The Role of science in unlocking the past Dating Methods

Dating The Past One of the most important functions of the archaeologist is to date a site and the objects in it. There are two techniques for dating in archaeological sites: relative and absolute dating. Relative dating: is based on the principles of stratigraphy and typology to establish approximate dates for archaeological finds. This type of dating can give archaeologists a general idea of the date of an object but not an exact date. This is the less scientific method of dating and involves comparing finds to different finds to help establish how old it is.

Stratigraphic Dating Archaeologists keep a careful record of the object found in each layer or stratum. Since each layer represents a different period of human occupation at the site, they are able to work out which objects are earlier or later than each other. This method of dating different levels to establish a chronological sequence is called stratigraphic dating. It is a form of relative dating because it states whether something is older or later than another object based on what stratum it is found in. It can be unreliable because layers can be disturbed and objects can be moved to a different layer. Natural processes including erosion and the down-slope movement in soils in colder climates can also change the artefact’s original context.

Stratigraphy

Typology Also involves placing objects in a chronological sequence. Groups of artefacts such as pottery, tools and weapons can be arranged in a sequence from the earliest to latest based on: All artefacts belonging to a particular period have distinctive features based on the materials from which they are made and on their shape and decoration. Over a long period of time, these artefacts will show gradual changes, or evolution, in style and decoration. The material from which they are made and their production technique may also vary. As a general rule, the earliest samples in the sequence tend to be simple, while the latest samples will be more elaborate.

Pottery can be a good general guide – an object’s shape or decoration indicates its age. Different shapes indicate different periods. Some pottery sequences show the main changes in styles over time and are reasonably accurate. The chronology for Greek pottery ranges from c. 2000 to 100BC. Often the city of production and even the individual workshop can now be identified.

Seriation Dating Pioneered by Sir William Flinders Petrie (1853-1942) At Naqada in Egypt in 1894-95, Petrie excavated a prehistoric cemetery containing over 2000 graves. Many of these were impossible to date by the stratigraphic method alone. He decided to arrange all the artefacts in typological sequences. He arranged groups of similar finds in a succession or serial order now know as seriation dating. Using this method he was able to establish a chronological sequence of the graves he excavated based on the artefacts.

The Three Age System Devised in 1836 by C.J. Thomsen (1788-1863). As a result of widespread excavation, he established a 3 part organisation of ancient artefacts based on the technology of stone, bronze and iron. Excavations confirmed that stone artefacts came before bronze ones and iron ones came after bronze. Stone bronze iron These were then divided into early, middle and late periods that more clearly show specific technological advancement during each age. Eg: Paleolitic (paleo = old + lithic = stone) Mesolitic (meso = middle) Neolitic (neo = new)

The Three-Age System This simple concept that something is older or younger in relation to something else is the basis of relative dating. Despite scientific advancements in the dating of artefacts, this basic ordering of finds on a site is still the most common method archaeologists apply when they excavate.

Absolute Dating As we have seen, relative dating methods can only give us an approximate date – whereas archaeologist and historians are always searching for exact or absolute dates. Therefore, new methods of dating were required. Absolute dating: uses strictly scientific methods to determine the exact or absolute dates for archaeological finds.

Radiocarbon (C-14) can date organic material that is between 50 000 and 400 years old. It works on the principle that all living things (organic material) interact constantly with the atmosphere, absorbing C-14, which is a radioactive isotope of carbon. At the time of the living thing’s death the process of absorption stops and the Carbon C-14 that the organism has absorbed during its life is ‘frozen’. It then begins to decay at a known and constant rate. After 5730 years, half of the carbon 14 has decayed. After about 40 000 to 50 000 years, the level of Carbon-14 is almost zero. Scientists can measure the radioactive emissions of the carbon in a sample of the organism to determine whether the organism died. In this way they can estimate its age (based on how much C-14 is left). Radiocarbon dating

Radio Carbon Dating The following are some of the many different types of organic materials that can be radiocarbon dated. Pollen Bone Hair Bird eggshell Leather Corals Peat Blood residues Paper and parchment Lake muds and sediment Charcoal, wood, twigs and seeds Soil Resins and glues Marine, estuarine and riverine shells Coprolites (samples of preserved faeces)

AMS radiocarbon dating A more sophisticated version of radiocarbon dating developed in the 1970s. It stands for Accelerator Mass Spectrometry It is an improvement on radiocarbon: It can date material back to about 70 000 years It is more accurate and requires much smaller samples (eg a grain of rice, a single human hair) Materials which might contaminate the samples and produce inaccurate results can be more easily reduced It produces results in a much shorter time.

Dendrochronology (tree-ring dating) Developed in the 1920s First method of scientific dating to be used by archaeologists. Often used to with radiocarbon dating to double-check dates. How it works: The age of the tree (or anything made of timber) can be determined by counting the growth rings in a cross section of timber. The outer ring represents the year it was cut down, and the innermost ring represents the first year of its growth. The rings vary in thickness depending on factors such as fluctuations in climate and the age of the tree. As the tree ages, its rings become progressively thinner.

Dendrochronology Dendrochronologists measure these rings and record their findings in diagrams that show the successive rings in a particular tree. Older trees of the same species growing in the same region will show a similar pattern of ring-growth. By comparing the overlapping of such ring patterns, scientists can work out a long, continuous or master sequence for a region going back thousands of years. Archaeologists can match the pattern of growth rings in timber artefacts from an excavation site with the master sequence for that region. This enables them to calculate a date for the stratum in which the timber artefacts were found.

Potassium-argon dating Potassium-argon (K-Ar) dating is used by geologists to date volcanic rock up to millions of years old. It has help to date fossils, both human and animal, embedded in volcanic rock as fossils. Such areas included Olduvai Gorge in eastern Africa, where scientist dating the various rock layers have been able to date human fossils there to over 3 million years old.

Uranium-series dating Most commonly used to date rocks containing calcium carbonate that can be found in stalagmite and other formation in caves. Since caves often contain remains of early human occupation such bones and artefacts embedded in the carbonate rocks, this method is another valuable too for dating human evolution. Especially useful for periods between 50 000 and 500 000 years old – which is beyond the reach of radiocarbon dating.

Fission-track dating An application of radioactive dating – it is used to determine the thermal age of artefacts containing uranium-bearing materials like: Natural and manufactured glass Stones one used in hearths or for boiling Any fired ceramic such as pottery or tile Dates between twenty and one thousand million years ago can be determined. It is often used to verify dates obtained from potassium-argon dating.

Thermoluminescence dating TL is another form of radioactive dating – it can date inorganic material over 50 000 years old. It is used for dating pottery and clay figurines and it is more accurate than typology dating. Can be used when radiocarbon dates can’t be obtained or where C14 dates may be unreliable – it also helps to establish authenticity of possible fake artefacts.

Thermoluminescence dating How it works The microscopic structure of some minerals and ceramics traps nuclear radioactive energy. This energy is in constant motion with the minerals or sherds (fragments of broken pottery) Most of the energy is escapes as heat, but sometimes this energy separates electrons from the molecules that make up the mineral or ceramic. As time passes, more and more electrons become trapped in the imperfections in the clay, so the older the pot, the more displaced electrons it will contain. When a sample of clay is heated, the electrons come back to their original form and when this happens an energy is released and it appears in the form of light. Scientist measure how much light comes out, and together with the radioactive content of the sample, they can determine the original date when the object was fired (made).

Other Methods of Scientific Dating Dating Method Application Electron-spin resonance (ESR) Bone, teeth, shell Archaeomagnetic dating Fireplaces, areas of burnt earth Obsidian hydration Volcanic glass, tools, etc – Palaeolithic sites Amino-acid racemisation Human or animal bone – up to 100 000 years old Carbon-ratio dating Rock carvings and engraving Flourine-uranium-nitrogen dating (FUN) Bone, teeth, antlers, ivory