Dating is a technique used in archeology to ascertain the age of artifacts, fossils and other items considered to be valuable by archaeologists.Archaeologists use different dating techniques to determine the age of a particular artifact, fossil,site, or part of a site. Two broad categories of dating or chronometric techniques that archaeologists use are called relative and absolute dating . Relative dating determines the age of artifacts or site, as older or younger or the same age as others, but does not produce precise dates.Absolute dating methods produce specific chronological dates for objects and occupations.This method is not available to archaeology until well into the 20th century.
Relative dating
Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age.Methods for relative dating were developed when geology first emerged as a formal science.Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things,it is the science of rock strata, or layers.Stratigraphy is based on the law of superposition-like a layer cake, the lowest layers must have been formed first .Layering occurs in sedimentary rocks as they accumulate through time, so rock layers hold the key to deciphering the succession of historical events in Earth’s past.In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers. Cross-dating of sites is still used today in which one compares geologic strata at one site with another location, and extrapolates relative ages in that manner.This method is used when sites are far too old for absolute dates to have much meaning.
However, geological strata are not always found to be in a neat chronological order. Wind and water erode strata and some areas are uplifted or even tilted. These processes result in geological unconformities , or breaks in the original stratigraphic sequence. In addition, people and other animals dig holes, resulting in a mixing of material from different strata.All of these processes confuse the stratigraphic record. In many cases, however, it is possible to reconstruct the original sequence of strata so that they can be used for relative dating.
For instance, if we find a fossil bone below the strata 3 rock level shown in the illustration above, we assume that the animal most likely lived at a time before that layer was formed. However, we must be careful to note whether or not the fossil comes from the mixed strata zone of the filled in hole.
When two objects are found in the same strata of a site, it is usually assumed that they date to the same time period. This is an application of the principle of association. However, the assumption of contemporaneity may not always be correct. This is due to the fact that one or both of the objects may have been moved or redeposited into a different location. In other words, they may no longer be in their primary context.
Biostratigraphy
Biostratigraphy is the use of fossils in stratigraphy. It relies on the study of in situ fossil distributions. Various fossil groups can be found in different sedimentary environments. The two main environments are land (terrestrial) and sea (marine).When the bones of our early ancestors are found in the same geological strata as those of other animals that are known to have lived only during a specific time period in the past, we assume that these ancestors must also have come from that time. This is referred to as dating by association with index fossils (also known as guide fossils, indicator fossils or zone fossils), or biostratigraphy .
Fluorine Analysis
When bones, teeth, or antlers are found at a site, fluorine analysis can be used to tell us whether or not the animals they were from actually lived at about the same time. This relative dating method is based on the fact that there are specific progressive chemical changes in skeletal remains that result from burial underground. As time passes, the organic components of bone (mostly fats and proteins) are lost primarily through bacterial action. Since these components contain nitrogen, there is a progressive loss of that element. At the same time, percolating ground water deposits trace amounts of fluorine and other elements, such as uranium, into the bone. As a result, the amount of fluorine and other trace elements progressively increase. If the bones of two animals are buried at the same time in the same site, they should have the same relative amount of nitrogen and fluorine. If they do not, they most likely come from different eras, despite the fact that they were found in association with each other.
Fluorine analysis can be used only as a relative dating method because the rate of decay and the amount of dissolved minerals in the ground water varies from site to site.Fluorine analysis is primarily used for verifying whether or not two fossils in the same strata at a site were in fact contemporaneous. If not, then at least one of them must be physically out of context.
A good example of the value of fluorine analysis was in bringing to light the Piltdown Man hoax. In 1912, Charles Dawson, an amateur paleontologist, found what was thought to be an early human skull and jaw in the Piltdown gravel deposits of England. Because it had an ape-like jaw and was found in association with the bones of extinct animals, this "Piltdown Man" was also believed to be a very ancient human. It was popularly referred to as "the missing link" in human evolution. In 1949, the Piltdown bones were finally tested for fluorine content by Kenneth Oakley and the fraud became apparent. After reexamining the strata at the Piltdown site, the evidence of a hoax was published in 1953. This was verified through the use of X-ray fluorescence examination. The skull and jaw clearly were not from the same time period. The jaw was likely to have come from a modern young adult orangutan. It had been cleverly carved to fit the skull and stained to look ancient. In addition, the associated bones from extinct animals had much older fluorine and nitrogen ratios than either the jaw or the human skull.
Geochronology
A relative dating method based on the association of early human sites with changing features of the land, such as the advance and retreat of glaciers or the rise and fall of sea levels. When these events are well dated, geochronology could be considered a reliable calibrated relative dating technique.
Artifact time marker
An artifact type that was made by a particular culture during a limited time period. When discovered clearly in association with ancient humans in an archaeological site, they are an indication of at least the relative time of the occupation. When the independent dating of the artifact types is reliable, this can be considered a calibrated relative dating method.
Seriation
When a stratigraphic sequence is lacking, another relative dating technique known as seriation may be applied. This technique dates a site based on the relative frequency of types of artifacts whose dates of use or manufacture are known. The basic assumption underlying seriation is that the popularity of culturally produced items (such as clay pipes or obelisk gravestone markers in America) varies through time, with a frequency pattern that has been called the "battleship curve." An item is introduced, it grows in popularity, then its use begins to wane as it is replaced by another form. Certain types of artifacts have been identified as particularly useful temporal markers, for example, gravestones, projectile points, lamps, pottery sherds.The frequency of artifact types in a stratum can be compared to known frequency changes previously recorded for an ancient culture. In this way, the stratum can be dated relative to other strata or sites. When a seriation sequence has been cross-calibrated with reliable chronometric dating methods, it can be considered a calibrated relative technique. Before being able to interpret materials found at a site, an archaeologist faces the task of sorting the artifacts into manageable units for analysis. This is frequently a difficult task. Sorting is usually based on form and function. What does it look like? What is it made of? Is it decorated in any way? Have you ever seen it before?
Patination
Patination is a technique involving the measuring of the patina on an artifact. The patina is the outermost surface of the artifact that differs in color, texture, luster or composition from the rest of the artifact. This difference is the result of chemical, physical or biological change in response to the surrounding soil and environmental condition. Although it is not an actual dating technique, patination is used when multiple artifacts of the same type are found in the same area and under the same conditions. The use of this technique is to determine the age of the artifacts, relative to the others, by comparing the thickness of the patina on them. There are many variables that have to be calculated, and this makes dating lithics from patina formations a relative dating technique.
Palaeontolgy - Dating by analysing Animal Remains
This method is based on the assumption that changed climate will bring about the occurrence of different animals and plant species i.e., with change in climate,some species become extinct.This assumption helps in establishing relative dates.
