The Record of Time: Chronometric Techniques: Part II

Most of the chronometric dating methods in use today are radiometric

. That is to say, they are based on knowledge of the rate at which certain radioactive isotopes decay or the rate of other cumulative changes in atoms resulting from radioactivity. Isotopes

are specific forms of elements. The various isotopes of the same element differ in terms of atomic mass but have the same atomic number.

The decay of radioactive elements occurs at different rates, depending on the specific isotope. These rates are stated in terms of half-lives. One half-life is the amount of time required for ˝ of the original atoms in a sample to decay. Over the second half-life, ˝ of the atoms remaining decay, which leaves Ľ of the original quantity, and so on. In other words, the change in numbers of atoms follows a geometric scale as illustrated by the graph below.

The decay, or fissioning, of atomic nuclei provides us with a reliable clock that is unaffected by normal forces in nature. The rate will not be changed by intense heat, cold, pressure, or moisture.

Radiometric Dating--how known radioactive decay rates for uranium are used for dating
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The most commonly used radiometric dating method is radiocarbon dating. It is also called carbon-14 and C-14 dating. This technique is used to date the remains of organic materials. Dating samples are usually charcoal, wood, bone, or shell, but any tissue that was ever alive can be dated.

Radiocarbon dating is based on the fact that cosmic radiation from space constantly bombards our planet. It collides with atoms in the atmosphere resulting in the release of neutrons. When the nucleus of a nitrogen (¹⁴N) atom in the atmosphere captures one of these neutrons, the atom subsequently changes into carbon-14 (¹⁴C) after the release of a proton. The carbon-14 quickly bonds chemically with atmospheric oxygen to form carbon dioxide. That gas drifts down to the earth's surface where much of it is taken in by green growing plants, and the carbon-14 is used to build new cells by photosynthesis

. Animals eat plants or other animals that have eaten them. Through this process, carbon-14 spreads through all living things and is incorporated into their proteins and other organic molecules.

drawings illustrating the natural production of carbon-14 and its entrance into the food chain from the atmosphere to green growing plants and ultimately to animals that eat them

Natural production of carbon-14 in the atmosphere and its entrance into the food chain

As long as an organism is alive, it takes in carbon-14 and other carbon isotopes in the same ratio as exists in the atmosphere. Following death, however, no new carbon is consumed. Progressively through time, the carbon-14 atoms decay and once again become nitrogen-14. As a result, there is a changing ratio of carbon-14 to the more atomically stable carbon-12 and carbon-13 in the dead tissue. That rate of change is determined by the half-life of carbon-14, which is 5730 ± 40 years. Because of this relatively rapid half-life, there is only about 3% of the original carbon-14 in a sample remaining after 30,000 years. Beyond 40-50,000 years, there usually is not enough left to measure with conventional laboratory methods.

Half-Lives	Years Past	C-14 Atoms	C-12 Atoms
0	0	1 N	1 N
1	5,730	1/2 N	1 N
2	11,460	1/4 N	1 N
3	17,190	1/8 N	1 N
4	22,920	1/16 N	1 N
5	28,650	1/32 N	1 N
6	34,380	1/64 N	1 N
7	40,110	1/128 N	1 N
N = some specific number of atoms

The conventional radiocarbon dating method involves burning a sample in a closed tube containing oxygen. The carbon containing gas that is produced is then cooled to a liquid state and placed in a lead shielded box with a sensitive Geiger counter. This instrument registers the radioactivity of the carbon-14 atoms. Specifically, it detects the weak beta particles

released when carbon-14 nuclei decay. The age of a sample is determined by the number of decays recorded over a set period of time. Older samples have less carbon-14 remaining and, consequentially, less frequent decays.

A relatively new variation of the radiocarbon dating method utilizes an accelerator mass spectrometer

, which is a large device usually used by physicists to measure the abundance of very rare radioactive isotopes. When used for dating, this AMS method involves actually counting individual carbon-14 atoms. This allows the dating of much older and smaller samples but at a far higher cost. Although, organic materials as old as 100,000 years potentially can be dated with AMS, dates older than 60,000 years are still rare.

Paleoanthropologists and archaeologists must always be aware of possible radiocarbon sample contamination that could result in inaccurate dates. Such contamination can occur if a sample is exposed to carbon compounds in exhaust gasses produced by factories and motor vehicles burning fossil fuels such as coal or gasoline. The result is radiocarbon dates that are too old. This has been called the Autobahn effect, named after the German high speed roadway system. Archaeologists in that country first noted this source of contamination when dating samples that were found near the Autobahn. The effect of global burning of fossil fuels on radiocarbon dates was verified and calibrated by Hans Suess of the University of California, San Diego when he radiocarbon dated bristlecone pine tree growth rings that were of known chronometric ages. Subsequently, it is also called the Suess effect.

Another source of error in radiocarbon dating that is understood and compensated for stems from the assumption that cosmic radiation enters earth's atmosphere at a constant rate. In fact, the rate changes through time, resulting in varying amounts of carbon-14 being created. This has become known as the de Vries effect because of its discovery by the Dutch physicist Hessel de Vries.

Radiocarbon dates that are too young can result if a sample is impregnated with tobacco smoke or oils from a careless researcher's hands.

There are a number of other radiometric dating systems in use today that can provide dates for much older sites than those datable by radiocarbon dating. Potassium-argon (K-Ar) dating is one of them. It is based on the fact that potassium-40 (⁴⁰K) decays into the gas argon-40 (⁴⁰Ar) and calcium-40 (⁴⁰Ca) 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.

Dating samples for this technique are usually geological strata of volcanic origin. While potassium is a very common element in the earth's crust, potassium-40 is a relatively rare isotope of it. However, potassium-40 is usually found in significant amounts in volcanic rock and ash. In addition, any argon that existed prior to the last time the rock was molten will have been driven off by the intense heat. As a result, all of the argon-40 in a volcanic rock sample is assumed to date from that time. When a fossil is sandwiched between two such volcanic deposits, their potassium-argon dates provide a minimum and maximum age. In the example below, the bone must date to sometime between 1.75 and 1.5 million years ago.

drawing illustrating the dating of volcanic ash stata above and below a bone sample to determine a minimum and a maximum age for the bone

Dating volcanic ash strata above and below a bone
sample to determine a minimum and a maximum age

Potassium-argon dates usually have comparatively large plus or minus factors--they may be on the order of .25 million years for a 2 million year old date. In addition, this dating technique usually is of use only where there is rock rich in potassium. Essentially, it is used only where there has been local volcanic activity. Theoretically, however, it can be used for samples that date from the beginning of the earth (4.55 billion years) down to 100,000 years ago or even more recently. Paleoanthropologists use it mostly to date sites in the 1-5 million year old range.

A relatively new related dating technique compares the ratios of argon-40 to argon-39 in volcanic rock. This provides more accurate dates and allows the use of smaller samples.

Another radiometric method that is used for samples from early human sites is fission track dating. This is based on the fact that a number of crystalline or glass-like minerals, such as obsidian, mica, and zircon crystals, contain trace amounts of uranium-238 (²³⁸U), which is an unstable isotope. When atoms of uranium-238 fission, 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.

drawing of fission tracks in obsidian seen through an optical microscope

Fission tracks in obsidian as they would appear 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 4.5 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. Artifacts made out of obsidian and mica are not fission track dated because it would only tell us when the rocks cooled from a molten state, not when they were made into artifacts by our early human ancestors.

Thermoluminescence (TL) dating is a radiometric method based on the fact that trace amounts of radioactive atoms, such as uranium and thorium, in some kinds of rock, soil, and clay produce constant low amounts of background ionizing radiation. The atoms of crystalline solids, such as pottery and rock, can be altered by this radiation. Specifically, the electrons of quartz, feldspar, diamond, or calcite crystals can become displaced from their normal positions in atoms and trapped in imperfections in the crystal lattice

of the rock or clay molecules. These energy charged electrons progressively accumulate over time. When a sample is heated to high temperatures in a laboratory, the trapped electrons are released and return to their normal positions in their atoms. This causes them to give off their stored energy in the form of light impulses (photons). This light is referred to as thermoluminescence (literally "heat light"). 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, in turn, varies with the age of the sample and the amount of trace radioactive elements it contains.

drawing illustrating thermoluminescence release resulting from rapidly heating a clay sample to high temperatures in a special electric oven

Thermoluminescence release resulting from rapidly heating a crushed clay sample

What is actually determined is the amount of elapsed time since the sample had previously been exposed to high temperatures. In the case of a pottery vessel, usually it is the time since it was fired in a kiln. For the clay or rock lining of a hearth or oven, it is the time since the last intense fire burned there. For burned flint, it is the time since it had been heated in a fire.

The effective time range for TL dating is now from a few decades back to about 300,000 years, but it is most often used to date things from the last 100,000 years. Theoretically, this technique could date samples as old as the solar system if we could find them. However, the accuracy of TL dating is generally lower than most other radiometric techniques.

Another relatively new radiometric dating method related to thermoluminescence is electron spin resonance (ESR). It is based also on the fact that background radiation causes electrons to dislodge from their normal positions in 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 (or spin) of the atoms. Since the magnetic field progressively changes with time in a predictable way as a result of this process, 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, which allows samples to be dated more than once. ESR is used mostly to date calcium carbonate in limestone, coral, fossil teeth, mollusks, and egg shells. It also can date quartz and flint. Paleoanthropologists have used ESR mostly to date samples from the last 300,000 years. However, it potentially could be used for much older samples.

Whenever possible, paleoanthropologists prefer to use as many dating samples from a human occupation site as possible and to employ a variety of dating methods. In this way, the confidence level of the dating is significantly increased. The methods that are used depend on the presumed age of the site from which they were excavated. For instance, if a site is believed to be over 100,000 years old, dendrochronology and radiocarbon dating could not be used. However, potassium-argon, fission track, amino acid racemization, thermoluminescence, electron spin resonance, and paleomagnetic dating methods would be considered.

EFFECTIVE TIME RANGE OF THE MAJOR CHRONOMETRIC DATING METHODS

In addition to the likely time range, paleoanthropologists must select dating techniques based on the kinds of datable materials available. Dendrochronology can only date tree-rings. Any organic substances can be used for radiocarbon and amino acid racemization dating. Calcium rich parts of animals such as coral, bones, teeth, mollusks, and egg shells can be dated with the electron spin resonance technique. In addition, ESR can date some non-organic minerals including limestone, quartz, and flint. Burned clay and volcanic deposits are materials used for paleomagnetic dating. Glassy minerals, such as mica, obsidian, and zircon crystals are datable with the fission track method. Pottery and other similar materials containing crystalline solids are usually dated with the thermoluminescence technique. The potassium-argon or argon-argon methods are used to date volcanic rock and ash.


A radiocarbon sample being prepared for dating with the AMS technique