Anthropology Paper I

Syllabus Section: 1.8 (a) Principles of Prehistoric Archaeology Chronology: Relative and Absolute Dating methods.

Accurate knowledge about the age of artifacts or archaeological remains is one of the major tasks in archaeological investigations. A fundamental factor in interpreting and reconstructing the cultural past is by calculating the age of artifacts and other related cultural remains. Absolute dating methods are the best ways to establish pre-historic chronology.

Major Approaches to Establishing Pre-Historic Chronology

• There are two approaches to establishing the chronology – relative dating and absolute dating or chronometric dating. Relative dating methods do not establish precise dates for archaeological remains.
• The absolute or chronometric dating method scientifically establishes accurate and precise dates of cultural remains of the past.
• Stratigraphy, fluorine analysis, Seriation, and pollen analysis are some of the relative dating methods.
• Dendrochronology, Radio-Carbon method, Potassium-Argon method, and Thermo luminescence method are some of the absolute dating methods.
• Aerial Surveys (Remote sensing), Ground surveys with modern types of equipment, Computer-Aided Mapping, and Under Water Archaeology are the modern trends in archaeological excavation.

 

1. Relative dating methods

Some of the dating methods are Stratigraphy, fluorine analysis, Seriation, and pollen analysis. Let us discuss some of the relative dating methods.

• Stratigraphy: Stratigraphy is based on the principle of the ‘superposition of layers’. The law of superposition says sedimentary layers are deposits in a time sequence. The layers on the bottom will be the oldest and the layers on the top will be the youngest. That means in undisturb situations, the oldest layers are deposits first and the newest layers are deposits last. Likewise, archaeological materials found in the lowest stratum will be the oldest, whereas, the materials found in the uppermost stratum will be the recent ones. However, some geological activities like earthquakes and volcanic eruptions, shift the position of materials found in layers or stratum. This will adversely affect the dating of the material, and hence, the need for alternative dating methods.
• Fluorine Analysis: Fluorine is a non-metallic element in the form of pale yellow highly reactive gas. Bones lying in the earth absorb fluorine dissolved in percolating water and result in the formation of stable Fluorapatite. Therefore, the older a fossil is, the higher its fluorine content. This method cannot give an absolute chronological age, because the amount of fluorine differs from soil to soil. But this method is suitable for the relative dating of bone materials found at the same site.
• Seriation: Pottery-making technology was not known to humans for a pretty long time. Usually, pottery has been found in the sites of agricultural practices while hunting and gathering people seldom exhibit this. Handmade pottery differs in size, shape, surface finish, and decoration. The style of craftsmanship changed from time to time. Therefore, it is possible to arrive at certain conclusions regarding where the ceramics had been in use for a long time. Thereby, the relative age of the pottery can be establish.
• Pollen Analysis: Pollens are the microscopic grains containing the male reproductive cells, discharge from the anther of flowers. These pollens are very small in size and relatively a small part of pollen is releas from the flowering plants. But these pollens are quite durable under certain conditions. They remain preserve in peat bogs, lake mud, and desert soil. Each type of tree has its own recognizable shape of a pollen grain. Analysis of tree pollen reflects the tree composition of a particular area during the period of formation of the deposit. By studying the earlier strata of peat deposits, one can find out the changes that took place in the flora pattern from one period to another. Pollen analysis is also helpful in detecting climatic changes. It is also helpful in reconstructing the stages of forestation after the glaciations in Europe. It is understood that birch trees came first, follow by pine and oak respectively. So a kind of relative dating is possible with Pollen grains. Pollens also bear the evidence of agricultural activities in Neolithic sites.

 

2. Absolute Dating Methods

Some of the absolute dating methods are Dendrochronology, Radio-Carbon method, Potassium-Argon method, and Thermo luminescence method. Let us examine these methods in detail.

• Dendrochronology: A tree builds up a new layer on its trunk every year. By counting the annual layers one can easily find out the age of the tree when it is cut down. The width of the rings and the relative distance between the rings reflects climatic variations over a period of time. For example, some rings look thick and some others are thin. During the years of plenty of rainfall, a tree absorbs more nutrients, moisture, etc., and tree rings grow wide. Likewise, during drought season tree rings will become narrow. Since all trees in an area are affected in the same manner, the ring sequences follow a general pattern. By comparing the tree ring layers in two areas, one can differentiate the climatic conditions between the two regions. But its method is complicated and trained personnel are needs. Experience in handling, recording, and interpreting the sequence and competence to take decisions on the absolute date is necessary. Dendrochronology cannot be apply to all kinds of trees in all kinds of environments. Samples from the same environment may be use.
• Radio-Carbon method: Radio-carbon dating is a very valuable and widely used method of dating. It is based on the measurement of the decaying rate of radioactive carbon, known as Carbon-14 (C14). In 1941, Willard F Libby discover the radioactive atom of carbon. The element of carbon has three isotopes, namely Carbon-12, Carbon-13, and Carbon-14. The first two isotopes are more or less stable and Carbon 14 is unstable and unsteady. It gives out radio-active rays and changes. All living matter possesses a certain amount of a radioactive form of carbon (C14). The quantity of C14 normally presents in organisms has been estimate. Radioactive carbon is absorb from the air by plants and then consume by animals that eat the plants. After an organism dies, it no longer takes any of the radioactive carbon. Carbon 14 decays at a steady and slow pace. The rate at which the carbon decays is known as “half-life”. C14 has a half-life of 5730 ± 40 years. The materials to be dates are burning at very high temperatures in laboratories and reduced to exact carbon and that is measures. The objects like leather, hair, cloth, charcoal, wood, and bone can be date using this method. The radiocarbon method is not free from disadvantages. First of all, it is not possible to cover a long span of time, not more than 60,000 years. Secondly, the radio-carbon level is generally calculate in terms of the present-day atmosphere, but we are not sure whether the same level of radiocarbon was present in the past.
• Potassium-Argon method: This method also follows the principle of the radiocarbon method. A radioactive form of potassium-40 is utilize here the rate of decay of which is known. After disintegration, it produces Argon-40 and Calcium-40. Therefore the ratio of Potassium-Argon may be measure to ascertain the date of minerals and rocks in a deposit. It does not date fossil specimens directly. The half-life of radioactive potassium is 1330 million years. It works well in case of the sites which are 500,000 years or more. So, this method is quite useful to date with very old pre-historic materials. Very old archaeological sites have been date by this method. This method is also not free from drawbacks. This method of dating can be apply to rocks and sediments which are rich in potassium. Such types of rocks are available only in volcanic areas. Now, you know why the potassium-argon method cannot be apply to the sites of South Africa, while East Africa yields good results.

Thermo luminescence method:

This method is relates to pottery and minerals. Thermo-luminescence is the emitting light from pottery, which can be measures. If the ground-up pottery is heat to about 500 degrees Celsius, some sort of light comes out. This phenomenon is the result of the radioactive influence of metallic elements like uranium and potassium present in the clay and surrounding soil. The geological thermo-luminescence was driven out at the time of the original heating of pottery. But, as the pottery remain further expose to steady natural radiation, it again revives the capacity of thermo-luminescence Age of pottery can be determine by measuring this thermo-luminescence.

Read more: Anthropology Decoded