François Lévêque

High resolution magnetic survey for soil monitoring: detection of drainage and soil tillage effects

Abstract This paper presents the results of high resolution magnetic cartography realized with a caesium vapor magnetometer on marshland plots in Western France. This site is interesting because of the clayey substratum homogeneity and the historical knowledge of plot exploitation. A wooden truck running on PVC rails allowed acquisition of accurate signals with dynamics of about 2 nT. Natural grassland presents weak decametric variations correlated to a micro-toposequence. Conversely, plots recently tilled show linear magnetic anomalies. These anomalies are interpreted as the effect of tractor tyre soil compaction which modify soil properties. In addition, the study of a drained plot, by a wavelet transform, reveals the presence of a frequency band close to the drain intervals. Moreover, the magnitude of the linear magnetic anomalies is higher above drains. This suggests that the compaction influence on the magnetic carriers is more important in drained, hence aerated, zones.

Early Neanderthal constructions deep in Bruniquel Cave in southwestern France

Very little is known about Neanderthal cultures, particularly early ones. Other than lithic implements and exceptional bone tools, very few artefacts have been preserved. While those that do remain include red and black pigments and burial sites, these indications of modernity are extremely sparse and few have been precisely dated, thus greatly limiting our knowledge of these predecessors of modern humans. Here we report the dating of annular constructions made of broken stalagmites found deep in Bruniquel Cave in southwest France. The regular geometry of the stalagmite circles, the arrangement of broken stalagmites and several traces of fire demonstrate the anthropogenic origin of these constructions. Uranium-series dating of stalagmite regrowths on the structures and on burnt bone, combined with the dating of stalagmite tips in the structures, give a reliable and replicated age of 176.5 thousand years (±2.1 thousand years), making these edifices among the oldest known well-dated constructions made by humans. Their presence at 336 metres from the entrance of the cave indicates that humans from this period had already mastered the underground environment, which can be considered a major step in human modernity.