Dr. Johnson C. Philip
In this third article we take a look at the absolute dating techniques. As mentioned before, the following are the more important Absolute Dating techniques used in archeology.
Absolute Dating Techniques
- Archeomagnetism
- Astronomical Dating
- Dendrochronology
- Electron Spin Resonance
- Fission Track
- Opacity Stimulation Luminescence
- Oxidizable Carbon Ratio
- Racemization
- Thermoluminiscence
- Radio-Carbon Dating
Archeomagnetism: Everyone knows that the earth has a magnetic field, and that this field is bound to create some effect upon things on the earth. It has been discovered that this magnetic field definitely affects materials, and even leaves an “imprint” on many of them. Using suitable equipment, that imprint can be discovered.
It has also been found via careful observations over the last four centuries, that the earth’s magnetic field varies from time to time and place to place. The intensity (strength) and direction of the field are two factors that show significant variation. Charts have been made of such variation for different places, and for different eras. Once the magnetic factors of a new artifacts is determined, it can be compared with the magnetic charts for the location it was found, and its age can be determined with accuracy.
If no charts are available, the result would give a relative dating. But with scientists working in many fields around the world interested in archeomagnetism, Master Charts are being made and improved continually. This in turn makes the absolute as well as relative dating more accurate.
Magnetic materials, specially if they were melted and cooled in the ancient world, record the orientation and strength of the magnetic field accurately. On obtaining such material, often from harths, their orientation in relation to the current North Pole is recorded accurately. Then they are removed, and studied for the direction recorded in them, and also the strength of the field. In this manner reasonable accurate dating can be obtained for the entire period of human habitation of the earth.
Astronomical Dating: Astronomy is one of the most exact sciences, and astronomical dates of ancient events such as solar and lunar eclipse can today be calculated with precision. On the other hand, if any of the ancient records mentions such an event, its date can be calculated with unusual precision. Similarly, the date of construction of astronomically aligned buildings can also be calculated with accuracy, because with changes in earth’s imaginary axis, the alignments are changing, providing a scale to determine dates.
Tables of known eclipses and their dates, tables of known angles and their dates, etc have been tabulated in great detail. Missing information can be filled in with great accuracy because here we are dealing with astronomical information that can be calculated precisely. Once available astronomical information from an archeological finding is compared with these tables, dates are known with a level of accuracy that will meet the strict demands of modern scientific investigation.
Dendrochronology: Many kinds of trees show somewhat concentric rings when cut across, polished and examined under microscopes. Generally these trees add one ring per year, though on rare occasions they can add more than one or none at all. However, since addition of one ring per year is the common behavior, these rings found on trees or their remains from archeological sites furnishes investigators with a very useful scale for determining time.
Over the years investigators have studied a large number of samples, of many varieties of trees, originating in a number of places, and spread over almost 5000 years. There is an attempt to extend the range, but the paucity of materials and other problems have made it difficult to obtain reliable dates beyond 5000 years BPE [before the present era].
Bristlecone Pines found in parts of America have provided a good amount of primary data to establish the calendar because these trees are able to survive the assault of the elements of nature for thousands of years. There has been much comparison and cross checking to solve the problem of multiple rings and missing rings.
The calendar is not as accurate as the Astronomical Dating system, but is sufficiently objective to date archeological discoveries with fascinating accuracy. Thus remains of trees from archeological sites are carefully cleaned, sliced, rings examined and compared, and reasonably accurate dates are established. Much work yet remains to be done, but what has already been done is extremely helpful in coming up with good results.
Electron Spin Resonance: There are random radiations all around, and electrons are liberated from atoms when they strike matter. In certain kinds of crystals these free electrons are trapped, and one can detect them using the method of ESR.
The older the sample, the larger will be the number of electrons trapped. Using samples of known age, extensive charts and lists have been compiled and standardized. Using this method, ages of a large number of quartz samples, fossilized teeth, flint, calcium carbonate in limestone, coral, and egg shells have been determined.
ESR dating has a very special advantage over many other methods in that it is non-destructive. Methods such as Thermoluminiscence and Opacity Stimulated Luminescence destroy the sample when it is dated. Thus there is no scope for retesting the sample. ESR, on the other hand, leaves the sample as it is, and the experiment can be repeated any number of times, giving a greater chance to eliminate experimental error.
Fission Track Dating: This is a radioactive dating method, used in archeology to determine the ages of artifacts that contain uranium-bearing materials.
The atoms of many elements with high atomic weight, such as Uranium and Radium, break down spontaneously and this phenomenon is called Radioactivity. The breakdown is also called fission. This fission produces two particles [alpha, beta] and an electromagnetic radiation [gamma]. The product of fission knock down electrons, and produce ionization in a series when they pass through substances. Using special techniques, these paths can be seen as tracks, called “fission tracks”. Obviously, the older a sample is, the greater would be the number of fission-tracks in it.
Natural and manufactured glass, stones used in hearths and in heating, and fired ceramics and tiles. They start with zero fission tracks at the time they are formed, and accumulate tracks in proportion to their age. Using special methods, these tracks are then detected, counted and the age is determined.
Opacity Stimulation Luminescence: Many minerals within sediments release electrons when they are exposed to certain kinds of light waves. When the number of electrons in a given sample, after exposing them to suitable kind of light, are compared with the number in a sample of known age, the age of the first sample can be determined.
OSL is suitable for more recent samples that have been deposited by silty and sandy sediments, glacial deposits, etc. that have never been heated, and which have not had much exposure to light.
This is a more recent dating method, and is still undergoing refinement and calibration, but is already yielding good results.
Oxidizable Carbon Ratio: Less accurate than the methods discussed previously, it is still one of the more useful methods of absolute dating. The method depends upon Humus and Charcoal, that are gradually oxidized but only slowly and in a strictly predictable manner.
Thus when suitable samples are discovered, the ratio of carbon left with oxidizable carbon is found out and compared with calibrated charts to find out dates. This method can be used not only to find the ages of individual finds, but also to establish the age-relationship of many biological objects.
OCR and Radio Carbon methods have also been worked carefully, giving greater accuracy to OCR, and this method of dating has therefore become quite common.
Racemization: This technique is used to find ages of things that had an organic origin [humans, animals, plants]. All organic life has Amino Acids as its constituent. These acids are of two type, the L and D.
All living organisms essentially have the L type amino acids, which begin to change to D type after they die. This is known as racemization, and it the rate at which this happens is known accurately. Thus the level of racemization in a given sample can be compared with calibrated charts to find when the particular sample died. This method can be applied to human and animal bones, teeth, plants, ostrich egg shells, mollusks, marine sediments, freshwater and marine shells, oyster shells, carbonate shells, calcareous sediments, and peats.
Some care is required because rate of racemization is affected by temperature. Thus though this is a method of absolute dating, it needs to be used in conjunction with other methods.
Thermoluminiscence: This is a method of absolute dating, but still in a state of development. All natural materials are exposed to natural radiation, which dislodges electrons from the materials. Most of these electrons recombine with atoms, but a few of them remain trapped in locations that are known in solid-state physics as “imperfections”. Heating of these samples releases these electrons, which produce a certain kind of light on doing so.
The method is used for dating ceramics, cooking hearths, rocks lying in hearths, and rocks that have deliberately been heated [such as flint or chert]. Materials made of quarts, feldspar, diamond and calcite.
Radio Carbon Dating: There is practically none who has not heard of this method. So important has this discovery been that WF Libby was given the Nobel Prize for it.
Carbon is a stable element with an atomic weight of 12. However, a small amount of carbon with the atomic weight of 14 is produced in the atmosphere due to cosmic ray bombardment. C14 is radioactive, and decays at a known rate. All air-breathing organisms have a definite C12, C14 ratio in their bodies. However, once they die, the amount of C14 begins to decrease due to radioactive breakdown. Thus if we can determine the ratio of C12 to C14 in an organic sample, one can compare it with calibrated charts and find the age when that organism died [or when the tree was cut down].
Radio-carbon dating gives highly precise dates in samples upto 5000 years BPE, and unreliable dates in samples older than that. The methods gives erroneous results in those samples which have an abnormally low or high accumulation of C14, but with increasing investigations and calibrations such abnormal cases can be ruled out in most cases.
Radio Carbon dating has turned out to be the most helpful and accurate method of dating in discoveries that yield organic samples. Archeology would never have been the same were it not for Radio Carbon Dating.
