ADA - Absolute Dating of Archaeomaterials
Knowledge of our past is both the starting point and the reason for seeking methods suited to date archaeomaterials. This word originally designated samples of archaeological interest, but has recently been extended to designate materials of Cultural Heritage at large, including landscapes and the environment. The selection of a particular technique is determined by the type of material to be dated and by the questions being investigated, which originate in all cases within interdisciplinary projects involving professionals with different expertise. LDL&BBCC Lab is currently one of the better equipped and competent European laboratories for dating ceramics, geological sediments and carbonaceous materials and is the reference facility in Southern Italy for the setup of TL authenticity tests. Since the time this activity started, protocols and procedures for measurements and data analysis have evolved both as optimization of the existing ones and as their extension to methods and techniques whose application has been motivated by the large variety and complexity of the physics questions posed by the properties of the samples and the conditions of their sites of origin. The TL, OSL, and EPR dating methods require the experimental determination of two physical quantities: paleodose — the total amount of radiation
accumulated by the crystalline inclusions present in the material to be dated since the time of the last zeroing, and the annual dose — the mean amount of radiation received every year from the radioactive elements naturally preset in the sample and its surroundings at the site of origin. The goal of our research activity is diversifying the chronological approaches by using a variety of samples of different nature, and applying different dating methods each time the sites allow this approach.
Dating via Stimulated Luminescence
Our main activity in the field of luminescence applications to dating consists in the improvement of precision and significance of age measurements and the extension of the scope of this method. Over the years, precision has improved from a
typical uncertainty of ~15% of the results of our early datings to ~5% obtained in the most recent campaigns. This dramatic improvement testifies the methodological effort accomplished. Test of samples whose age was known, coming from excavations performed all over the Mediterranean Sea basin, have been followed over the years by dating campaigns on ceramics and terracotta coming from different sites, such as Milena and Ramacca in Sicily, the Crete island in Greece, as well as from different architectonic structures: the so-called “Cubas” of Eastern Sicily, the Sant’Agata la Vetere complex in the center of Catania, the Church of Saint Philbert de Gran Lieu in France. Some of these campaigns have been conducted in collaboration with researches of other areas and Countries. Thanks to inter-calibration programs, the undertaken projects have confirmed the importance of inter-lab and inter-method comparisons both from the applicative and the methodological points of view and led in the year 2005 to the formation of GDRE, an European dating research group of which LDL&BBCC is part. This project, currently still in operation, entails the application of different methods to the same materials following independent procedures, in order to obtain a more significant evaluation of the time intervals involved (Figure 1). In the last few years research in the field of TL—a method which has become routine for dating materials that have undergone thermal annealing- have been focused on the solution of the question of dating specimens already excavated for which no information is available about the contribution of the environmental dose. This method, known as subtraction dating, consists in performing TL measurements on fractions of different granulometry and aims to solve chronological problems concerning entire cultures excavated in the past, whose remains lay in
authority offices and museums. OSL has afforded to solve chronological problems related to archaeosediments and geological sediments whose bleaching is due to sunlight. This led to strengthen collaborations between research projects pursued in Italy and in other Countries (such as France, Libya, Greece, Thailand) and has afforded to extend the range of measurable ages from thousand of years for ceramics to hundreds of thousands of years for some sedimentary deposits. Among the projects completed, particularly relevant has become the simultaneous dating of volcanic and alluvial sediments of the same stratigraphy in the Flegrean vulcanites of Crati (Calabria), an example of application of integrated TL and OSL coarse grain methods. Results obtained with OSL have also allowed to extend the field of application to a material which is of historical as well as artistic interest, but until recently had not been considered by the scientific community: mortars taken from historical buildings. This research topic is being pursued in order to recover samples previously considered hard to date, but also with the aim of making possible a direct dating of the construction of architectonic structures. This problem lays in the frame of one of the main current research lines of our lab, which concerns the constructions at large, including determination of the time of the building materials were manufactured and the time the structure was built. TL dating of building material is an indirect measurement which provides a terminus post quem, as the production of bricks or terracotta elements not necessarily is simultaneous to the epoch the building was constructed. The possibility that some materials got reused—a rather common practice in the past—make the interpretation of dating results even harder. With OSL instead, it is the time the mortars were spread that is determined, thus the date the structure was built.
Electron Paramagnetic Resonance and Time
The first step for the application of this method is the determination and characterization of EPR signals of the crystalline fractions separated from the samples to be dated. Our activity in the field of EPR dating has two goals: improve the confidence on the results obtained by luminescence techniques and increase the variety of datable samples. To this end, our studies concern both EPR centres of quartz inclusions, to which TL and/or OSL can also be applied, and calcite crystals, which at the moment can be dated only by EPR. Finding the centres whose response in terms of signal intensity shows a regular behaviour, experimentally reconstructed, with absorbed dose, allows to determine the paleodose (the total accumulated dose from time ‘zero’) with a technique both complementary and alternative to stimulated luminescence. For quartz inclusions extracted from ceramics and terracotta or from geological sediments, the obtained values refer to the instant of last heating at high temperature or the last exposure to sunlight, respectively; for these samples independent datings performed on the same mineral fraction leads to an improvement of the accuracy of the chronological data for a given sample or site under study. For materials currently not datable by means of luminescence, such as speleothemes coming from karst caves (Grotta Monello near Siracusa, and Grotta Zinzulusa near Lecce), studies illustrate the capabilities of this method in order to determine the total accumulated dose since the formation of the calcite crystals which constitute the stalactites and stalagmites of those caves. These results, together which those obtained for fossil shells found in quaternary stratigraphies of the Taranto Gulf and from marine terraces of Southern Calabria have shown the need to determine, for each measurement series, the signals really useful for dating, as they can vary quite a lot from one sample to another.
GRUPPO DI RICERCA:
Sebastiano Olindo Troja