DATING METHODS (Ay 304)

           

  According to Winfried (2007:312), Chronometric dating is also known as chronometry or absolute dating method that gives a result in a calendar year before the present time. Archaeologist and scientist use absolute dating method on samples ranging from pre-history fossil to artifact from relatively recent history. Also is a process of determining an appropriate computed age in archaeology and geology. In archaeology, absolute dating is usually based on the physical or chemical properties of the material of artifacts, buildings or other items that have been modified by humans. Absolute dates do not necessarily tell us precisely when a particular event happened, but when taken as part of the overall archaeological record. They are invaluable in constructing a more specific sequence of event.

               According to Fagan (1985:132) argued that although historical record provide for much of the past 5000 years, archaeologist rely heavily on chemical and physical chronometric dating method. Therefore the following are the main chronometric dating method in archaeology;

               According to Fagan (1985:132-139), radiocarbon dating method is the most widely used method. It can be applied at sites from between 7500 and 400 years ago. Based on the rate at which carbon 14 decay to nitrogen in organic object, it can be used to date many such materials as charcoal and bone and even skin lather. The occurrence of radiocarbon dating is subject to statistical error, owing to past variation in the carbon 14 content of the atmosphere and thus has to be calibrated against tree rings chronology. J. R Arnold and W. F Libby was the one who published a paper in science from object of known age by their radiocarbon content. According to them radiocarbon dating method is based on the fact that cosmic radiation produce neutrons that enter the earth atmosphere and react with nitrogen. They produce carbon 14, a carbon isotope with fourteen rather than the usual twelve neutrons in the nucleus. With these conditional neutrons the nucleus is unstable and is subject to gradual radiocarbon decay. Willard Libby calculated that it took 5568 years for half the carbon 14 in any sample to decay; it’s so called half –life. It believed that carbon 14 exactly behave like ordinary carbon from a chemical standpoint and together with ordinary carbon it enters into carbon dioxide of the atmosphere. Plant got radiocarbon 14 through the process of photosynthesis and other living organism got it by eating plant in which the very short life time of individual plant is negligible compared with the half-lie of radio carbon. As soon as organism dies no further radiocarbon is incorporated into it. The radiocarbon present in the dead organism will continue to disintegrate slowly so that after 5568 years only half the original amount will be self; after about 11,100 years, only a quarter, and so on. Thus if you measure the rate of disintegration of carbon 14 to carbon 12, you can get an idea of the age of the specimen being measured. The initial amount of radiocarbon in a sample is so small that limit of delectability is soon reached. Samples ear lies than 75,000 years contain only miniscule quantities of carbon 14, Grootes (1978), in Fagan (1985:139).

            Radiocarbon dates can be taken from samples of many organic materials. About a handful of charcoal, burnt bone, shell, hair, skin, wood or other organic substance is needed. The samples themselves are collected with meticulous care during excavation from impeccable stratigraphic context, so that and exact location, specific structure or even a hearth is dates. The first stage in the dating procedure is physical examination of the sample; the material is then converted into gas, purified to remove radioactive contaminants and then piped into proportional contaminants and the piped into a proportional counter. In which this sample can be read into radiocarbon years, not calendar years. Corrections must be applied to make this absolute date.

               The limitation of radiocarbon is that can be obtained only from organic materials, which means that relatively few artifacts can be dated. All radiocarbon dates are, of course, statistical computations, and if uncalibrated, merely the radiocarbon age that is statistically mostly likely.

                 Also Renfrew and Bahn (1996:134) one of the basis assumptions of the radiocarbon method has turned out to be not quit correct. Libby assumed that the concentration of carbon 14 in the atmosphere has been constant through time; but we now know that it has varied, largely due to changes in the earth magnetic field. The method that demonstrated the inaccuracy tree-ring dating has also provided the means of correcting or calibrating radiocarbon dates.

               According to Fagan (1985:135), potassium argon is the dating method technique used for dating the earliest human being. These can be used to determine dates from the origin of the earth up to 400,000 years ago. This radioactive counting method is based on measuring accumulations of argon 40 volcanic rocks. Geologist used this radioactive counting technique to date rock as old as 2 billion years and recent as 400,000 years ago. Potassium (K) is one of the most abundant element in the earth crust and is present in nearly every mineral in it natural form, potassium contain a small proportion of radioactive 40k atoms. For every 100 40k atoms that decay, 11 become argon 40, an inactive gas that can easily escape from its material by diffusion when lava and other igneous rock are formed. As volcanic rock form by crystallization, the concentration of argon 40 drops to almost nothing. But regular ad reasonable decay of 40k will continue, with half life of 1.3 billion years. It is possible then to measure the concentration of argon 40 that has accumulated since the rock formed with a spectrometer. It is possible to date many archaeologist sites by associated to human settlement. Potassium Argon dating it is restricted to volcanic rock no more recent that around 100,000 years. Renfrew and Bahn (1996:139).

                Also McDougall and Harrison (1999:10), is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (k) into Argon (Ar). Potassium is a common element found in many materials, such as micas, clay mineral, tephra and evaporates. In these materials, the decay product 40ar is able to escape the liquid (molten) rock solidifies (recrystallizes). Time since recrystallization is calculated by measuring the ratio of amount of 40Ar accumulated to the amount of 40K remaining. The long half-life of 40K allows the method to be used to calculate the Absolute age of samples older than a few thousand years.

                 According to Fagan (1985:136), potassium argon dates have been obtained from many igneous minerals of which the most resistant to later argon diffusion are biotite muscovite and sanidine. Microscopic examination of the rock is essential to eliminate the possibility of contamination by recrystallization and other process. The samples are processed by crushing the rock, concentrating it, and treating it with hydrofluoric acid to remove any atmospheric argon from the sample. The age of the sample is then calculated using the argon 40 and 40k content and a standard formula. The resulting dates are quarter of a million years.

             Also Renfrew and Bahn (1996:139-140), example Potassium argon it has been used in Olduvai Gorge in Tanzania for study of hominid evolution, as it has fossil remain of Australopithecus (Paranthropus) boisei, Homo habilis, and Homo erectus as well as large number of stone artifacts and bones, and it has been an important method in dating other early east African sites, such as Hadar in Ethiopia.

                The limitation of Potassium argon is that it results are generally accompanied by an error estimate as in the case of other radioactive based methods. And also the principle limitation of the technique are that it can only be used to date sites buried by volcanic and that is rarely possible to achieve and accuracy of better than one percent. Potassium argon dating has nevertheless provided a key tool in areas where suitable volcanic materials are present.

               On top of that Fagan (1985:135) maintained that potassium argon dates can be taken only from volcanic rock, and preferable from actual volcanic flaws. This laboratory technique is so specialized that only trained geologist should take the sample in field.

               Fission track is a radiometric dating technique created by Shulen G. Goodman based on analyses of the damage trails or tracks left by fission fragments in certain uranium bearing minerals and glasses.

              According to Fagan (1985:133,143-144), fission track is done by measuring the uranium context of many minerals and volcanic glasses and examining the fission track left in the material by fragmentation of massive concentration o energy changed particles. It can be applied in site between a million and 100,000 year old, where volcanic rocks are found in human occupied levels. The principle of fission track dating is that many minerals and natural gases such as obsidian contain very small quantities of uranium that undergoes slow spontaneous decay. Most of uranium atoms decay of about one atom in every two million. The fission decay rates and its extent are constant, and the date of mineral containing uranium can be obtain by measuring the amount of uranium in the sample, which is done by counting the fission track in the material. The older the sample, the more track it has. It is possible to examine fission track under high magnification and to calculate the samples age by establishing the ratio between the density of the track and uranium content of the sample. Fission track technique is still new in archaeology, but it promises to become a fairy prices means o dating samples between 100,000 and 1000,000 years old. In archaeology it applicable only to sites that were subjected to volcanic activities just before or shortly after occupation, example the Great Rift Valley in East Africa, the method has obvious applicable. Also few result from the fission track method has been published, example volcanic pumice from Bed 1 at Olduvai Gorge, where the early hominid fossils were found, was dated to 2.03 +- 0.28 million by fission track method.

             Also Renfrew and Bahn (1996:144), the fission track is the most useful for early Paleolithic sites especially where the potassium argon method cannot be applied. Also it helped to settle the controversy over the date of the KBS Tuff and associated hominid remains and artifact at East Turkana sites and Kenya. Also the fission truck technique is mostly easily applied to natural occurring mineral such as pumice and obsidian but mineral with rock formation example Zircon and apatite which contain high amount of uranium. Example Micas from Zimbabwe in Africa have been dated back to more than 2500 million or (2.5 billion) years ago.

            On top of that Fagan (1895:144) argued that the limitations of fission track dating are much the same as those of argon dating. Only volcanic rock contemporary with a human settlement and formed at the time that was occupied can be measured.

              Archaeomagnetic dating is the study and interpretation of the signatures of the earth’s magnetic field at past times recorded in archaeological materials, these paleomagnetic signatures are fixed when ferromagnetic material such as magnetite cool below the curie point, freezing the magnetic moment of the material in the direction of the local magnetic field at that time, According to Eighmy et al (1990).

                  Also Fagan (1985:133 ,154, 155), archaeomagnetic dating can be used to date samples from fumiest and other features, by meaning the thermoremanent magnetism of the clay and correlating it with record of changes in the earth magnetic field. It is mainly used to date pottery kilns and similar structure dating to the last 500 years, for which changes in magnetic field have been record. Also archaeomagnetic dating is potential useful from 2000 years to present.  

                  On top of that Traling, (1971, 1975) in Fagan (1985:153) argued that by using archaeomagnetic dating it is possible to date any suitable sample of clay material known to have been heated by correlating the thermoremanent magnetism of the heated clay with record of the earth’s magnetic field. Archeologist frequently discover structure with well baked clay floor, ovens, kilns and iron – smelting furnaces, to name only a few, whose burned clay can be used for archaeomagnetic dating. In archaeological point of view, archaeomagnetic has but limited application, because systematic record of the secular variation in the earth’s magnetic field has been for only a few areas. Declination and dip have been recorded in London for 400 years, and a very accurate record of variation covers for period A.D 1600. At the moment the method is limited, but as local variation curves are recorded from more useful for the more recent period of pre history, when kilns and other burned features were in use.

              According to Renfrew and Bahn (1996:128, 130), tree ring dating or dendrochronology was developed by an American astronomer A.E Douglass, in the early decades of this century, although many of the principles had been understood long before that. Working on well preserved timbers in the arid American South West, by 1930 Douglass could assign Absolute dates to many of the major sites there, such as Mesa Verde, and Pueblo Banito. But it was not until the end of the 1930’s that the technique was introduced to Europe and only in the 1960’s that the use of statistical procedures and computers laid the foundations for the establishment of the long tree-ring chronologies now the fundamental to modern archaeology. Today dendrochronology has two distinct archaeological uses, one as a successful means of calibrating or correcting radiocarbon dates and second as an independent method of absolute in its own right.

                 Most of tree produces a ring of new wood each year and this circle of growth can easily be seen in across-section of the trunk of a felled tree. Dendrochronological used by people in the past be taking timber from species such as oak that today forms one of dendrochronological sequence, one can obtain an archeological useful absolute dating by matching the preserved timber with part of the master sequence. In central and western Europe the oak master sequence now allow the equality precise dating of the development of Neolithic and Bronze Age lake village such as Curtailed established in Switzerland, Another example is that in the German Rhine land, close to the village of Kiickhoven recently discovered timber from the wooden supporting frame of well have provided three tree ring dates of 5090 BC, 5067 BC, and 5055 BC. The timbers were associated with shreds of the linearbandkeramik culture and thus provide an absolute date for the early practice of agriculture in Western Europe.

             Also Fagan (1985:148,151-152) argue that Dendrochonology or treeing dating was originated in Arizona by Dr. A. E Douglass in about 1913 (Bannister, 1969). One of the most remarkable applications of tree-ring dating was carried out by Jeffrey Deah, who collected numerous samples from wooden beams at Betatakin, a cliff dwelling in North eastern Arizona dating to A.D 1270. Dean ended up with no fewer than 292 samples, which are used to reconstruction a history of the cliff dwelling room by room (Dean, 1970). He found that three clusters were built in 1267 and a fourth was added a year later. In 1269 the inhabitants trimmed and stock picked beams for later use. These beams were not actually used until 1275, when ten more room cluster was added to Betatakin. Dean also found that the site was abandoned between 1286 and 1300 such intrasite dating are possible only when large number of sample can be found.

                Also McGovern PJ et al (1995:79-142), dendrochronology is the scientific method based on analysis of patterns of tree – rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, many type of wood, to the exactly calendar year, it was developed during the first half of 20^th c and it has three main areas of application, first  in paleoecology, where it is used to determine certain aspect of past ecologies (most prominently climates. Second in archaeology and history of art and architecture, were it is used to date old panel paintings on wood and buildings. And third in radiocarbon dating where it is used to calibrate radio carbon age, in some areas of the world, it is possible to date wood back a few thousand years or even many thousand, currently the maximum for fully anchored chronologies is little over 11,000 years from present

                On top of that Renfrew and Bahn (1996:131) argue that unlike radiocarbon, dendrochronology is not o worldwide dating method because of two basic limitations First, it applies only to trees in regions outside the tropics where pronounced differences between the seasons produce clearly defined annual rings, second for a direct tree-ring date it is restricted to wood from those species have yielded a master sequence back from the present and people actually used in the past.

                According to Renfrew and Bahn (1996:150), obsidian technique was first developed by the American geologist Irving Friedman and Robert L. Smith. It is based on the principle that when Obsidian (the volcanic glass often used rather than like flint to make tools) is fractuated, it starts absorbing water from its surrounding , forming a hydration layer that can be measured. In a section through an obsidian tool viewed under the optical microscope, the layer appears as a distinct zone at the surface. If the layer increases in thickness in a linear way, then assuming we know the rate of growth and present thickness we ought to be able to calculate the length of time elapsed since growth began. It is relevant to site and artifacts of the last 10,000 years (the postglacial period), Obsidian hydration has given acceptable dates of around 120,000 years of middle paleoclimatic materials from East Africa. One of the boldest applications of the method as far has been by one of the pioneers of obsidian dating, Joseph Michels in his study of the rural hinterland around the importance ancient center of Kamina Ijuyu in Guatemala. The site was difficult to date from pottery found on the surface, which was much abraded, so an attempt was made to date them by measuring the hydration layer on at least two of the obsidian dates fell within on chronological phases (ranging from early formative C. 2500 BC to late post classic C. AD 1500) the site was assigning to that place.

             Also Fagan (1985:144-146), maintain that obsidian is a natural glass substance often formed by volcanic activity. It has long been prized for its sharp edge and other excellent qualities for tool making. Projectile heads, hand axes, blades and even mirrors were made from this widely traded material in both the new world and old world. And also he add that obsidian hydration is potentially as useful as Radiocarbon dating, but is still suffers from grave limitations especially when one is dealing with very early sites. Obsidian itself is of very limited distribution, hydration layers more than30 microns thick are known to peel off and reaching this thickness may take longer in some areas than others.                          

            Also he add that, this method has been tried experimentally with East African settlements 300,000 to 780,000 years old, should the method be validated for sites this old, then it will have far wide application than potassium argon and radiocarbon dating.

              Uranium-series dating, according to Renfrew and Bahn(1996:140-141) argue that uranium-series dating is a dating method based on the radioactive decay of isotope of uranium. It have proved particularly useful for the period of 500,000-50,000years ago, which lies outside the time range of radiocarbon dating. In Europe, where there are few volcanic rock suitable for dating  by the potassium-argon technique, uranium-series dating may be the method of first choice for clarifying when a site was occupied by early humans

            Two radioactive isotopes of the element uranium (238u and 235u) decay in a series of stages into daughter elements. Two of these daughter elements, thorium (230^Th and also called “ionium” a daughter  of 238u) and protactium (231Pa, a daughter of 235u) themselves also decay with half lives useful for dating, the essential point is that the parent uranium isotope are soluble in water, where as the daughter product are not. This means for instance that only the uranium isotopes are present in water that seeps into limestone caves.

          The method is used to date rock rich in calcium carbonate, often those deposited by the action of surface or ground waters around lime-rich springs or by seepage into limestone caves. As early humans used caves and overhanging rocks for shelter, artifacts and bones often become embedded in a layer of calcium carbonate or another type of sediment between two layers of the calcareous deposit.

          According to Renfrew and Bahn (1996:141) argue that it’s difficult to determining the correct order of deposition in a cave is one of the reason why uranium-series method is prone to give ambiguous results. For this and other reasons, several layers of deposit in a cave need to be sampled and the geology meticulously examined.

             By concluding; absolute dating or chronometric dating is importance because it is able to establish the age of an object or events in a calendar year and it can be utilized several times on a given sample.

           

                                                Reference;

Eighmy, Jeffery, Sternberg, Robert (editors) (1990). “Archaeomagnetic Dating.”  The University of Arizona Press.

Evans, Susan Toby; David L, Webster, (2001). Archaeology of ancient Mexico and Central America: an encyclopedia. New York.   

Fagan, B.M (1985). In the Beginning. An introduction to archaeology. Little Brown and company Boston Toronto.

Henke, Winfred (2007). Handbook of paleoanthropology. New York: Springer.

Renfrew, G. and Bahn, P. (1996). Archaeology; Theory, Method and Practice. London: Thames and Hudson.