Les insectes subfossiles des archives du sol : un nouveau proxy pour caractériser la paléobiodiversité et l'histoire Holocène des espaces montagnards d'Europe occidentale (Pyrénées, Alpes et Apennins)

The study of subfossil insects as a means of documenting Holocene vegetation dynamics in relation to human practices has so far focused primarily on waterlogged, anoxicarchives. In contrast, unsaturated soils, widely employed in geoarchaeology and pedoanthracology, have rarely been explored in paleo-entomology, due to their perceived unfavorable preservation conditions. The aim of this dissertation is to fill this gap by extending the study of subfossil insects to mountain soils. Two main field contexts were considered: (1) ancient and mature montane forests (in the Pyrenees, Apennines, and Alps) exhibiting varying degrees of human impact; and (2) montane and subalpine grasslands (in the Pyrenees and Alps) influenced by pastoralism and past mining activities, the latter justifying the inclusion, for comparison, of a mining technosol. Four strategies were implemented to evaluate the relevance of paleoecological reconstructions based on insects preserved in mountain soils. First, the present-day composition of ground beetle (Carabidae) communities was analyzed across the Pyrenean sites, with the aim of identifying species indicative of dense forest cover or open habitats. These current assemblages were then compared with the taxa present in the studied soils. Second, the preservation conditions of insect remains were examined in soils from eleven sites with distinct biotic characteristics. Third, the ancientness of insect remains was investigated using two complementary approaches: radiocarbon dating of individual specimens, and estimation of their relative age through the development of a taphonomic assessment tool: the Time since death index (TSD). Fourth, the paleoecological information provided by the insects was compared with pedo-anthracological, archaeological, and historical data. Analysis of modern assemblages sampled in forest, ecotone, and grassland habitats shows that certain species may serve as reliable bioindicators within subfossil assemblages. Insect preservation depends both on the physicochemical properties of the soil and on taxon specific characteristics such as size, resistance to degradation, and ecological traits. Forest soils, due to the microclimate created by the canopy and their higher C/N ratios relative to grasslands, are especially favorable to the long-term preservation of insect remains. Radiocarbon dating of individual sclerites proved unreliable, as their low carbon mass (mgC) tends to produce artificially young ages. In the absence of robust absolute dates, the TSD index allowed the distinction between potentially ancient remains and more recent ones buried by bioturbation. At the scale of the upper Argentières valley, this approach confirms both the high sensitivity of insects and the very fine spatial resolution of the reconstructions. The presence of certain taxa effectively reveals episodes of landscape opening and ancient pastoral activities. However, insect remains did not provide information on forest stand maturity, as saproxylic species were not preserved in deeper horizons. Entomological reconstructions record environmental disturbances also detected through charcoal analysis, although the unreliability of radiocarbon dating prevented the establishment of chronological synchrony between the two archives. Despite limitations mainly linked to the lack of reliable absolute dating, this research identifies key factors promoting the long-term preservation of insect remains in forest soils. It thereby opens new perspectives for the broader development of both archaeoentomology and paleoentomology.