Using palaeobotanical and geochemical investigations to disentangle complex relations between human and landscape: The lacustrine record of Lake Petit (2200m, Southern Alps)

Archaeological studies carried out in the European Alps have documented ancient human occupation of mountains, characterized by hunting, pastoral activities and mining. High resolution multi-proxy analyses, including sedimentological, palynological and geochemical investigations, were carried out on a 144 cm-long sediment core from the Lac Petit (2200m, Mediterranean French Alps) in order to reconstruct past interactions between humans, the environment and the climate over the last 5000 years. Three main phases were identified, evidencing a progressive destabilization of the local environment. The first phase (ca. 4800 to 4300 cal. BP) is characterized by a relatively stable environmental conditions, as attested by a high lake productivity with the deposition of pure diatomite, the local presence of trees (conifer stomata, macro-remains), and well developed soils on slopes. The second phase (ca. 4300 to 1500 cal. BP) corresponds to a drastic decrease in arboreal pollen abundance, interpreted as the result of deforestation. In the same time, the lake geochemical record highlights an abrupt switch in sediment source marked by an input of terrestrial organic and mineral matters. Regular occurrences of anthropogenic pollen assemblages suggest early pastoral activity in the lake vicinity, while wetter conditions may have triggered hillslopes destabilization. The most recent phase (since ca. 1500 cal. BP) is marked by a degradation of the environment: the lake productivity dramatically decreased contemporaneously with the fall of the proportion of arboreal pollen. The highest values of anthropogenic pollen taxa and the highest concentrations of lead and mercury are recorded. Abundant terrigenous inputs also suggest that high-intensity rainfall events might have occurred, while agro-pastoralism and local mining activities continuously weakened the slopes. Even if the climate could have triggered such drastic changes, a discreet but continuous human impact since 1550 cal.BP might be the main factor of slopes degradation and ecosystem destabilization in the catchment.