latewood, are produced later in the growing season. Simply counting the rings gives a measure of age of the wood, but also all trees of the same species in an area usually have roughly the same pattern of growth. Since weather patterns tend to run in cycles of a number of years, the sequence of tree-rings in a region will also reflect the conditions for growth in each season. Factors affecting tree ring growth are:

1. Slope Gradient
2. Soil properties
3. Temperature
4. Wind
5. Sun
6. Snow Accumulation

When the climate is particularly moist it will produce wider rings and in the dry years, narrow rings. The changing patterns thus formed can be matched from tree to tree in an area, giving a sequence going back as far as old wood can be found. Due to severe weather, trees may not produce a ring every year. Each region has its own unique master sequence since weather patterns are not the same from one area to another. It gives a natural calendar that is notionally accurate to a single year. Ancient log samples can be compared with the master tree-ring sequence to date them to the year that they were cut down. By looking at a species with a known sequence of growth they can look for matching patterns in the unknown.

Dry weather, water logging or fossilization preserve the wood for hundreds or thousands of years, yielding ancient wood to continue sequwnces into the past. Tree species are most sensitive to environmental change at the latitudinal and elevational limits of its range. Unfortunately, no tree-ring sequence yet goes back much further than 10,000 years. In the American Southwest bristlecone pine chronologies now extend 8,500 years. Work done in Germany and Northern Ireland has expanded the European oak and pine chronologies to over 11,000 years. Work in the Aegean over the past twenty years has produced about 6,000 years of chronologies over the past 9,500 years. As a result, dendrochronology, is primarily used for comparatively recent sites and for checking the reliability of other chronometric methods.

Varve Analysis

Baron de Geer in 1878 invented varve analysis, counting varves or annually laid down sediments. When a glacier reaches a lake, it drops layers of sediment from its melting fringe. A varve consists of two layers, a thick light colored layer of silt and fine sand which forms in the spring and summer and a thin dark colored layer of clay forming in the fall and winter. Since this process repeats in a seasonal cycle, in good conditions the sediments can be counted like tree rings. It provides detailed chronological information about the composition, displacement, and climate of the place, but seems to happen only in near freezing water, and not in oceans or temperate and tropical lakes.

By making a bore hole in the sediment, a vertical sequence of sediment can be drawn, the older, the deeper. Even in glacial waters the varves are not always clear cut, notably sometimes seeming to double in some years perhaps due to unseasonal cold or hot spells, leading to error. Digital methods using computers and colour and hue gradations have proved to increase accuracy. Pollen analysis, first produced by the Swedish geologist Von Post in about 1916, is another help. Each varve can be examined for pollen grains under a high-powered microscope. From the pollen diagram, the analyst can infer sea level, vegetational, and climatic changes. Statistical analysis is usually needed.

Geomagnetic Reversal Time Scale

Another chronometric method, called variously geomagnetic reversal time scale (GRTS) dating, archaeomagnetic dating, and paleomagnetic dating, is based on changes in the earth’s magnetic field.

The field of study concerned with ancient geo-magnetic phenomena and the use of archaeological material in determining past variation in the earth’s magnetic field is called archaeomagnetism. The fact of variations in direction and intensity of the earth’s magnetic field have been recorded in London, Paris and Rome over the past four centuries, and are the basis of archaeomagnetic dating. This variation leaves maks in natural material and is called fossil magnetism.

Declination, inclination and intensity of the earth’s magnetic field at any point on the earth’s surface shifts. At present the declination for London changes by approximately 1 degree every decade. The angle of dip is also subject to shifting. So the time of acquisition of a particular magnetic character can be traced by comparing the determined magnetic character with records of the past magnetic field direction where the specimen was found.

Because observatory studies of the geomagnetic field only extend back for 400 years at the most, only relatively recent material can be dated by direct comparison. So for older specimens, archaeomagnetic dates are determined by finding a rate of the geomagnetic field by comparing the pole location of an archaeomagnetic sample with a master curve of a polar movement constructed from an average of many independently dated samples.

Magnetism occurs in different forms, the most frequent of which are not considered magnetic by most people because they are used to the strong magnetism of substances like iron. In substances with the lesser forms of magnetism, the very weak magnetic fields of individual particles are randomly oriented, but heating to above 600° C causes them to align their fields with any magnetic field present at the time. The magnetic field present everywhere is the earth’s own. After cooling, magnetism will remain trapped as a permanent record of the direction of magnetic north at that time until the material is reheated or broken up.

Such a condition can occur in a pottery kiln, a bonfire, or a burning house. Likewise, it can occur in molten rock from a volcano. Baked clay, used for thousands of years in the construction of hearths, ovens and kilns, has had the random orientation of its magnetic domains in its pre-heated state oriented by the earth’s field each time it is strongly heated. Cooling traps the aligned domains and gives the clay a slight magnetism. The slight magnetization thus caused can be measured to determine the magnetic intensity and declination at the time of its last cooling.

Datable materials include volcanic rock, fired clay pots, and other forms of clay or rock that have been exposed to high temperatures. Before the sample is taken it must be marked with its exact orientation to geographic or magnetic north, and so items that might have been moved after its last firing are no good. Fixtures such as the floor or wall base of a kiln or oven are ideal. Its thermoremnant magnetism is measured with a magnetometer. The direction of magnetic north slowly wanders about the earth. Thermoremnant magnetism records these movements. By comparing these data, a researcher can determine the direction of magnetic north at the last time the sample had been exposed to a high temperature.

Researchers have created a map of the locations of magnetic north during the last 10,000 years. This was based primarily on charcoal from fire hearths associated with thermoremnant magnetic samples. With this map, it is now possible to determine the age of new samples that date to within this time range. Archaeomagnetism yields good results up to ages of 10,000 years.

At times the north and south magnetic poles reverse. There have been eight reversals in the last 2.43 million years, at 0.69, 0.89, 0.95, 1.9, 2.0, 2.1, and 2.43 million years ago. Lava and volcanic ash deposits often contain the thermoremnant magnetic records of these reversals. When the fossils of early humans or their ancestors are found in association with such deposits, they can be roughly dated by them. However, this dating method is less useful than some others since at best it only tells us that a fossil dates to sometime between two reversals. Paleoanthropologists have found magnetic pole reversals to be useful for dating geological deposits in association with even earlier pre-human fossils going back 10,000,000 years.

Potassium-Argon Dating

The Potassium-Argon (K-Ar) dating method is the measurement of the accumulation of argon in a mineral. Potassium-40 decays into argon-40 and calcium-40 at a known rate. The half-life of potassium-40 is approximately 1.25 billion years. Measurement of the amount of argon-40 in a sample is the basis for age determination.

Argon is an inert gas, and being unreactive remains trapped in the crystal. The time elapsed is like that of thermoluminescence, the time since the sample was treated in some way that reset the K-Ar clock by releasing any argon previously accumulated. Heating is a common way of zeroing the clock. Archaeologists can find how long ago a heat-treated arrow head was made, or a cooking pot was last used.

For more ancient samples, when a fossil is sandwiched between volcanic rock or ash deposits with comparatively large amounts of potassium, their potassium-argon dates provide a minimum and maximum age.

Potassium-argon dates have errors of about 15% of the date, and is useful only where rock is rich in potassium, mostly elated to volcanic activity. Paleoanthropologists use it mostly to date sites in the 1-5 million year old range. Finding the ratios of argon-40 to argon-39 in volcanic rock gives more accurate dates and requires smaller samples.

Thermoluminescence Dating

Thermoluminescence (TL) dating is used to date rocks, minerals and pottery between the years 300-10,000 BP. All natural minerals are thermoluminescent. Trace amounts of radioactive atoms, such as uranium and thorium, in soil and clay produce constant low amounts of background ionizing radiation. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. These energy charged electrons progressively accumulate over time. Heating releases the trapped electrons, producing light.

When a sample is heated to high temperatures in a laboratory, the trapped electrons are released and give off their stored energy in the form of photons of light, which can be measured by photomultiplier tubes and optical wavelength filters. A microcomputer controls the heating and collects the data. In practice, emitted light intensity is measured as a function of the temperature of the sample, typically up to 500 C. A similar effect can be brought about by stimulating the sample with infrared light. The intensity of thermoluminescence is directly related to the amount of accumulated changes produced by background radiation, which depends on the age of the sample and the amount of trace radioactive elements it contains.

In archaeology, thermoluminescence is best for ceramics, cooking hearths, accidentally fire-cracked rocks and deliberately fire treated rocks such as flint or chert. What is measured is the amount of time since the sample was last heated to 350 C, meaning, for pottery, when it was fired, or, for the clay or rock lining of a hearth or oven, the last time a fire burned there. The last time a crystal was reheated and its electrons were released is known as a clock resetting event. The effective time range for TL dating is now about 300,000 years down to a few decades. The accuracy of TL dating is lower than most other radiometric techniques, and it is not yet accurate enough for archaeological dating of pottery. It is only about 15% accurate for a single sample and 7 to 10% accurate for a suite of samples in a single context. The steps are:

Electron Spin Resonance Dating

Electron spin resonance (ESR) dating is based, like TL, on the fact that background radiation causes electrons to separate from their atoms and become trapped in the crystalline lattice of the material. When odd numbers of electrons are separated, there is a measurable change in the magnetic field of the material. Since this magnetic field progressively changes with time in a predictable way, it provides another atomic clock, or calendar, that can be used for dating purposes. Unlike thermoluminescence dating, however, the sample is not destroyed with the ESR method. Electron spin resonance is used to date minerals, especially calcium carbonate in limestone, coral, fossil teeth, mollusks, and egg shells. ESR has been used to provide dates going back roughly ½ billion years.

Fission Track Dating

Fission track dating is based on the fact that some crystalline or glass-like minerals, such as obsidian and mica, contain trace amounts of uranium-238, which is an unstable isotope. When atoms of uranium-238 fission and become lead-206, there is a release of energy-charged alpha particles which burn narrow fission tracks, or damage trails, through the glassy material. These can be seen and counted with an optical microscope.

The number of fission tracks is directly proportional to the amount of time since the glassy material cooled from a molten state. Since the half-life of uranium-238 is known to be approximately 4.51 billion years, the chronometric age of a sample can be calculated. This dating method can be used with samples that are as young as a few decades to as old as the earth and beyond. However, paleoanthropologists rarely use it to date sites more than several million years old.

With the exception of early historic human made glass artifacts, the fission track method is usually only employed to date geological strata. Obsidian and mica artifacts are not fission track dated because it would only tell us when the rocks cooled, not when they were made into artifacts by our early human ancestors.

Three methods are used for dates down to about 10,000 BC—dendrochronology, radiocarbon (C-14), and archaeomagnetic dates based on the wandering of magnetic north around the rotational north pole. Dating events down to 70,000 BP is done with radiocarbon dating, amino acid racemization, thermoluminescence, electron spin resonance and fission track dating. Before that potassium-argon and fission track have usually been used.

Uranium-Thorium Dating

Uranium-Thorium dating is an absolute dating technique which uses the properties of the radio-active half-life of Uranium-238 and Thorium-230. The half-life of uranium-238 is 4,470 million years. The half-life of thorium-230 is only 75,380 years. When the amounts of uranium and thorium are compared an accurate estimation of the age of an object can be obtained.

The methods used are Isotope Dilution Mass Spectrometry (IDMS), Secondary Ion Mass Spectrometry (SIMS) and IDMS-Thermal Ionization Mass Spectrometry (TIMS). The technique has been checked with C-14 dating and is accurate.

Uranium-Thorium dating was first used on fossil bones in 1956, however, it had been used for dating wood before this. This dating technique has been used effectively on marine sediment, bone, wood, coral, stone and soil. One of the benefits of uranium-thorium dating is that the sample sizes can be less than 20 grams, in fact bone samples can be 3-5 grams for an accurate date.

Fluorine Dating

Fluorine is an element that is found in most ground water around the world, and can be used for relative dating. Skeletal remains buried in the earth can occur when percolating ground water inundates the bone remains with a solution of minerals drawn from local soils. A change in the mineral composition of the bone in which hydroxyl ions are substituted by fluoride ions, making insoluble fluorapatite. The degree of the change is a measure of elapsed time.

Fluorine dating is chiefly shows whether bone implements or human skeletal remains found in association with other bones were buried at the same time. Fluorine dating exposed the Piltdown hoax.

Obsidian Hydration Analysis

Developed in 1960, Obsidian Hydration Analysis (OHA) is an inexpensive technique for dating. Obsidian is a natural glass, usually black in color, an igneous rock formed when volcanic magma cools quickly, and found in lava flows. Being glassy, it has little to no crystalline structure,and fractures conchoidally leaving sharp edged sherds. Obsidian was a common rock used in stone tool making, a favorite material like flint for knapping since the beginning of stone tool production, and is found at archaeological sites around the world. Hydration of the newly fractured surface at a steady rate offers an easy method of dating the fracture.

When obsidian is newly exposed to the atmosphere, its surface begins to absorb water from the air. Irving Friedman and R L Smith in 1960 discovered the hydration rate of obsidian depended on the composition of the obsidian, temperature, and relative humidity. Erosion and burning could also reduce the thickness of the hydration layer. Soil type, climate and geochemistry were also relevant.

The hydrated layer at the surface, known as the rind, is visible under a microsope and its thickness can be measured using polarized light, white light, or both according to the flake’s translucency. Several measurements on each rind are taken, and the samples are often checked after a week.

Amino Acid Racemization Dating

A newer chronometric method, known as amino acid racemization dating, relies on the fact that amino acids, the building blocks of all proteins, exist in two mirror image forms, both of which otherwise have the same chemical structures. The L-amino acid molecule form has an extension to the left, while the D-amino acid form has an extension to the right. The L-amino acids occur exclusively in life but change to D-amino acids steadily—they racemize—following death. As a result, remains of organisms that died long ago will have more D-amino acids than ones that died recently. Aspartic acid, one of the 20 amino acids, is usually extracted from fossil bones or shells for this dating technique.

Dates as old as 200,000 years have been obtained. Racemization rates can vary with different soil temperatures and possibly other environmental factors, and since these have not yet been fully explored paleoanthropologists consider this dating technique not yet fully reliable, and useful mainly as a relative methed.

Continue: Greeks & Persians: Zoroastrian Influence on Greek Philosophy

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