The Oldest Neanderthal Fire Deep in Cave Revealed by Geomagnetic Survey in Bruniquel Cave on the Speleofacts Structures Covered by Calcite Concretions

The Bruniquel Cave structures are composed of speleothems or fragments of speleothems (speleofacts), aligned and superimposed or accumulated, more than 300 m from the cave entrance.The construction of these structures has been dated to 176.5 ky ±2.1 (Jaubert et al. 2016). Evidence of fire can be seen locally in various parts of the structures not covered by calcite. Since the first geomagnetic surveys conducted in 2015, the area covered has been extended and the protocols have been adapted to the context. A sensor for measuring the magnetic field intensity is associated with a laser reflector and suspended at the end of a telescopic pole that can be extended to almost 5 m and fixed on a tripod. While the first sensor is moved by sweeping the space during the deployment of the pole, a second fixed sensor is located nearby, less than 5 m away, to allow correction of the temporal variation of the local magnetic field. The positioning of the mobil measurements is ensured by laser tracking of the reflector using a total station. The magnetic field strength measurements are carried out continuously at a rate of 10 Hz while the reflector coordinates are determined at a rate of 20 Hz. The resulting magnetic field intensity point cloud identifies dipole anomalies, some of which coincide with fire indices. Other dipole anomalies correspond to areas covered by calcite. These structures correspond to fires masked by the calcite drape. Negative anomalies reflect holes left in the clay sediment of the cave by the pulling out of speleothems. The intensity of the magnetic anomalies produced by the thermomagnetic impact on the detrital elements present in the speleothem calcite seems high but could be explained by the presence of clay coating the speleothem in place. Thus, the source of the thermo-remanent magnetizations would not correspond to the impurities in the calcite of the speleothems, but to the magnetic enrichment produced by the reduction of the goethite present in the wet clayey sediments of the cave which partially covered the speleothems at a time in the history of the cave, before the structures were built by the Neanderthals. The presence of dipole geomagnetic anomalies on speleofact structures dated at 176.5 ky demonstrates that the hearths were made in situ on the structures most likely contemporaneously with their construction.