6 Relative dating methods

K. Polley

1. INTRODUCTION

Dating is an indispensable tool in the field of evolutionary investigations. It helps researchers to make a better assessment of time of any relict objects like fossils, artifacts and also geological deposits. There are two different approaches to dating, namely- Absolute or Chronometric Dating and Relative Dating.

Relative Dating gives information about the age of an object in relation to other objects. In another way, it can be said that this dating method can determine whether any object is older or younger than other objects. With the help of this method of dating archaeologists can group or rank various archaeological materials like artifacts, fossils etc. according to their relative age. Thus, the relative method of dating produces only comparisons of age or date, not actual or calendrical date of an object. Stratigraphic analysis is an essential component of this method of dating. Besides Fluorine test method help to determine the relative age of different objects belonging to the same stratum. In possible cases, the technique of Palynology and Pedology is also applied to achieve a more or less correct date for an object.

The approach of relative dating has its root in the science of Geology, particularly in Stratigraphy. British geologist William Smith (1800) formulated the two basic principles of stratigraphy. The first of which is known as the Law of Superposition, it means if one series of rock lies above another then the upper series was formed after the lower series. The second principle is related to the Biostratigraphy; this principle indicates that each bed or each group of beds contains a characteristic assemblage of fossils (Walker, 2005:165). On the basis of the aforesaid laws various Stratigraphic units can be placed in relative order of age. In the field of archaeology use of the said laws of stratigraphy have been used since the time of Antiquerians (Green, 2003:18-23). However, after 1950’s use of more advanced techniques of stratigraphy came in the appearance (Green, 2003:108-113) and have been increasingly applied in the field of archaeology.

2. RELATIVE DATING METHODS: A SHORT SURVEY

Within the class of relative dating methods, there are mainly four different methods of dating, namely, Typological dating of ancient artifacts, Stratigraphic dating, Fluorine analysis method of dating and Pollen analysis method of dating. Most of the said relative dating methods require limited technological sophistication as well as funding also. Beside the afore said methods other relative dating methods like Obsidian hydration method, Amino Acid Racemization Method etc. are recently developed and being applied in the field of archaeological research. Brief descriptions of all these methods of dating are given below.

2.1. Typological Method of Dating:

Classification is one of the most fundamental interpretative activities in the field of natural sciences. In archaeology classification of various cultural materials is known as typology. Typological classification of material remains of ancient culture has a long history in archaeology; it probably dates back to the beginning of archaeology itself. Besides making classification or subdivision of various data during archaeological excavation and exploration, typological classification often helps archaeologists to determine the relative age of various artifacts. In the nineteenth century, observations about the types of artifacts, collected from different layers led to the creation of a time frame for prehistory, often known as the ‘Three Age System’. It was based on the introduction of tools made from stone, the bronze and then iron. Despite the presence of many flaws in this scheme, the terms are still used today to distinguish different periods of the past (Grant et al, 2008:98). In the typological method of dating at first various attributes of an artifact like shape, design, raw material or fabric etc. are selected to characterize or classify that artifact into a particular ‘form’ or artifact ‘type’, after that the typified or classified artifact is compared to the other type of artifacts to allocate the newly typified artifact in a particular time period. In this way once a good typological sequence for an area is established it can be referred to when new finds and sites are discovered and used to cross date them (Andrefsky, 2005:66; Grant et al, 2008:98). As an example to this scheme it can be said that if someone finds a piece of pottery and takes it to a museum, typology will help him to classify the potsherd to a particular time period.

The typological scheme of classification of human culture and various archaeological materials has its root in the time of ‘Antiquarianism’, or at the time of ‘Enlightenment’ period. It was that time, when the dominant concern of the archaeologists was to bring order and system to the mass of relict objects, collected by excavations or explorations. This scheme of classification was mainly influenced by the ‘System of Nature’ of Linnaeus. In later time influence of Linnaeus and other naturalists provided a methodological foundation for the later development of artifact classification (Ellis, 2008, cited in Pearsal, 2008:1019). Now a day’s typological classification of artifacts has been developed a lot and newer techniques like seriation technique, study of the curation of artifacts etc. are used by modern archaeologists to classify an artifact of a more accurate sense.

2.2. Stratigraphic Dating Method:

Stratigraphy is a sub-discipline of Geology. It is the study of the origin, physical characteristics and spatial relationships of stratified rocks, primarily to understand the history of events documented in the strata (Delson et al, 2004). Usually sedimentary rocks are studied in stratigraphy, however layered igneous rocks and even layered metamorphosed rocks can also be studied using principles of stratigraphy. The science of stratigraphy is mainly governed by three basic principles or laws; they are Principle of Superposition, Principle of original Horizontality and Principle of Lateral Continuity. The principle of superposition says that means if one series of rock lies above another then the upper series was formed after the lower series (Walker, 2005). The principle of original horizontality says that strata are horizontal or nearly so when they are deposited; according to the principle of lateral continuity it can be said that all parts of a stratum, disrupted by lateral activity, were once joined in a single connected layer (Delson et al, 2004:1384).

Stratigraphy is used to date rocks and fossils of an area on the basis of the principle of superposition. This principle helps geologists and archaeologists to put various historical events in order. Succession of events in one area can be correlated with other on the basis of original horizontality and principle of lateral continuity. Sometimes it can be done by comparing fossils of two areas or by the use of the knowledge of biostratigraphy.

In Stratigraphy rock strata are classified according to their nature and age of origin, lithological components and biological components. On the basis of the said features, three different branches of stratigraphy- lithostratigraphy, biostratigraphy and chronostratigraphy have been developed. Lithostratigraphic studies are based only on the nature of rocks and minerals of stratigraphic units alone. Various lithostratigraphic units can be classified as laminae, bed, formation, groups etc. on the basis of their lateral extent and relative thickness in a particular area. Biostratigraphy classifies various Stratigraphic units on the basis of the fossils found in each of them. Biostratigrapher tries to correlate various Stratigraphic units on the basis of evolutionary relationship or absolute age of the fossil remains found in various strata. Chronostratigraphy tries to correlate various Stratigraphic units on the basis of their relative age. The major objective of chronostratigraphy is to gather all strata of the same age into the same chronostratigraphic unit (Delson et al., 2004).

2.3. Fluorine Analysis:

Fluorine analysis is one of the most widely used relative dating methods. This method of dating is used to compare age of fossilized bones buried underground for a long time. Like other relative dating methods discussed previously this dating method cannot determine the actual age of any bone but it can compare the age of the fossilized bone buried under the soil for a longer time. Studies have shown that fluorine is widely distributed in the form of soluble fluorides which occur in trace quantities in ground water. When fluorine ions come into contact with buried bones and teeth, they are absorbed and become locked in it by replacing hydroxyl ions of the bone in the ultramicroscopic crystals of hydroxyapatite and as a result of this, those hydroxyapatite ions changes into fluorapatite ions. When fluorine atoms have once been fixed in bone, they are not readily dissolved out. In course of time buried bones and teeth deposit fluorine progressively. The rate at which fluorine deposits occur in bone varies from one region to the other; however bones that are deposited in soil in a same period of time will have approximately same amount of fluorine content within them. As the fluorine content in a fossilized bone is fixed in amount, a fossilized bone which has been washed out of a much older deposit and re-deposited in a younger one will have higher fluorine content than fossil bones of younger deposit. Fluorine to phosphate ratio in a fossilized bone is established by X-Ray diffraction technique. This method is particularly useful when bones have accumulated in sand and gravel deposits, but difficulties have been found in lime stone deposits where it has been observed that the accumulation of fluorine ions on the bone is inhibited whereas the decay of organic matters in bone proceeds at normal rates (Lowe and Walker, 1984:272-273). Fluorine dating was used to uncover the Piltdown hoax, in which a skull was fabricated from bones of different species and age and allegedly represented a human ancestor (Truncer, 2008, cited in Pearsal, 2008:1081).

2.4. Pollen Analysis:

This method of dating is sometimes known as Palynology. It is the microscopic identification of the pollen grains of particular species of plants in soil samples recovered from a particular site. By identifying the plants growing in a locality at some time in the past, and by counting the relative number of pollen grains from different plants in a Stratigraphic sequence, it becomes possible to develop pollen diagram which summarizes the shifting of vegetation pattern over time. Work on pollen began in Scandinevia in the 1920s and it confirmed the general pattern of climatic change that had been proposed from visible plant remains. In this way Palynologists became able to develop a model, reflecting changes in past climatic condition.

The value of this technique for archaeology lay in the fact that climatic phases were likely to have been fairly uniform for a particular region. Thus, plant species found in a sample of pollen from an archaeological site could be fitted into the climatic sequence. Correlations could be established between sites belonging to similar climatic phases in different countries. With the help of this technique even individual artifacts could be dated if they were found in peatbogs, or if they had sufficient soil attached to them for the identification of pollen. For example, a Mesolithic bone harpoon dredged from the bottom of the North Sea was placed into the period when pine was declining in favour of trees that preferred warmer conditions around 7000 BC (Zeuner 1946, cited in Green, 2003:112)

2.5. Obsidian Hydration Method of Dating:

Obsidian hydration method of dating is based on the fact that a freshly broken surface of Obsidian, a variety of naturally volcanic glass, will absorb from its surroundings to form a measurable hydration layer, known as ‘Perlite’ (Friedman and Long, 1976, cited in Lowe and Walker, 1984:273). This layer is not visible to the naked eye and should not be confused with the ‘Patina’, which develops on many materials as a result of alteration or chemical weathering. The hydration begins at the moment the surface is exposed by chipping or flaking and the penetration of water into the artifact continues at a known rate until the present time. Thus, when ancient man chipped or flaked a piece of Obsidian, he exposed a fresh surface on which hydration started. Measurement of the depth of penetration of hydration on the artifact depicts a measure of time that elapsed since the manufacture of the artifact (Lowe and Walker, 1984).

Calculating the hydration rate can be difficult, however, since the rate is dependent on composition, moisture, temperature, and pressure. Even so, obtaining sufficient estimates for these attributes is far from impossible, and many successful applications of obsidian hydration have been made. In a relative-dating role, obsidian hydration offers the possibility of identifying and ordering assemblages of mixed age. In addition, because the chemical composition of obsidian differs significantly from outcrop to outcrop, provenance studies have been very successful in accurately characterizing different sources and allocating artifacts to them. Such provenance information, when combined with the chronological information of obsidian hydration, provides a powerful set of tools to document change in the archaeological record (Truncer, 2008, cited in Pearsal, 2008:1081).

2.6. Amino Acid Racemization:

This dating method is a potentially valuable tool for dating fossilized bone fragments. Racemizaion dating is based on the fact that amino acids, which make collagen, a fibrous protein that forms 90-95% of organic material in bone, consist of only L-isomers (molecules that rotate in polarized light to the left). Over long periods of geological time, these L-isomers of the amino acids slowly racemize (a process of change in the rotation direction of polarized light) and change into D-isomers (molecules that rotate polarized light to the right). Both L and D isomers of amino acids are found in fossil bones. Racemization process is sensitive to the temperature change. Usually before amino acid racemization dating records of past temperature change in a region is recorded with the help of pollen analysis and other sophisticated methods and a detail database of this is constructed. At the time of dating this database is used for the ‘calibration’ of the data coming out from the racemization dating. Alternatively sometimes Radio carbon dates are used in the calibration process of racemization dating. Finally D: L ratio is used for the racemization dating of any fossilized bone sample (Lowe and Walker, 1984; Walker, 2005).

The most common amino acid, used in racemization dating is Aspartic acid, which has one of the fastest racemization rates of all the stable amino acids. Racemization half life of Aspartic acid is about 15,000 years at a temperature of 20oC, which means that the technique can be used to date bone samples much older than 60,000 years to 50,000 years limit of C14 dating. Since only a few grams of fossil bones are needed, amino acid racemization can be used to date samples too small to be dated by C14 dating (Lowe and Walker, 1984).

The ranking of fossils and their associated sediments on the basis of amino acid ratios is referred to as aminostratigraphy, and this provides a basis for relative dating and for inter-site correlation. When the amino acid racemization dates are calibrated by various other dating methods (C14, Uranium series dating, Palynology etc) that provide an independent timeframe and an amino acid geochronology can be developed that is applicable beyond the calibration site, and in some cases relative amino acid ages may range up to 1 million years ( Walker, 2005:188).

3. SUMMARY

• The relative dating method only establishes the relative order of antiquity of fossils and Stratigraphic Units.
• The development of relative dating method has its root in the field of Geology, particularly in Stratigraphy.
• Relative dating method comprises various dating techniques like- Stratigraphic Method of dating, Typological Dating, Palynology, Fluorine Analysis method of dating, Obsidian Hydration method and Amino Acid Racemization Method.
• Most of relative dating methods are less sophisticated and require less time and funding also.
• Typological dating method dates an ancient artifact of the basis of its characteristics like shape, raw material, decoration, probable function etc.
• Stratigraphic dating method dates geological deposits, fossils and artifacts on the basis of their relative position in Stratigraphic section of a particular region.
• Fluorine dating method measures the relative age of the fossilized bone samples on the basis of the amount of fluorine deposits within them.
• Pollen analysis method of dating estimates age of any geological deposits by the study of pollen grains preserved in the deposit. This method of dating can produce pollen diagram of a region which is used to understand changes in the climate in a particular region.
• Obsidian Hydration method of dating can measure relative age of various Obsidian made artifacts on the basis of the thickness of hydration layers on them.
• Amino acid Racemization dating can compare age of fossilized bone samples on the basis of relative proportion of D- and L- isomers of amino acids in the fossilized bone samples.

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