Caractérisation thermique des sédiments chauffés des foyers de la Grotte des Fraux (Dordogne, France) Mise en place d'une approche couplée entre tehrmoluminescence, susceptibilité magnétique, expérimentation et simulation
Résumé
In the cave of Les Fraux (Dordogne, France), the numerous traces of fires raise the questions of function of the site and of the way of life of the Bronze Age human groups inside the cave. The study of hearths is one possible way to answer those questions. This paper presents the methodology developed for the first time in the cave of les Fraux for the study of hearths. It consists in three parts: (1) the thermal characterization of sediment of the hearth; (2) the experimentation of fires; (3) the simulation of propagation of heat in the sediment. The thermal characterization is based on the examination of the thermoluminescence (TL) of quartz grains (from localized samples) and a map of magnetic susceptibility which is mainly brought by iron oxides contained in sediments. First, a map of magnetic susceptibility was built on the surface of the studied hearth and allowed the location of the heated zones. Then samples were extracted from the sediment along gradient of magnetic susceptibility located on the map. After the extraction of quartz grains, measurements of TL gave the paleotemperature attained by the sediment samples. This determination is based on the comparison between 'natural' TL curves of samples and 'simulated natural' TL curves of thermal references. The latter were made by heating, in laboratory, sediment from the cave with the same geological origin than hearth samples. They were heated according to an arbitrary and reproducible thermal cycle: heating at 20°C/min, one hour at the maximal temperature and subsequently cooling at 5°C/min. Thirteen thermal references were created with their maximal temperature ranged between 200°C and 650°C, keeping an unheated sample. The comparison allowed to associate one temperature to each sample. Then, the trend between paleotemperature and magnetic susceptibility permits to express magnetic susceptibility as a function of the paleotemperature and to obtain a map of temperature of the studied hearth. These results give some data about intensity of fires. To compare the fire effect observed on archaeological heated sediment and paleotemperature found, experimentations of fires were made. A substratum of sandstones and clay extracted from non-archaeological zones of the cave of Les Fraux was built. Temperatures in different positions and depths were recorded during firing and matched quite well with the surface paleotemperatures of archaeological hearth. Even the changing of sediment color seems to be identical. Finally, some tests of simulation of heat propagation were made in a homogeneous substratum. After a validation of the numerical modeling with the experimental data, it is possible to observe at different times the propagation of the heat. The methodology based on in situ and laboratory measurements, experimentations and simulations need to be supplement. The thermal characterization of a second hearth is begun, completed by a complete study of magnetic properties of the sediment in order to see if it is possible to obtain a new paleothermometer from magnetic mineralogy. A second step of experimentations of fires in underground environment and simulations in heterogeneous substratum are planned. Finally, it is hope that these reciprocal approaches will give clues to understand the function of the hearths and thus data on occupation of the site.
Les nombreuses traces de feux visibles au sein de la grotte des Fraux (Dordogne, France) posent des questions sur la fonction du site et sur les conditions de vie des hommes de l'Âge du Bronze à l'intérieur de la grotte. L'étude des foyers est une des voies pour répondre à ces questions. Cet article présente la méthodologie mise en place pour la caractérisation thermique des foyers. Elle est fondée sur l'observation de la thermoluminescence (TL) de grains de quartz (provenant d'échantillons ponctuels) et sur une carte de susceptibilité magnétique, laquelle est principalement portée par les oxydes de fer contenus dans les sédiments. La carte de susceptibilité magnétique permet de repérer et délimiter les zones chauffées. La TL permet d'obtenir une température équivalente maximum (paléotempérature) atteinte par le sédiment, en comparant les courbes TL des échantillons avec celles de références thermiques (réalisées à partir de sédiment de la grotte chauffé en laboratoire selon un cycle thermique arbitraire et reproductible). La tendance observée entre la paléotempérature et la susceptibilité magnétique permet d'exprimer la susceptibilité magnétique comme une fonction de la température. Ces informations nous renseignent sur l'intensité des feux. Des expérimentations de feux et simulations de la propagation de la chaleur ont été réalisées pour observer et comparer l'effet thermique des feux sur le substrat. Cette approche croisée apportera dans le futur des indices pour comprendre la fonction des foyers et à travers elle, la fonction du site
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