Identification du rythme annuel de précipitation des carbonates pariétaux pour un calage micro-chronologique des occupations archéologiques pyrogéniques : cas de la Grotte Mandrin (Malataverne, Drôme, France)

Abstract

In rock shelters and caves, the geo-archaeological reading of sediments can allow, in favourable cases, a micro-chronological study of traces of anthropogenic activities and in particular in the use of fire. And if the recurrence in the use of fireplaces can sometimes be identified by a micromorphological study of the structures, it is very uncertain that all the different combustion episodes can be identified. It turns out that paleo-fire events can be recorded elsewhere than in the hearths. For instance, they can be recorded as soot marks trapped in speleothems, which are witnesses of fires in cavities. They can be the object of a fuliginochronological study (lat. fuliginosus, fuligo: soot), which consists in studying the succession of soot deposits trapped in a matrix. Some limestone concretions have another advantage, which is to be annually laminated. When this is the case, the joint study of soot films and calcite doublets makes it possible to set paleo-fire chronicles on a micro-chronological scale of measured time. 
 In this study, we demonstrate, through the joint analysis of crystalline fabric alternations and seasonal variations of strontium (Sr) in concretions, that the calcite doublets observed in the fine parietal carbonated crusts of the Grotte Mandrin archaeological site are indeed annual. To do so, we use laser-induced plasma microspectroscopy (LIBS - Laser Induced Breackdown Spectroscopy), which allows to reveal variations of minor and traces elements in speleothems' carbonates on an annual to sub-annual scale through the analysis of transects or maps. Thus, soot film sequences can be indexed to the annual carbonate precipitation calendar and the joint study of soot film and calcite doublets makes it possible to set paleo-fire chronicles on a micro-chronological scale of measured time (annual resolution). The study of the rhythmicities of the occupations on this site then becomes accessible.

Editorial. Les carbonates archéologiques : la mémoire de l’eau

“…Par ailleurs, dans une optique chronologique à très haute résolution, le rythme de précipitation des carbonates permet le plus souvent d'en extraire un signal annuel ; ces enregistrements permettent alors de proposer des histoires dont le calendrier s'avère précis à une échelle (infra)annuelle, définissant ainsi une échelle micro-chronologique (Petit et al, 2018). Dans le champ de la Préhistoire, les encroutements de paroi qui ont conservé des films de suies, issues de feux réalisés dans les abris sous roche, permettent d'aborder les rythmes de fréquentation de ces espaces par les sociétés de chasseurs-collecteurs nomades (Vandevelde et al, 2021). Pour l'époque antique, l'analyse des dépôts laminés conservés, au moins partiellement, dans certaines structures hydrauliques permet de leur attribuer des durées de fonctionnement des aqueducs (Dubar, 2006a ;Bobée et al, 2010), ou de bains thermaux (Broise & Curie, 2014).…”