1

HOW DO ARCHAEOLOGISTS DATE ANCIENT ARTIFACTS?

Excavation site at Gran Dolina in Spain

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!

2

1. Stratigraphy

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 help 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.

4. Dendrochronology

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

3

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.

6. Potassium-Argon

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,

and 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.

4

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.

9. Thermoluminescence

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, thermo

luminescence 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

5

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 thermo luminescence.

11. Rehydroxylation

Like thermo luminescence, 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.

12. Paleomagnetism

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

6

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.

16. Tephrochronology

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

7

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.

Please leave a comment.

http://kosmo.hubpages.com/hub/How-Do-Archaeologists-Date-Past-Events