The study of past societies (sociocultural systems) and the reconstruction of their paleoenvironments requires the creation of a chronological frame of reference that allows to situate them in time. The accuracy and nature of their construction vary according to the methods used. Archaeology usually refers to two major groups of procedures to date the reality it investigates.
One of them, and for decades the only one available, is called relative dating and consists of placing the facts in time with each other, through the establishment of relations of anteriority, contemporaneity and posterity. This procedure uses, among other methods, stratigraphy, that is, the order of succession of the geological layers represented in the archaeological sites, and also the typological or stylistic sequences, which also intent to establish a logical temporal ordering applicable either to artefacts or to artistic representations.
The second major pathway for dating the past is called absolute dating and intends to situate the events within a universally recognized numerical time scale (calendar). Established since the second half of last century with the invention of the radiocarbon method, this domain experienced, in the last half century and through the progress made especially in the fields of physics and chemistry, a huge development.
Having today a very diverse and comprehensive range of methods, each based on the analysis of phenomena and/or different processes, they can cover, when combined with each other (because none can do it alone), the entire chronological spectrum of prehistorical times.
Within the investigations conducted in the Côa area, the two methods used so far are in the field of absolute radiometric dating. One, radiocarbon, is based on measuring the physical phenomenon of the disintegration (decay) of C14 in organic matter. The other, luminescence, seeks to assess the amount of energy captured, in the form of radioactive elements, in the imperfections of the crystalline structure of certain minerals.
Invented in the late forties of the twentieth century by an American physicist named W. F. Libby, radiocarbon was the first absolute dating method to be developed. Its principle is based on the quantitative evaluation of the regression rate (decay or disintegration) of the radioactive isotope of carbon (C14) present in organic matter.
In its natural state, carbon has three isotopes; two of them are stable, the C12 (the vast majority) and C13 (present in a percentage of just over 1%), and one is radioactive, therefore unstable, the C14. Formed in the upper layers of the atmosphere by the action of cosmic radiation, the radiocarbon is subject to a fast oxidation process, turning into radioactive carbon dioxide, which is dispersed through the atmosphere and is also absorbed in vast amounts by the oceans (hydrosphere).
When it integrates the natural carbon cycle, it is also assimilated by all living things (biosphere) that is, by plants (photosynthesis) and animals (breathing, food chain).
Even though it is constantly being produced, the rate of representation of C14 in the atmosphere remains constant, since its disintegration takes place at the same velocity it is formed.
The same process occurs in living beings, which remain in equilibrium with the environment, and this cycle is interrupted only at their death. When this occurs, and the metabolic exchanges with the environment are ultimately interrupted, the unstable nature of the radioactive C14 determines that their presence in organisms begins a process of gradual decay.
When he invented the method, W. Libby calculated that, by comparison with the rate of C12, the amount of radiocarbon initially present in an organism declined by half at the end of 5,568 years after his death (this figure came to be known as "half life"). Thus, if the value of the present radiation corresponds to one fourth, this will mean that 11,136 years have elapsed, or the equivalent of two "half lives", and so on. So, the measurement of residual C14 and its proportionality in relation to the presence of C12 is sufficient to find the age of a sample of organic matter, represented, for example, by fragments of wood, charcoal, peat, seeds, bones and shells (though some provide a greater accuracy of results). The limit for dating is close to 50,000 years, since from this time the measurement instruments can no longer assess the residual radioactivity in the samples.
This method, however, and like all others, has its limitations.
On the one hand, the fact that radioactive decay corresponds to a spontaneous and random phenomenon determines that its quantification must always admit a margin of error. Thus, the numerical expression of a radiocarbon dating is always constituted by two values: the first represents the estimated date, say the average value; and the second, the margin of error admitted around this value (standard deviation). A date of C14 with a value of 5568 ± 30 BP means that the event it seeks to date has a 68% probability of being within the range of time between 5538 and 5598 years, with the notation BP (Before Present) corresponding to year 1950 of our era, which was adopted internationally as the standard reference for all dating by this method.
Another difficulty that quickly came across this method resulted from the confrontation of dates provided by it with others historically known and established through calendars, in particular the one of the Egyptian civilization. This comparison resulted, invariably, that radiocarbon datings provided a rejuvenation of facts in comparison to their actual age.
Meanwhile, the study of the growth rings of trees, the object of attention of botanists and, in particular, of specialists in Dendrology, demonstrated, through the differences observed in their varied growth rates, that the amount of atmospheric carbon dioxide had not remained stable over time. Now, this observation questioned, thoroughly, one of the basic assumptions admitted by W. Libby in the establishment of his method, according to which the production and proportion of atmospheric C14 would have been kept constant over time, which, as we know today, is far from the truth. This evidence has determined the need to find procedures to redress, as far as possible, the deviation provided by radiocarbon datings.
The solution to overcome this difficulty would be supplied by another method of absolute dating, developed in the meantime, which was based precisely on the researches in the field of Dendrology.
In fact, from the comparison of the rate of annual growth of certain tree species characterized by their longevity (the giant American sequoia, the European oak, one particular species of pine) and the similarities found, for each species, in their growth rings, it was possible to recognize several stages of coating, which reflected, over time, an analogous behavior. For these stages was awarded the designation of dendrochronological sequences, which, with a high degree of reliability, allowed, at first, to establish rigorous dating for the past 10,000 years, but which, presently, already exceed 20,000 years.
The assistance provided by Dendrochronology, in order to correct the radiocarbon method, consisted in the collection of samples from growth rings that it accurately dated, and submit them to C14 dating. Successive deviations found between the dates allowed the construction of so-called calibration curves, which today are used to correct (calibrate) the radiocarbon datings. The conventional C14 dates (non calibrated) still contain the notation BP, whereas calibrated dates are identified with the title "cal BP", "cal BC (Before Christ) or" cal AD "(Anno Domini).
The method of radiocarbon dating has known, from the late seventies and early eighties of last century, important developments in its technical procedures.
The measurement of residual radioactive carbon present in the samples subjected to dating was held, until then, by measuring the β-ray emission (Beta) during the radioactive decay (conventional method).
A new technology has since emerged and today, fully tested and developed, proceeds to the direct count of the number of C14 atoms present in the material submitted to dating, using a particle accelerator. It is therefore known as AMS - Accelerator Mass Spectrometry.
The development of this new method promised to bring three important improvements for radiocarbon dating: a substantial reduction in the size of the samples (expressed in grams before, nowadays in less than a milligram); an equally significant reduction in the time required to obtain the results (this time was once measured in weeks, and can now be reduced to a few days); and, finally, an extension of the time limit of the method itself (it promised to approach the barrier of 100,000 years). If the first two advantages were fully realized, the last still remains well short of the promised (40,000 and 50,000 years), due to the difficulty in controlling the contamination problems of micro-samples.
The foundation of the methods of luminescence dating translates into a basic principle: the amount of retained energy in the form of particles of negative electrical charge (electrons) in the imperfections of the crystal lattice of some minerals or rocks (such as, for instance, quartz, feldspar, calcite, clay, flint), depends on the amount of radiation received and increases with time. Therefore, the more time has passed, the greater the value of stored energy.
The source responsible for the radiation is represented, first, by the radioactive elements that, in a vestigial way, integrate the minerals and rocks (uranium, thorium, potassium). The radiation naturally emitted by them (α, β, and γ - Alpha, Beta and Gamma) will trigger the ionization of the atoms that make up the crystalline structure of the mineral, destabilizing it and, simultaneously, triggering the release of electrons. These will lodge in the natural imperfections of the structure itself, or in those created in the meantime by that very same phenomenon.
Thus, the accumulation of these electrons will be a constant process until, for example, a sample of rock or mineral is heated in laboratory. This causes the release of these particles, accompanied by production of energy in the form of an ultraviolet ray emission (luminescence).
The number of accumulated electrons and, hence, the intensity of the light emission associated with its release, as a result of heating, will be all the more intense as longer the radiation dose is received, so the more time has elapsed in the meantime.
In the context of this phenomenon, three steps assume a particular relevance in its use for the domain of dating:
a) the moment in which irradiation begins, after the material has been subjected to heat (about 500° C.), which has erased the effects of any previous radiation, is what we call "zero moment" and may correspond, for example, to the heating of the stones used to build a fireplace, or the baking of clay to make a figurine.
b) the accumulation in the mineral of a latent energy represented by the action of radiation α, β, and γ.
c) the laboratory-induced heating responsible for issuing of a thermoluminescence (TL), which is proportional to the radiation dose accumulated by the mineral, since its ability to retain electrons had been set to zero (when the stones were heated by fire produced in the fireplace, for example). Insofar as the sources of radiation did not change with time, the registered TL is proportional to the time that has elapsed since that zero moment.
Therefore, from the first heating, the mineral has been receiving a certain dose of radiation, which corresponds to the so-called "archaeological dose". In order to be determined, it is necessary to compare the natural thermoluminescence of the material (rock or mineral) with its artificial thermoluminescence, which is triggered and evaluated in a laboratory by irradiating another sample of it with a controlled source of β radiation. Ascertaining the induced TL by a known dose, it is possible then, almost by the appliance of a simple rule of three, to establish the dose stored in an archaeological sample, which took place since its last heating.
To complete the procedures that support the process of dating, it is still necessary to know the annual dose of radiation absorbed by the rock or mineral. As we have seen, this results, on the one hand, from the internal radiation (α and β rays) inherent to the presence of radioactive elements and, secondly, to external radiation (γ rays). These come from the same elements, but this time represented in the sediments that contextualize the archaeological materials and from cosmic radiation itself. The measurement of this radiation is established by placing at the site a set of thermoluminescent dosimeters that, for about a year, will accumulate and record it, its full amount being calculated later in laboratory.
Finalmente, e uma vez na posse de toda esta informação, é possível enunciar a simples fórmula que possibilita o estabelecimento de uma datação. A idade de uma amostra (t) é igual à razão da dose arqueológica (DA) pela dose anual (Da).
Finally, and once in possession of all this information, it is possible to state the simple formula that allows the establishment of a dating. The age of a sample (t) is equal to the ratio of the archaeological dose (AD) by the annual dose (aD).
The method of dating by optically stimulated luminescence (OSL), recently developed, can be considered a variant of the previous technique. Applied primarily to the direct dating of sediments rich in quartz and feldspar, its main difference is the fact that the phenomenon of luminescence is not caused by a phenomenon of rapid heating to high temperatures, but proceeds from the exposure of the sample to a source of green light (green laser) or ultraviolet rays.
BICHO, N. F. (2006) – Manual de Arqueologia Pré-Histórica, Edições 70, Lisboa, 525 p.