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How Do Archaeologists Date Ancient Artifacts?
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In Archaeology, timing is everything
In times past, things that appeared old were simply considered old, maybe as old as Atlantis, the biblical flood or the earth itself. But nobody knew for sure how old. It wasn’t until the late nineteenth century that archaeologists began using dating techniques, specifically relative dating, which began to provide an acceptable degree of accuracy for dating old things.
Then in the early twentieth century scientists began using absolute dating techniques, perhaps the most prominent of which is carbon-14. It would be hard to imagine modern archaeology without this elegant and precise timing method. Now with carbon-14 and other modern dating techniques we have a very good idea how old things are.
The following is a list of dating techniques used in archaeology and other sciences. It is more or less in the order of discovery of each procedure. Please enjoy!
Stratigraphy is the most basic and intuitive dating technique and is therefore also the oldest of the relative dating techniques. Based on the law of Superposition, stratigraphy states that lower layers should be older than layers closer to the surface, and in the world of archaeology this is generally the case, unless some natural or manmade event has literally mixed up the layers in some fashion. Most archaeological sites consist of a kind of layer cake of strata, so figuring out how old each layer is comprises the basis for the dating of the site itself and also helps date the artifacts found within these layers as well.
For instance, the site of Hisarlik in western Turkey comprises a manmade earthen mound, also known to archaeologists as a tell, which is covered by of nine layers of strata, the lowest of which appears to be the oldest. Interestingly, Hisarlik could be the site of Homeric Troy (circa 1300 B.C.E.), though this possibility has not been conclusively proven.
2. Seriation or Artifact Sequencing
Invented by preeminent archaeologist Sir William Flinders-Petrie in the late nineteenth century, seriation, another form of relative dating, is based on the idea that over time artifacts such as gravestones and ceramics undergo changes in style, characteristics and use. Seriation is particularly useful when layers of strata are not available, such as at ancient cemeteries.
3. Chronological Markers
The first and simplest method of absolute dating, chronological markers pertain to artifacts with dates inscribed upon them, such as coins, documents or inscriptions on buildings announcing historical events. Roman coins are excellent for this usage, as they often show dates, as well as the likeness of the emperor in power when the coins were minted. The only problem with this dating method is that when these markers are discovered out of context, their value is greatly diminished.
Dendrochronology or tree-ring dating was developed in the early 1900s by Tucson astronomer A. E. Douglass, who hypothesized that the growth rings in trees are influenced by solar flares and sunspot activity. This theory turned out to be true, of course, because solar activity affects virtually every living thing on the planet! These growth rings can be used to date slices of wood or logs, sometimes to the exact calendar year. Douglass’ research led to the dating of Native American puebloan ruins through the American Southwest.
The limitation with this method is that growth rings only pertain to the climate in a particular region; thus, comparing growth rings from different areas is often useless. Also, some trees show no growth rings. Nevertheless, the International Tree Ring Database has contributions from 21 countries, providing researchers with comparative regional data.
5. Radiocarbon or Carbon-14 Dating
The development of radiocarbon dating in the 1940s started a scientific revolution. The scientific basis of radiocarbon dating is that every living organism contains carbon and absorbs the radioactive isotope carbon-14 (C14) from the atmosphere during its life cycle. (C14 forms by the bombardment of cosmic rays from space.) When the organism dies, the C14 begins to decay at a rate that appears constant. The half-life of this decay is about 5,730 years. Thus the age of the organism when it died can be calculated with great accuracy. This dating method remains accurate for about 57,000 years.
The limitations to radiocarbon dating is that the ratio of C14 to other carbon isotopes such as C13 doesn’t remain constant, as the amount of cosmic radiation hitting the earth can vary over time. Also, since the advent of the Industrial Revolution, the amount of carbon spewed into the atmosphere has increased dramatically. More carbon, means more C14. Therefore, refinements and calibrations of the technique are a constant concern.
Potassium-argon dating, like radiocarbon dating, involves the decay of radioactive elements in a sample. It is based on the decay of an isotope in potassium which then forms the element argon. Potassium is found in material such as micas, clay minerals, tephra and evaporates. Moreover, since argon is a noble gas it doesn’t bind with other elements once it is formed, so measuring the amount of it is straightforward.
Potassium-argon dating is mostly limited to dating volcanic materials at sites between 50,000 and two billion years old. This technique has been used greatly at Olduvai Gorge in Africa, helping date the hominid fossils found there. A recent modification is argon-argon dating, which has been used at sites such as Pompeii.
7. Fission Track
Fission track dating was developed in the mid 1960s. This method is based on the knowledge that damage tracks in minerals and glasses are created when small amounts of uranium are present in a sample. Such damage tracks are accumulated at a fixed rate that can be measured. This dating method has been used at hominid fossil sites such as Zhoukoudian in China, where Peking Man - later labeled as a specimen of Homo erectus - was discovered in 1920s.
8. Obsidian Hydration
Obsidian is a volcanic glass used by early ancestors of man primarily during the Paleolithic era. Once obsidian is exposed to the air, such as after it was used to make spear points, arrowheads or knives, it begins to absorb water. The resultant rim or rind can be measured using many different techniques such as microscopic depth profiling or, the most sensitive of the options, secondary ion mass spectrometry. Thus, the larger the rind, the longer the artifact has been exposed to the air.
Unfortunately, some factors can change the hydration rate of an obsidian artifact: the higher temperatures experienced at lower elevations, differences in water vapor pressure and the intrinsic qualities of various obsidian samples.
Thermoluminescence (TL) is used for dating inorganic material, particularly pottery or other ceramics, hallmarks of ancient civilization from Mesopotamia to Mesoamerica. TL can also be used to date sediments. Invented by physicists around 1960, thermoluminescence operates on the principle that upon being heated electrons are trapped in the minerals within the material. When this material is then re-heated in the laboratory the electrons in the minerals emit light (or luminesce). This light is then measured to find the date the ceramic was fired (or when the sediment was exposed to sunlight). The effectiveness of TL is from 300 to 100,000 years ago.
Controversy using TL centers on the possibility that if a particular pottery vessel has been subjected to heat more than once, dating can present inaccurate data.
10. Optically Stimulated Luminescence
Optically stimulated luminescence (OSL) measures the complex process when minerals in sediments are exposed to sunlight, which frees electrons trapped within the mineral lattice. When the amount of electrons is measured, dating is accomplished. This method primarily dates sediments containing minerals such as quartz, feldspar and calcite. OSL is often used in conjunction with thermoluminescence.
Like thermoluminescence, rehydroxylation (RHX) is used to date ceramics. According to scientific observation, once a ceramic is fired it immediately begins to absorb moisture from the atmosphere at a measurable rate – the fourth root of the time elapsed since firing, actually. To test a sample, it is weighed and then heated to 500 degrees Celsius until it is completely dehydrated. Then the amount of water loss can be measured, showing the age of the ceramic.
The drawback to RHX is that scientists need to know the temperature history of the site where the ceramic is found. Also, natural events such as wild fires could completely dehydrate a sample, thereby resetting its clock.
As applied to archaeology, paleomagnetism is the study of the magnetic history of rock samples. Over time, the location of the earth’s north magnetic pole shifts position; also, the poles can reverse entirely from time to time. This magnetic orientation through millennia can be measured in rocks.
Studies using paleomagnetism led to the discovery of continental drift, which validated the theory of plate tectonics. And the resultant revolution in geophysical studies hasn’t let up!
13. Magnetic Properties of Lead
Lead is another element or mineral that tends to absorb water and oxygen from the atmosphere over time. When lead is subjected to cryogenic temperatures it becomes a superconductor, but when corrosion sets in it loses it superconductivity. This difference in conductivity can be measured with some accuracy. Dating lead is useful to archaeologists because it was widely used in antiquity, particularly in places such as Israel.
14. Amino Acid Dating
Also known as racemization dating, amino acid dating relies on the principle that all biological tissues contain amino acids. These amino acids have two different configurations “D” or “L” (left-handed). With a few exceptions, all organisms contain the L configuration. But after the organism dies this configuration tends to even out over time, a process known as racemization, and this tendency toward equilibrium can be measured.
However, racemization tends to happen more quickly in warm, wet climates, so knowing the climate history of the area where the artifacts were found is a critical issue. Acidity and alkalinity can also affect racemization. Amino acid dating is accurate from 5,000 to 1,000,000 years ago.
15. Oxidized Carbon Ratio
Oxidized carbon ratio dating is used to date organic material going back 35,000 thousand years. This dating method is based on the measurable difference between oxidizable and organic carbon. Over time carbon tends to oxidize with exposure to the atmosphere. Dating is achieved by measuring the difference between the two carbons.
The oxidized carbon ration method is considered experimental and is therefore subject to further testing, evaluation and refinement. Of course, this could be said of many dating methods! At present, most scientists opt for C-14 dating instead.
Tephrochronology is a geochronological dating technique that relies on the fact that every eruption of volcanic ash – or tephra – has a unique chemical fingerprint, establishing a “tephra horizon” that is easily dated. The advantage to such dating is that layers of volcanic ash are easy to spot and spread over a wide area. The device used to read the chemical composition of the tephra is an electron microprobe.
The limitation to tephrochronology is that tephra chemistry, especially that of basaltic tephra, can be altered over time. This technique has been used at Mt.Vesuvius and the island of Santorini, where a volcanic eruption in 1650 B.C.E. may have ended the great Minoan civilization.
17. Oxygen Isotope Chronostratigraphy
Oxygen Isotope chronostratigraphy relies on the fact that oxygen has three different stable isotopes, the ratio of which changes over time and therefore can be dated in the rocks present in any particular area. The ratio tends to change during long cold or warm periods of time.
18. Uranium-Thorium Dating
Uranium and thorium are radioactive elements that accumulate in some rock formations. Uranium has a half-life of billions of years, while thorium’s is only 75,000 years. Scientists use a mass spectrometer to measure the ratio between uranium-238 and thorium-230, thereby establishing a date the radioactive breakdown began. This technique has been used to date rock formations found in caves, especially speleothems such as stalagmites and stalactites, some of which have formed in the recent past, just thousands of years ago.
As shown in the article “Diving Ice Age Mexico” in the May/June 2011 issue of Archaeology magazine, uranium-thorium dating has been used to date rock formations in the numerous limestone caves that pocket the Yucatan Peninsula in Mexico. Many cave dwellers lived in these caves during the Pleistocene, when sea levels were hundreds of feet lower than they are today.
19. X-Ray Diffractometry or Crystallography
This method can determine the arrangement of atoms and molecules in crystals in both organic and inorganic material, including salts, metals, minerals and semiconductors. It can also be useful determining the structure of DNA.
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© 2011 Kelley